Detection and Enumeration of Clostridium difficile Spores in Retail Beef and Pork

Recent studies have identified Clostridium difficile in food animals and retail meat, and concern has been raised about the potential for food to act as a source of C. difficile infection in humans. Previous studies of retail meat have relied on enrichment culture alone, thereby preventing any assessment of the level of contamination in meat. This study evaluated the prevalence of C. difficile contamination of retail ground beef and ground pork in Canada. Ground beef and ground pork were purchased from retail outlets in four Canadian provinces. Quantitative and enrichment culture was performed. Clostridium difficile was isolated from 28/230 (12%) samples overall: 14/115 (12%) ground beef samples and 14/115 (12%) ground pork samples (P = 1.0). For ground beef, 10/14 samples (71%) were positive by enrichment culture only. Of the 4 ground beef samples that were positive by direct culture, 20 spores/g were present in 2 while 120 and 240 spores/g were present in 1 each. For ground pork, 10/14 (71%) samples were positive by enrichment culture only. Of the 4 ground pork samples that were positive by direct culture, 20 spores/g were present in 3 while 60 spores/g were present in 1. Ribotype 078 predominated, consistent with some previous studies of C. difficile in food animals. Ribotype 027/North American pulsotype 1 was also identified in both retail beef and pork. This study has identified relatively common contamination of retail ground beef and pork with C. difficile spores; however, the levels of contamination were very low.Clostridium difficile is an important cause of enteric disease in humans. It is the most commonly diagnosed cause of hospital- and antimicrobial agent-associated diarrhea in people, and recent evidence suggests that it may be emerging as an important community-associated pathogen (2, 5). In addition to humans, C. difficile can be found in the intestinal tracts of a variety of animal species, including food animals, such as cattle and pigs (7, 10, 13). Clostridium difficile has also been found in retail meat (11, 12, 17), and concerns about the role of food in the epidemiology of community-associated C. difficile infection (CA-CDI) have been expressed (5, 8, 15).Initial studies have reported isolation of C. difficile from 4.6 to 45% of retail meat samples (11, 12, 17). However, all studies have used broth enrichment protocols, which could detect very low spore numbers and provide no information about the number of organisms present in a sample. No studies have evaluated numbers of C. difficile spores in food. While the infectious dose is not known, an understanding of the level of contamination may be an important factor in determining the relevance of contamination of food. Additionally, the use of different methods between studies hampers comparison of results. Recently a study was performed to evaluate different methods for qualitative and quantitative detection of C. difficile (21). This study determined that the detection threshold of enrichment culture could be at least as low as 10 spores/g of meat. It also determined that quantitative culture can accurately determine the level of contamination in experimentally inoculated meat samples, albeit with a higher detection threshold. The objective of this study was to determine the prevalence of C. difficile contamination of retail ground beef and ground pork using both qualitative and quantitative methods.     

Proteomic and Genomic Characterization of Highly Infectious Clostridium difficile 630 Spores

Clostridium difficile, a major cause of antibiotic-associated diarrhea, produces highly resistant spores that contaminate hospital environments and facilitate efficient disease transmission. We purified C. difficile spores using a novel method and show that they exhibit significant resistance to harsh physical or chemical treatments and are also highly infectious, with <7 environmental spores per cm² reproducibly establishing a persistent infection in exposed mice. Mass spectrometric analysis identified ∼336 spore-associated polypeptides, with a significant proportion linked to translation, sporulation/germination, and protein stabilization/degradation. In addition, proteins from several distinct metabolic pathways associated with energy production were identified. Comparison of the C. difficile spore proteome to those of other clostridial species defined 88 proteins as the clostridial spore “core” and 29 proteins as C. difficile spore specific, including proteins that could contribute to spore-host interactions. Thus, our results provide the first molecular definition of C. difficile spores, opening up new opportunities for the development of diagnostic and therapeutic approaches.Clostridium difficile is a gram-positive, spore-forming, anaerobic bacterium that can asymptomatically colonize the intestinal tracts of humans and other mammals (3, 30, 39). Antibiotic treatment can result in C. difficile overgrowth and can lead to clinical disease, ranging from diarrhea to life-threatening pseudomembranous colitis, particularly in immunocompromised hosts (2, 4, 7). In recent years, C. difficile has emerged as the major cause of nosocomial antibiotic-induced diarrhea, and it is frequently associated with outbreaks (21, 22). A contributing factor is that C. difficile can be highly infectious and difficult to contain, especially when susceptible patients are present in the same hospital setting (13).Person-to-person transmission of C. difficile is associated with the excretion of highly resistant spores in the feces of infected patients, creating an environmental reservoir that can confound many infection control measures (29, 44). Bacterial spores, which are metabolically dormant cells that are formed following asymmetric cell division, normally have thick concentric external layers, the spore coat and cortex, that protect the internal cytoplasm (15, 42). Upon germination, spores lose their protective external layers and resume vegetative growth (24, 27, 36). Bacillus spores and the spores of most Clostridium species germinate in response to amino acids, carbohydrates, or potassium ions (24, 36). In contrast, C. difficile spores show an increased level of germination in response to cholate derivatives found in bile (40, 41). Thus, spores are well adapted for survival and dispersal under a wide range of environmental conditions but will germinate in the presence of specific molecular signals (24, 36).While the spores of a number of Bacillus species, such as Bacillus subtilis and Bacillus anthracis, and those of other Clostridium species, such as Clostridium perfringens (15, 20), have been well characterized, research on C. difficile spores has been relatively limited. A greater understanding of C. difficile spore biology could be exploited to rationalize disinfection regimes, molecular diagnostics, and the development of targeted treatments such as vaccines. Here we describe a novel method to isolate highly purified C. difficile spores that maintain their resistance and infectious characteristics, thus providing a unique opportunity to study C. difficile spores in the absence of vegetative cells. A thorough proteomic and genomic analysis of the spore provides novel insight into the unique composition and predictive biological properties of C. difficile spores that should underpin future research into this high-profile but poorly understood pathogen.     

Inhibiting the Initiation of Clostridium difficile Spore Germination using Analogs of Chenodeoxycholic Acid,a Bile Acid

To cause disease, Clostridium difficile spores must germinate in the host gastrointestinal tract. Germination is initiated upon exposure to glycine and certain bile acids, e.g., taurocholate. Chenodeoxycholate, another bile acid, inhibits taurocholate-mediated germination. By applying Michaelis-Menten kinetic analysis to C. difficile spore germination, we found that chenodeoxycholate is a competitive inhibitor of taurocholate-mediated germination and appears to interact with the spores with greater apparent affinity than does taurocholate. We also report that several analogs of chenodeoxycholate are even more effective inhibitors. Some of these compounds resist 7α-dehydroxylation by Clostridium scindens, a core member of the normal human colonic microbiota, suggesting that they are more stable than chenodeoxycholate in the colonic environment.Clostridium difficile is a Gram-positive, spore-forming, anaerobic bacterium that is pathogenic for both humans and animals (33, 44). Infections caused by C. difficile range from mild diarrhea to more life-threatening conditions, such as pseudomembranous colitis (33). In the classic case, prior antibiotic treatment that disrupts the normally protective colonic flora makes patients susceptible to C. difficile infection (CDI) (35, 53). Other antibiotics, such as vancomycin and metronidazole, are the most commonly used treatments for CDI (54). However, because these antibiotics also disrupt the colonic flora, 10 to 40% of patients whose symptoms have been ameliorated suffer from relapsing CDI (15, 24). The annual treatment-associated cost for CDI in the United States is estimated to be between $750 million and $3.2 billion (8, 9, 16, 31). Moreover, the number of fatal cases of CDI has been increasing rapidly (14, 39). Thus, there is an urgent need to find alternative therapies for CDI.C. difficile infection likely is initiated by infection with the spore form of C. difficile (12). C. difficile elicits disease through the actions of two secreted toxins, TcdA and TcdB (48). TcdB was recently shown to be critical for pathogenesis in an animal model of disease (18). Since the toxins are produced by vegetative cells, not by spores (17), germination and outgrowth are prerequisites for pathogenesis.Spore germination is triggered by the interaction of small molecules, called germinants, with receptors within the spore inner membrane. These germinants vary by bacterial species and can include ions, amino acids, sugars, nucleotides, surfactants, or combinations thereof (43). The recognition of germinants triggers irreversible germination, leading to Ca²⁺-dipicolinic acid release, the uptake of water, the degradation of the cortex, and, eventually, the outgrowth of the vegetative bacterium (43). The germination receptors that C. difficile uses to sense the environment have not been identified. Based on homology searches, C. difficile germination receptors must be very different from known germination receptors (42), but they appear to be proteinaceous (13).Taurocholate, a primary bile acid, has been used for approximately 30 years by researchers and clinical microbiologists to increase colony formation by C. difficile spores from patient and environmental samples (3, 49, 51, 52). This suggested that C. difficile spores interact with bile acids along the gastrointestinal (GI) tract and that spores use a host-derived signal to initiate germination.The liver synthesizes the two major primary bile acids, cholate and chenodeoxycholate (40). These compounds are modified by conjugation with either taurine (to give taurocholate or taurochenodeoxycholate) or glycine (producing glycocholate or glycochenodeoxycholate). Upon secretion into the digestive tract, bile aids in the absorption of fat and cholesterol; much of the secreted bile is actively absorbed and recycled back to the liver for reutilization (40). Though efficient, enterohepatic recirculation is not complete; bile enters the cecum of the large intestine at a concentration of approximately 2 mM (30).In the cecum, bile is modified by the normal, benign colonic flora. First, bile salt hydrolases found on the surfaces of many bacterial species remove the conjugated amino acid, producing the deconjugated primary bile acids cholate and chenodeoxycholate (40). These deconjugated primary bile acids are further metabolized by only a few species of intestinal bacteria, including Clostridium scindens. C. scindens actively transports unconjugated primary bile acids into the cytoplasm, where they are 7α-dehydroxylated, converting cholate to deoxycholate and chenodeoxycholate to lithocholate (21, 40). The disruption of the colonic flora by antibiotic treatment abolishes 7α-dehydroxylation activity (41).Building upon the work on Wilson and others (51, 52), we demonstrated that taurocholate and glycine, acting together, trigger the loss of the birefringence of C. difficile spores (45). All cholate derivatives (taurocholate, glycocholate, cholate, and deoxycholate) stimulate the germination of C. difficile spores (45). Recently it was shown that taurocholate binding is prerequisite to glycine binding (37). At physiologically relevant concentrations, chenodeoxycholate inhibits taurocholate-mediated germination (46) and also inhibits C. difficile vegetative growth, as does deoxycholate (45). In fact, C. difficile spores use the relative concentrations of the various bile acids as cues for germination within the host (10).Since chenodeoxycholate is absorbed by the colonic epithelium and metabolized to lithocholate by the colonic flora (25, 40), the use of chenodeoxycholate as a therapy against C. difficile disease might be hindered by its absorption and conversion to lithocholate.Here, we further characterize the interaction of C. difficile spores with various bile acids and demonstrate that chenodeoxycholate is a competitive inhibitor of taurocholate-mediated germination. Further, we identify chemical analogs of chenodeoxycholate that are more potent inhibitors of germination and that resist 7α-dehydroxylation by the colonic flora, potentially increasing their stability and effectiveness as inhibitors of C. difficile spore germination in the colonic environment.     

Use of Purified Clostridium difficile Spores To Facilitate Evaluation of Health Care Disinfection Regimens

Clostridium difficile is a major cause of antibiotic-associated diarrheal disease in many parts of the world. In recent years, distinct genetic variants of C. difficile that cause severe disease and persist within health care settings have emerged. Highly resistant and infectious C. difficile spores are proposed to be the main vectors of environmental persistence and host transmission, so methods to accurately monitor spores and their inactivation are urgently needed. Here we describe simple quantitative methods, based on purified C. difficile spores and a murine transmission model, for evaluating health care disinfection regimens. We demonstrate that disinfectants that contain strong oxidizing active ingredients, such as hydrogen peroxide, are very effective in inactivating pure spores and blocking spore-mediated transmission. Complete inactivation of 10⁶ pure C. difficile spores on indicator strips, a six-log reduction, and a standard measure of stringent disinfection regimens require at least 5 min of exposure to hydrogen peroxide vapor (HPV; 400 ppm). In contrast, a 1-min treatment with HPV was required to disinfect an environment that was heavily contaminated with C. difficile spores (17 to 29 spores/cm²) and block host transmission. Thus, pure C. difficile spores facilitate practical methods for evaluating the efficacy of C. difficile spore disinfection regimens and bringing scientific acumen to C. difficile infection control.Clostridium difficile is a Gram-positive, spore-forming, anaerobic bacterium that is a major cause of health care-acquired infections and antibiotic-associated diarrhea (2). In recent years, several genetic variants of C. difficile have emerged as important health care pathogens (6). Perhaps most notable is the “hypervirulent” variant, commonly referred to as PCR ribotype 027/restriction endonuclease analysis (REA) group BI, that produces elevated levels of toxins TcdA and TcdB (17, 19). Other virulent ribotypes that display extensive heterogeneity among their toxin protein sequences (26) and gene activities (8) have emerged. Using whole-genome sequencing, we demonstrated that there are broad genetic differences between the entire genomes of several common variants, including ribotype/REA group variants 012/R, 017/CF, and 027/BI used in this study (12, 27, 31). In contrast, phylogeographic analysis of 027/BI isolates from Europe and the United States demonstrates that this clade is extremely clonal and implies recent transcontinental spread of hypervirulent C. difficile (12).C. difficile is distinct from many other health care pathogens because it produces highly infectious spores that are shed into the environment (25, 28). C. difficile spores can resist disinfection regimens that normally inactivate other health care pathogens, such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci, therefore challenging current infection control measures (2). A multifaceted approach is normally used to control C. difficile in health care facilities (32). Interventions include antimicrobial stewardship, increased clinical awareness, patient isolation (11), and enhanced environmental disinfection regimens based on hydrogen peroxide (H₂O₂) vapor (HPV) (4). While attempts to break the spore-mediated infection cycle and interrupt these efficient routes of transmission are important for infection control measures, there is little quantitative evidence indicating which interventions are most effective (7). Here we describe the exploitation of pure C. difficile spores (16) and a murine transmission model (15) in simple, practical methods to quantitatively monitor the impact of health care disinfection regimens on C. difficile viability. These methods can be used to optimize disinfection regimens targeted at C. difficile.     

Superdormant Spores of Bacillus Species Germinate Normally with High Pressure,Peptidoglycan Fragments,and Bryostatin

Superdormant spores of Bacillus cereus and Bacillus subtilis germinated just as well as dormant spores with pressures of 150 or 500 MPa and with or without heat activation. Superdormant B. subtilis spores also germinated as well as dormant spores with peptidoglycan fragments or bryostatin, a Ser/Thr protein kinase activator.Spores of Bacillus species are formed in sporulation, a process that is generally triggered by starvation for one or more nutrients (13, 19). These spores are metabolically dormant and extremely resistant to a large variety of environmental stresses, including heat, radiation, and toxic chemicals, and as a consequence of these properties, these spores can remain viable in their dormant state for many years (13, 18, 19). However, spores are constantly sensing their environment, and if nutrients return, the spores can rapidly return to growth through the process of spore germination (17). Spore germination is generally triggered by specific nutrients that bind to nutrient germinant receptors, with this binding alone somehow triggering germination. However, spore germination can also be triggered by many non-nutrient agents, including cationic surfactants such as dodecylamine, a 1:1 complex of Ca²⁺ with pyridine-2,6-dicarboxylic acid (dipicolinic acid [DPA], a major spore small molecule), very high pressures, specific peptidoglycan fragments, and bryostatin, an activator of Ser/Thr protein kinases (17, 19, 20). For nutrient germinants in particular, spore germination is also potentiated by a prior sublethal heat treatment termed heat activation (17).While normally the great majority of spores in populations germinate relatively rapidly in response to nutrient germinants, a small percentage of spores germinate extremely slowly. These spores that are refractory to nutrient germination have been termed superdormant spores and are a major concern for the food industry (8). Recently superdormant spores of three Bacillus species have been isolated by repeated germination of spore populations with specific nutrient germinants and isolation of remaining dormant spores (5, 6). These superdormant spores germinate extremely poorly with the nutrient germinants used in superdormant spore isolation, as well as with other nutrient germinants. All of the specific defects leading to spore superdormancy are not known, although an increased level of receptors for specific nutrient germinants decreases levels of superdormant spores obtained with the nutrients that are ligands for these receptors (5). Superdormant spores also have significantly higher temperature optima for heat activation of nutrient germination than the spore population as a whole (7).In contrast to the poor germination of superdormant spores with nutrient germinants, superdormant spores germinate normally with dodecylamine and Ca-DPA (5, 6). This is consistent with possible roles of nutrient germinant receptor levels and/or heat activation temperature optima in affecting spore superdormancy, since neither dodecylamine nor Ca-DPA triggers Bacillus spore germination through nutrient germinant receptors, and germination with these agents is also not stimulated by heat activation (11, 15, 17). However, the effects of high pressures, peptidoglycan fragments, and bryostatin, all of which almost certainly trigger spore germination by mechanisms at least somewhat different than triggering of germination by nutrients, dodecylamine, and Ca-DPA (2, 3, 11, 15, 20, 22, 23), have not been tested for their effects on superdormant spores. Consequently, we have compared the germination of dormant and superdormant spores of two Bacillus species by high-pressures, peptidoglycan fragments, and bryostatin.The spores used in this work were from Bacillus subtilis PS533 (16), a derivative of strain 168 that also carries plasmid pUB110, providing resistance to kanamycin (10 μg/ml), and Bacillus cereus T (originally obtained from H. O. Halvorson). Spores of these strains were prepared and purified as described previously (6, 10, 12). Superdormant spores of B. subtilis were prepared by germination following heat activation at 75°C for 30 min by two germination treatments at 37°C with 10 mM l-valine for 2 h, followed by isolation of remaining dormant spores, all as described previously (5, 10, 12). These superdormant spores germinated extremely poorly with 10 mM valine at 37°C, giving ≤10% germination in 2 h at 37°C, while the initial spore population exhibited >95% germination under the same conditions (data not shown). Superdormant B. cereus spores were isolated similarly, although heat activation was at 65°C for 30 min and the germinant was 5 mM inosine as described previously (6). These superdormant B. cereus spores exhibited <5% germination with inosine in 2 h at 37°C compared to the >95% germination of the initial dormant spores under the same conditions (data not shown).     

Occurrence and Molecular Characteristics of Methicillin-Resistant Staphylococcus aureus and Methicillin-Resistant Staphylococcus pseudintermedius in an Academic Veterinary Hospital

Recently, methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant Staphylococcus pseudintermedius (MRSP) have been increasingly isolated from veterinarians and companion animals. With a view to preventing the spread of MRSA and MRSP, we evaluated the occurrence and molecular characteristics of each in a veterinary college. MRSA and MRSP were isolated from nasal samples from veterinarians, staff members, and veterinary students affiliated with a veterinary hospital. Using stepwise logistic regression, we identified two factors associated with MRSA carriage: (i) contact with an identified animal MRSA case (odds ratio [OR], 6.9; 95% confidence interval [95% CI], 2.2 to 21.6) and (ii) being an employee (OR, 6.2; 95% CI, 2.0 to 19.4). The majority of MRSA isolates obtained from individuals affiliated with the veterinary hospital and dog patients harbored spa type t002 and a type II staphylococcal cassette chromosome mec (SCCmec), similar to the hospital-acquired MRSA isolates in Japan. MRSA isolates harboring spa type t008 and a type IV SCCmec were obtained from one veterinarian on three different sampling occasions and also from dog patients. MRSA carriers can also be a source of MRSA infection in animals. The majority of MRSP isolates (85.2%) carried hybrid SCCmec type II-III, and almost all the remaining MRSP isolates (11.1%) carried SCCmec type V. MRSA and MRSP were also isolated from environmental samples collected from the veterinary hospital (5.1% and 6.4%, respectively). The application of certain disinfection procedures is important for the prevention of nosocomial infection, and MRSA and MRSP infection control strategies should be adopted in veterinary medical practice.Methicillin-resistant Staphylococcus aureus (MRSA) is an important cause of nosocomial infections in human hospitals. The prevalence of hospital-acquired MRSA (HA-MRSA) infection among inpatients in intensive care units (ICUs) continues to increase steadily in Japan. Recently, cases of community-acquired MRSA (CA-MRSA) have been documented in persons without an established risk factor for HA-MRSA infection (14, 32, 36, 49).There has also been an increase in the number of reports of the isolation of MRSA from veterinarians and companion animals (5, 21, 23-26, 28, 31, 34, 38, 44, 50, 51, 53). Values reported for the prevalence of MRSA among veterinary staff include 17.9% in the United Kingdom (21), 10% in Japan (38), 3.9% in Scotland (13), and 3.0% in Denmark (28). Loeffler et al. reported that the prevalence of MRSA among dog patients and healthy dogs owned by veterinary staff members was 8.9% (21). In Japan, an MRSA isolate was detected in only one inpatient dog (3.8%) and could not be detected in any of 31 outpatient dogs (38). In the United States, MRSA isolates were detected in both dog (0.1%) and cat (0.1%) patients (31). The prevalence of MRSA among healthy dogs has been reported to be 0.7% (5). Hanselman et al. suggested that MRSA colonization may be an occupational risk for large-animal veterinarians (12). Recently, Burstiner et al. reported that the frequency of MRSA colonization among companion-animal veterinary personnel was equal to the frequency among large-animal veterinary personnel (6).In addition, other methicillin-resistant coagulase-positive staphylococci (MRCPS), such as methicillin-resistant Staphylococcus pseudintermedius (MRSP) and methicillin-resistant Staphylococcus schleiferi (MRSS), isolated from dogs, cats, and a veterinarian have been reported (11, 31, 38, 40, 52). MRSP isolates have also been detected among inpatient dogs (46.2%) and outpatient dogs (19.4%) in a Japanese veterinary teaching hospital (38). In Canada, however, MRSP and MRSS isolates were detected in only 2.1% and 0.5% of dog patients, respectively (11).Methicillin-resistant staphylococci produce penicillin-binding protein 2′, which reduces their affinity for β-lactam antibiotics. This protein is encoded by the mecA gene (48), which is carried on the staphylococcal cassette chromosome mec (SCCmec). SCCmec is a mobile genetic element characterized by the combination of the mec and ccr complexes (16), and it is classified into subtypes according to differences in the junkyard regions (43). SCCmec typing can be used as a molecular tool (22, 27, 30, 33, 36, 55) for examining the molecular epidemiology of methicillin-resistant staphylococci.In this study, we investigated the occurrence and characteristics of MRCPS isolates in a veterinary hospital in order to establish the transmission route of MRCPS in a veterinary hospital and with a view to preventing the spread of MRCPS infection. In addition, we evaluated the factors associated with MRCPS. Further, as Heller et al. have reported the distribution of MRSA within veterinary hospital environments and suggested the necessity to review cleaning protocols of hospital environments (13), we also attempted to isolate MRCPS from environmental samples collected in a veterinary hospital for an evaluation of MRSA transmission cycle though environmental surfaces in the veterinary hospital.     

Roles of Small,Acid-Soluble Spore Proteins and Core Water Content in Survival of Bacillus subtilis Spores Exposed to Environmental Solar UV Radiation

Spores of Bacillus subtilis contain a number of small, acid-soluble spore proteins (SASP) which comprise up to 20% of total spore core protein. The multiple α/β-type SASP have been shown to confer resistance to UV radiation, heat, peroxides, and other sporicidal treatments. In this study, SASP-defective mutants of B. subtilis and spores deficient in dacB, a mutation leading to an increased core water content, were used to study the relative contributions of SASP and increased core water content to spore resistance to germicidal 254-nm and simulated environmental UV exposure (280 to 400 nm, 290 to 400 nm, and 320 to 400 nm). Spores of strains carrying mutations in sspA, sspB, and both sspA and sspB (lacking the major SASP-α and/or SASP-β) were significantly more sensitive to 254-nm and all polychromatic UV exposures, whereas the UV resistance of spores of the sspE strain (lacking SASP-γ) was essentially identical to that of the wild type. Spores of the dacB-defective strain were as resistant to 254-nm UV-C radiation as wild-type spores. However, spores of the dacB strain were significantly more sensitive than wild-type spores to environmental UV treatments of >280 nm. Air-dried spores of the dacB mutant strain had a significantly higher water content than air-dried wild-type spores. Our results indicate that α/β-type SASP and decreased spore core water content play an essential role in spore resistance to environmentally relevant UV wavelengths whereas SASP-γ does not.Spores of Bacillus spp. are highly resistant to inactivation by different physical stresses, such as toxic chemicals and biocidal agents, desiccation, pressure and temperature extremes, and high fluences of UV or ionizing radiation (reviewed in references 33, 34, and 48). Under stressful environmental conditions, cells of Bacillus spp. produce endospores that can stay dormant for extended periods. The reason for the high resistance of bacterial spores to environmental extremes lies in the structure of the spore. Spores possess thick layers of highly cross-linked coat proteins, a modified peptidoglycan spore cortex, a low core water content, and abundant intracellular constituents, such as the calcium chelate of dipicolinic acid and α/β-type small, acid-soluble spore proteins (α/β-type SASP), the last two of which protect spore DNA (6, 42, 46, 48, 52). DNA damage accumulated during spore dormancy is also efficiently repaired during spore germination (33, 47, 48). UV-induced DNA photoproducts are repaired by spore photoproduct lyase and nucleotide excision repair, DNA double-strand breaks (DSB) by nonhomologous end joining, and oxidative stress-induced apurinic/apyrimidinic (AP) sites by AP endonucleases and base excision repair (15, 26-29, 34, 43, 53, 57).Monochromatic 254-nm UV radiation has been used as an efficient and cost-effective means of disinfecting surfaces, building air, and drinking water supplies (31). Commonly used test organisms for inactivation studies are bacterial spores, usually spores of Bacillus subtilis, due to their high degree of resistance to various sporicidal treatments, reproducible inactivation response, and safety (1, 8, 19, 31, 48). Depending on the Bacillus species analyzed, spores are 10 to 50 times more resistant than growing cells to 254-nm UV radiation. In addition, most of the laboratory studies of spore inactivation and radiation biology have been performed using monochromatic 254-nm UV radiation (33, 34). Although 254-nm UV-C radiation is a convenient germicidal treatment and relevant to disinfection procedures, results obtained by using 254-nm UV-C are not truly representative of results obtained using UV wavelengths that endospores encounter in their natural environments (34, 42, 50, 51, 59). However, sunlight reaching the Earth''s surface is not monochromatic 254-nm radiation but a mixture of UV, visible, and infrared radiation, with the UV portion spanning approximately 290 to 400 nm (33, 34, 36). Thus, our knowledge of spore UV resistance has been constructed largely using a wavelength of UV radiation not normally reaching the Earth''s surface, even though ample evidence exists that both DNA photochemistry and microbial responses to UV are strongly wavelength dependent (2, 30, 33, 36).Of recent interest in our laboratories has been the exploration of factors that confer on B. subtilis spores resistance to environmentally relevant extreme conditions, particularly solar UV radiation and extreme desiccation (23, 28, 30, 34 36, 48, 52). It has been reported that α/β-type SASP but not SASP-γ play a major role in spore resistance to 254-nm UV-C radiation (20, 21) and to wet heat, dry heat, and oxidizing agents (48). In contrast, increased spore water content was reported to affect B. subtilis spore resistance to moist heat and hydrogen peroxide but not to 254-nm UV-C (12, 40, 48). However, the possible roles of SASP-α, -β, and -γ and core water content in spore resistance to environmentally relevant solar UV wavelengths have not been explored. Therefore, in this study, we have used B. subtilis strains carrying mutations in the sspA, sspB, sspE, sspA and sspB, or dacB gene to investigate the contributions of SASP and increased core water content to the resistance of B. subtilis spores to 254-nm UV-C and environmentally relevant polychromatic UV radiation encountered on Earth''s surface.     

SleC Is Essential for Germination of Clostridium difficile Spores in Nutrient-Rich Medium Supplemented with the Bile Salt Taurocholate

Clostridium difficile is the major cause of infectious diarrhea and a major burden to health care services. The ability of this organism to form endospores plays a pivotal role in infection and disease transmission. Spores are highly resistant to many forms of disinfection and thus are able to persist on hospital surfaces and disseminate infection. In order to cause disease, the spores must germinate and the organism must grow vegetatively. Spore germination in Bacillus is well understood, and genes important for this process have recently been identified in Clostridium perfringens; however, little is known about C. difficile. Apparent homologues of the spore cortex lytic enzyme genes cwlJ and sleB (Bacillus subtilis) and sleC (C. perfringens) are present in the C. difficile genome, and we describe inactivation of these homologues in C. difficile 630Δerm and a B1/NAP1/027 clinical isolate. Spores of a sleC mutant were unable to form colonies when germination was induced with taurocholate, although decoated sleC spores formed the same number of heat-resistant colonies as the parental control, even in the absence of germinants. This suggests that sleC is absolutely required for conversion of spores to vegetative cells, in contrast to CD3563 (a cwlJ/sleB homologue), inactivation of which had no effect on germination and outgrowth of C. difficile spores under the same conditions. The B1/NAP1/027 strain {"type":"entrez-nucleotide","attrs":{"text":"R20291","term_id":"774925","term_text":"R20291"}}R20291 was found to sporulate more slowly and produce fewer spores than 630Δerm. Furthermore, fewer {"type":"entrez-nucleotide","attrs":{"text":"R20291","term_id":"774925","term_text":"R20291"}}R20291 spores germinated, indicating that there are differences in both sporulation and germination between these epidemic and nonepidemic C. difficile isolates.The Gram-positive anaerobe Clostridium difficile causes diarrheal diseases ranging from asymptomatic carriage to a fulminant, relapsing, and potentially fatal colitis (8, 30). This organism is resistant to various broad-spectrum antibiotics and capitalizes on disruption of the normal intestinal flora to colonize and cause disease symptoms through the action of toxins A and B (16, 40). While these toxins are the principal virulence factors, the ability of the organism to produce endospores is necessary for disease transmission.Clostridial spores are extremely resistant to all kinds of chemical and physical agents and provide the mechanism by which C. difficile can evade the potentially fatal consequences of exposure to heat, oxygen, alcohol, and certain disinfectants (35). Thus, the spores shed in fecal matter are very difficult to eradicate and can persist on contaminated surfaces in health care facilities for extended periods of time (35). This leads to infection or reinfection of cohabitating individuals through inadvertent ingestion of infected material (10, 32). Once in the anaerobic environment of the gut, spores presumably germinate to form toxin-producing vegetative cells and, in susceptible individuals, diarrheal disease.Spore germination is defined as the events that result in the irreversible loss of spore characteristics. However, current mechanistic knowledge of the germination process is based principally on data derived from studying Bacillus subtilis. Little is known about spore germination in clostridia and, in particular, in C. difficile. Germination is initiated when the bacterial spore senses specific effectors, termed germinants. These effectors can include nutrients, cationic surfactants, peptidoglycan, and a 1:1 chelate of pyridine-2,6-dicarboxylic acid (dipicolinic acid) and Ca²⁺ (CaDPA) (23, 34, 36). Spores of B. subtilis can germinate in response to nutrients through the participation of three sensory receptors located in the spore inner membrane, GerA, GerB, and GerK (23). After activation, the events include the release of monovalent cations (H⁺, K⁺, and Na⁺) and CaDPA (accounting for approximately 10% of the spore dry weight) (36). The third major step in germination involves hydrolysis of the spore peptidoglycan (PG) cortex. It is during this hydrolysis that the previously low water content of the spore is restored to the water content of a normal vegetative cell and the core is able to expand, which in turn allows enzyme activity, metabolism, and spore outgrowth (36).CwlJ and SleB are two specific spore cortex-lytic enzymes (SCLEs) involved in Bacillus cortex hydrolysis, which break down PG containing muramic-δ-lactam (28). SleB has been shown to localize in both the inner and outer layers of B. subtilis spores through interaction of the enzyme peptidoglycan-binding motif and the δ-lactam structure of the cortex (7, 19) and in association with YpeB, which is required for sleB expression during sporulation (4, 7). SleB is a lytic transglycosylase muramidase, but so far its mode of activation is unknown (21). CwlJ is localized to the spore coat during sporulation (3) and is required for CaDPA-induced germination in B. subtilis. Activation can be due to either CaDPA released from the spore core at the onset of germination or exogenous CaDPA (22). Neither enzyme is individually essential for complete cortex hydrolysis during nutrient germination, although inactivation of both cwlJ and sleB in B. subtilis results in a spore unable to complete this process (15). The role of SleL has recently been studied in Bacillus anthracis. Mutants unable to produce this enzyme are still able to germinate, but the process is retarded (18).The SCLEs of Clostridium are less well studied than those of Bacillus. The SCLEs SleC (20) and SleM (6) have been identified in Clostridium perfringens, and a recent study demonstrated that SleC is required during germination for complete cortex hydrolysis (26). Although SleM can degrade spore cortex peptidoglycan and inactivation of both sleC and sleM decreased the ability of spores to germinate more than inactivation of sleC alone did, SleM was not essential (26). It has also been shown that the germination-specific serine protease CspB is essential for cortex hydrolysis and converts the inactive pro-SleC found in dormant spores to an active enzyme (24). So far, there has been no detailed study of any gene responsible for spore germination in C. difficile, although genes showing homology to cwlJ and sleB of B. subtilis (CD3563) and sleC of C. perfringens (CD0551) have now been identified in the C. difficile 630 genome (33).With germinant receptors in C. difficile yet to be identified, the mechanism by which the spores sense a suitable environment for germination is unclear. Recent studies have suggested that this process may involve the interaction of C. difficile with bile. Taurocholate has been shown to enhance recovery of C. difficile spores in nutrient-rich medium (42), and it has been proposed that glycine and taurocholate act as cogerminants (38), while chenodeoxycholate inhibits C. difficile spore germination (39).The emergence of C. difficile B1/NAP1/027 strains has increased the burden on health care services worldwide. Such strains have been shown to produce higher levels of toxin in the laboratory than many other types of strains (41), although the mechanism behind this production is not fully understood. However, while the observed higher levels of toxin production is doubtless important, perhaps the recent attention given to B1/NAP1/027 strains has focused too much on toxins. As spores represent the infectious stage of C. difficile, processes such as spore germination may also contribute to the greater virulence of these strains. In this study we evaluated the sporulation and germination efficiencies of an “epidemic” B1/NAP1/027 C. difficile strain ({"type":"entrez-nucleotide","attrs":{"text":"R20291","term_id":"774925","term_text":"R20291"}}R20291, isolated from the Stoke Mandeville outbreak in 2004 and 2005) and the “nonepidemic” strain 630Δerm (14). We then constructed strains with mutations in CD3563 (a cwlJ/sleB homologue) and a sleC homologue to analyze the role of these genes in the germination of C. difficile spores.     

Abundance of Novel and Diverse tfdA-Like Genes,Encoding Putative Phenoxyalkanoic Acid Herbicide-Degrading Dioxygenases,in Soil

Phenoxyalkanoic acid (PAA) herbicides are widely used in agriculture. Biotic degradation of such herbicides occurs in soils and is initiated by α-ketoglutarate- and Fe²⁺-dependent dioxygenases encoded by tfdA-like genes (i.e., tfdA and tfdAα). Novel primers and quantitative kinetic PCR (qPCR) assays were developed to analyze the diversity and abundance of tfdA-like genes in soil. Five primer sets targeting tfdA-like genes were designed and evaluated. Primer sets 3 to 5 specifically amplified tfdA-like genes from soil, and a total of 437 sequences were retrieved. Coverages of gene libraries were 62 to 100%, up to 122 genotypes were detected, and up to 389 genotypes were predicted to occur in the gene libraries as indicated by the richness estimator Chao1. Phylogenetic analysis of in silico-translated tfdA-like genes indicated that soil tfdA-like genes were related to those of group 2 and 3 Bradyrhizobium spp., Sphingomonas spp., and uncultured soil bacteria. Soil-derived tfdA-like genes were assigned to 11 clusters, 4 of which were composed of novel sequences from this study, indicating that soil harbors novel and diverse tfdA-like genes. Correlation analysis of 16S rRNA and tfdA-like gene similarity indicated that any two bacteria with D > 20% of group 2 tfdA-like gene-derived protein sequences belong to different species. Thus, data indicate that the soil analyzed harbors at least 48 novel bacterial species containing group 2 tfdA-like genes. Novel qPCR assays were established to quantify such new tfdA-like genes. Copy numbers of tfdA-like genes were 1.0 × 10⁶ to 65 × 10⁶ per gram (dry weight) soil in four different soils, indicating that hitherto-unknown, diverse tfdA-like genes are abundant in soils.Phenoxyalkanoic acid (PAA) herbicides such as MCPA (4-chloro-2-methyl-phenoxyacetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) are widely used to control broad-leaf weeds in agricultural as well as nonagricultural areas (19, 77). Degradation occurs primarily under oxic conditions in soil, and microorganisms play a key role in the degradation of such herbicides in soil (62, 64). Although relatively rapidly degraded in soil (32, 45), both MCPA and 2,4-D are potential groundwater contaminants (10, 56, 70), accentuating the importance of bacterial PAA herbicide-degrading bacteria in soils (e.g., references 3, 5, 6, 20, 41, 59, and 78).Degradation can occur cometabolically or be associated with energy conservation (15, 54). The first step in the degradation of 2,4-D and MCPA is initiated by the product of cadAB or tfdA-like genes (29, 30, 35, 67), which constitutes an α-ketoglutarate (α-KG)- and Fe²⁺-dependent dioxygenase. TfdA removes the acetate side chain of 2,4-D and MCPA to produce 2,4-dichlorophenol and 4-chloro-2-methylphenol, respectively, and glyoxylate while oxidizing α-ketoglutarate to CO₂ and succinate (16, 17).Organisms capable of PAA herbicide degradation are phylogenetically diverse and belong to the Alpha-, Beta-, and Gammproteobacteria and the Bacteroidetes/Chlorobi group (e.g., references 2, 14, 29-34, 39, 60, 68, and 71). These bacteria harbor tfdA-like genes (i.e., tfdA or tfdAα) and are categorized into three groups on an evolutionary and physiological basis (34). The first group consists of beta- and gammaproteobacteria and can be further divided into three distinct classes based on their tfdA genes (30, 46). Class I tfdA genes are closely related to those of Cupriavidus necator JMP134 (formerly Ralstonia eutropha). Class II tfdA genes consist of those of Burkholderia sp. strain RASC and a few strains that are 76% identical to class I tfdA genes. Class III tfdA genes are 77% identical to class I and 80% identical to class II tfdA genes and linked to MCPA degradation in soil (3). The second group consists of alphaproteobacteria, which are closely related to Bradyrhizobium spp. with tfdAα genes having 60% identity to tfdA of group 1 (18, 29, 34). The third group also harbors the tfdAα genes and consists of Sphingomonas spp. within the alphaproteobacteria (30).Diverse PAA herbicide degraders of all three groups were identified in soil by cultivation-dependent studies (32, 34, 41, 78). Besides CadAB, TfdA and certain TfdAα proteins catalyze the conversion of PAA herbicides (29, 30, 35). All groups of tfdA-like genes are potentially linked to the degradation of PAA herbicides, although alternative primary functions of group 2 and 3 TfdAs have been proposed (30, 35). However, recent cultivation-independent studies focused on 16S rRNA genes or solely on group 1 tfdA sequences in soil (e.g., references 3-5, 13, and 41). Whether group 2 and 3 tfdA-like genes are also quantitatively linked to the degradation of PAA herbicides in soils is unknown. Thus, tools to target a broad range of tfdA-like genes are needed to resolve such an issue. Primers used to assess the diversity of tfdA-like sequences used in previous studies were based on the alignment of approximately 50% or less of available sequences to date (3, 20, 29, 32, 39, 47, 58, 73). Primers specifically targeting all major groups of tfdA-like genes to assess and quantify a broad diversity of potential PAA degraders in soil are unavailable. Thus, the objectives of this study were (i) to develop primers specific for all three groups of tfdA-like genes, (ii) to establish quantitative kinetic PCR (qPCR) assays based on such primers for different soil samples, and (iii) to assess the diversity and abundance of tfdA-like genes in soil.     

Engineering of Clostridium phytofermentans Endoglucanase Cel5A for Improved Thermostability

A family 5 glycoside hydrolase from Clostridium phytofermentans was cloned and engineered through a cellulase cell surface display system in Escherichia coli. The presence of cell surface anchoring, a cellulose binding module, or a His tag greatly influenced the activities of wild-type and mutant enzymes on soluble and solid cellulosic substrates, suggesting the high complexity of cellulase engineering. The best mutant had 92%, 36%, and 46% longer half-lives at 60°C on carboxymethyl cellulose, regenerated amorphous cellulose, and Avicel, respectively.The production of biofuels from nonfood cellulosic biomass would benefit the economy, the environment, and national energy security (17, 32). The largest technological and economical obstacle is the release of soluble fermentable sugars at prices competitive with those from sugarcane or corn kernels (17, 31). One of the approaches is discovering new cellulases from cellulolytic microorganisms, followed by cellulase engineering for enhanced performance on pretreated solid substrates. However, cellulase engineering remains challenging because enzymatic cellulose hydrolysis is complicated, involving heterogeneous substrates (33, 37), different action mode cellulase components (18), synergy and/or competition among cellulase components (36, 37), and declining substrate reactivity over the course of conversion (11, 26). Directed enzyme evolution, independent of knowledge of the protein structure and the enzyme-substrate interactions (6, 34), has been conducted to generate endoglucanase mutants, such as enhanced activities on soluble substrates (14, 16, 22), prolonged thermostability (20), changed optimum pH (24, 28), or improved expression levels (21). Here, we cloned and characterized a family 5 glycoside hydrolase (Cel5A) from a cellulolytic bacterium, Clostridium phytofermentans ISDg (ATCC 700394) (29, 30), and engineered it for enhanced thermostability.     

Characterization of Wet-Heat Inactivation of Single Spores of Bacillus Species by Dual-Trap Raman Spectroscopy and Elastic Light Scattering

Dual-trap laser tweezers Raman spectroscopy (LTRS) and elastic light scattering (ELS) were used to investigate dynamic processes during high-temperature treatment of individual spores of Bacillus cereus, Bacillus megaterium, and Bacillus subtilis in water. Major conclusions from these studies included the following. (i) After spores of all three species were added to water at 80 to 90°C, the level of the 1:1 complex of Ca²⁺ and dipicolinic acid (CaDPA; ∼25% of the dry weight of the spore core) in individual spores remained relatively constant during a highly variable lag time (T_lag), and then CaDPA was released within 1 to 2 min. (ii) The T_lag values prior to rapid CaDPA release and thus the times for wet-heat killing of individual spores of all three species were very heterogeneous. (iii) The heterogeneity in kinetics of wet-heat killing of individual spores was not due to differences in the microscopic physical environments during heat treatment. (iv) During the wet-heat treatment of spores of all three species, spore protein denaturation largely but not completely accompanied rapid CaDPA release, as some changes in protein structure preceded rapid CaDPA release. (v) Changes in the ELS from individual spores of all three species were strongly correlated with the release of CaDPA. The ELS intensities of B. cereus and B. megaterium spores decreased gradually and reached minima at T₁ when ∼80% of spore CaDPA was released, then increased rapidly until T₂ when full CaDPA release was complete, and then remained nearly constant. The ELS intensity of B. subtilis spores showed similar features, although the intensity changed minimally, if at all, prior to T₁. (vi) Carotenoids in B. megaterium spores'' inner membranes exhibited two changes during heat treatment. First, the carotenoid''s two Raman bands at 1,155 and 1,516 cm⁻¹ decreased rapidly to a low value and to zero, respectively, well before T_lag, and then the residual 1,155-cm⁻¹ band disappeared, in parallel with the rapid CaDPA release beginning at T_lag.Bacterial spores of Bacillus species are formed in sporulation and are metabolically dormant and extremely resistant to a variety of harsh conditions, including heat, radiation, and many toxic chemicals (37). Since spores of these species are generally present in foodstuffs and cause food spoilage and food-borne disease (37, 38), there has long been interest in the mechanisms of both spore resistance and spore killing, especially for wet heat, the agent most commonly used to kill spores. The killing of dormant spores by wet heat generally requires temperatures about 40°C higher than those for the killing of growing cells of the same strain (37, 43). A number of factors influence spore wet-heat resistance, with a major factor being the spore core''s water content, as spores with higher core water content are less wet-heat resistant than are spores with lower core water (15, 25). The high level of pyridine-2,6-dicarboxylic acid (dipicolinic acid [DPA]) and the types of its associated divalent cations, predominantly Ca²⁺, that comprise ∼25% of the dry weight of the core also contribute to spore wet-heat resistance, although how low core water content and CaDPA protect spores against wet heat is not known. The protection of spore DNA against depurination by its saturation with a group of α/β-type small, acid-soluble spore proteins also contributes to spore wet-heat resistance (14, 23, 33, 37).Despite knowledge of a number of factors important in spore wet-heat resistance, the mechanism for wet-heat killing of spores is not known. Wet heat does not kill spores by DNA damage or oxidative damage (35, 37). Instead, spore killing by this agent is associated with protein denaturation and enzyme inactivation (2, 7, 44), although specific proteins for which damage causes spore death have not been identified. Wet-heat treatment also often results in the release of the spore core''s large depot of CaDPA. The mechanism for this CaDPA release is not known but is presumably associated with the rupture of the spore''s inner membrane (7). In addition, the relationship between protein denaturation and CaDPA release is not clear, although recent work suggests that significant protein denaturation can occur prior to CaDPA release (7). Almost all information on spore killing by moist heat has been obtained with spore populations, and essentially nothing is known about the behavior of individual spores exposed to potentially lethal temperatures in water. Given the likely heterogeneity of spores in populations, in particular in their wet-heat resistances (16, 18, 39, 40), it could be most informative to analyze the behavior of individual spores exposed to high temperatures in water.Raman spectroscopy is widely used in biochemical studies, as this technique has high sensitivity and responds rapidly to subtle changes in molecule structure (1, 22, 31). In addition, when Raman spectroscopy is combined with confocal microscopy and optical tweezers, the resultant laser tweezers Raman spectroscopy (LTRS) allows the nondestructive, noninvasive detection of biochemical processes at the single-cell level (9, 10, 19, 46). Indeed, LTRS has been used to analyze the DPA level and the germination of individual Bacillus spores (5, 19, 30). In order to obtain information more rapidly, dual- and multitrap laser tweezers have been developed to allow multiple individual cells or particles to be analyzed simultaneously (11, 13, 24, 27), and the dual trap has been used to measure the hydrodynamic cross-correlations of two particles (24). In addition to Raman scattering, the elastic light scattering (ELS) from trapped individual cells also provides valuable information on cell shape, orientation, refractive index, and morphology (12, 45) and has been used to monitor spore germination dynamics as well (30).In this work, we report studies of wet-heat treatment of individual spores of three different Bacillus species by dual-trap LTRS and ELS. A number of important processes related to wet-heat inactivation of spores, including CaDPA release and protein denaturation, and the correlation between these processes were investigated by monitoring changes in Raman scattering at CaDPA-, protein structure-, and phenylalanine-specific bands and changes in ELS intensity.     

A Novel Spore Protein,ExsM, Regulates Formation of the Exosporium in Bacillus cereus and Bacillus anthracis and Affects Spore Size and Shape

Bacillus cereus spores are assembled with a series of concentric layers that protect them from a wide range of environmental stresses. The outermost layer, or exosporium, is a bag-like structure that interacts with the environment and is composed of more than 20 proteins and glycoproteins. Here, we identified a new spore protein, ExsM, from a β-mercaptoethanol extract of B. cereus ATCC 4342 spores. Subcellular localization of an ExsM-green fluorescent protein (GFP) protein revealed a dynamic pattern of fluorescence that follows the site of formation of the exosporium around the forespore. Under scanning electron microscopy, exsM null mutant spores were smaller and rounder than wild-type spores, which had an extended exosporium (spore length for the wt, 2.40 ± 0.56 μm, versus that for the exsM mutant, 1.66 ± 0.38 μm [P < 0.001]). Thin-section electron microscopy revealed that exsM mutant spores were encased by a double-layer exosporium, both layers of which were composed of a basal layer and a hair-like nap. Mutant exsM spores were more resistant to lysozyme treatment and germinated with higher efficiency than wild-type spores, and they had a delay in outgrowth. Insertional mutagenesis of exsM in Bacillus anthracis ΔSterne resulted in a partial second exosporium and in smaller spores. In all, these findings suggest that ExsM plays a critical role in the formation of the exosporium.Bacillus cereus and Bacillus anthracis are closely related members of the Bacillus cereus group (47). Although B. cereus is mainly an apathogenic organism, certain isolates can cause two different types of food poisoning, emetic syndrome and diarrheal disease (18). The emetic syndrome is caused by ingestion of cereulide, a heat-resistant toxin produced by vegetative cells contaminating the food (30), while the diarrheal disease occurs when spores germinate in the intestinal tract. Spores are also the infective agent in anthrax, a disease caused by B. anthracis (64).B. cereus and B. anthracis differentiate into spores when faced with nutrient deprivation. The spore is a dormant cell type that can remain viable for decades until favorable conditions induce germination and the resumption of vegetative growth. The remarkable resistance properties of the spore result from its unique architecture, consisting of a series of concentric protective layers (51). The spore core contains the genetic material and is surrounded by the cortex, a thick layer of modified peptidoglycan that promotes a highly dehydrated state. Encasing the core and the cortex, the coat is a multilayer protein shell that provides mechanical and chemical resistance. In addition, both the cortex and coat contribute to spore germination (17). Separated from the coat by an interspace, the exosporium encloses the rest of the spore, and it is composed of an inner basal layer and an outer hair-like nap (25).Being the most external layer of the spore, the exosporium interacts directly with the environment and as such provides a semipermeable barrier that may exclude large molecules, like antibodies and hydrolytic enzymes (3, 23, 24, 54). However, the exosporium does not appear to contribute to the typical resistance properties of the spore (6, 35, 60). Also, the exosporium is not necessary in anthrax pathogenesis when tested under laboratory conditions (7, 27, 59), although it is able to down-modulate the innate immune response to spores and mediate adhesion to host tissues (4, 8, 43, 44). The exosporium may also help the spore avoid premature germination in unsustainable environments, since it contains two enzymes, alanine racemase (Alr) and inosine hydrolase (Iunh), that can inactivate low quantities of the germinants l-alanine and inosine, respectively (6, 48, 55, 61). However, regulation of germination by the exosporium is poorly understood. Mutation of exosporial proteins has resulted in only negligible and inconsistent germination phenotypes (2, 5, 27, 28, 52, 54).The exosporium is composed of at least 20 proteins and glycoproteins in tight or loose association (48, 53, 57, 61, 65). These proteins are synthesized in the mother cell and always start self-assembly at the forespore pole near the middle of the mother cell, concurrently with the cortex and coat formation (42). Exosporium assembly is discontinuous and starts with a synthesis of a substructure known as the cap, which likely contains only a subset of the proteins present in the exosporium (55). After cap formation, construction of the rest of the exosporium requires the expression of ExsY (6). BclA is the main component of the hair-like nap on the external side of the exosporium, and it is linked to the basal layer through interaction with ExsFA/BxpB (54, 58). In addition, CotE participates in the correct attachment of the exosporium to the spore (27).Despite these findings, exosporium assembly continues to be a poorly understood process, and many questions remain regarding its composition and the regulation of its synthesis. In this study, we characterized a new spore protein, ExsM, which plays a key role in assembly of the exosporium. In B. cereus, inactivation of exsM resulted in spores with an unusual double-layer exosporium, and a similar phenotype was also observed in B. anthracis exsM null mutant spores. Finally, double-layer exosporium spores allowed us to study the role of the exosporium in germination and outgrowth.     

Factors Affecting Variability in Time between Addition of Nutrient Germinants and Rapid Dipicolinic Acid Release during Germination of Spores of Bacillus Species

The simultaneous nutrient germination of hundreds of individual wild-type spores of three Bacillus species and a number of Bacillus subtilis strains has been measured by two new methods, and rates of release of the great majority of the large pool of dipicolinic acid (DPA) from individual spores of B. subtilis strains has been measured by Raman spectroscopy with laser tweezers. The results from these analyses and published data have allowed a number of significant conclusions about the germination of spores of Bacillus species as follows. (i) The time needed for release of the great majority of a Bacillus spore''s DPA once rapid DPA release had begun (ΔT_release) during nutrient germination was independent of the concentration of nutrient germinant used, the level of the germinant receptors (GRs) that recognize nutrient germinants used and heat activation prior to germination. Values for ΔT_release were generally 0.5 to 3 min at 25 to 37°C for individual wild-type spores. (ii) Despite the conclusion above, germination of individual spores in populations was very heterogeneous, with some spores in wild-type populations completing germination ≥15-fold slower than others. (iii) The major factor in the heterogeneity in germination of individual spores in populations was the highly variable lag time, T_lag, between mixing spores with nutrient germinants and the beginning of ΔT_release. (iv) A number of factors decrease spores'' T_lag values including heat activation, increased levels of GRs/spore, and higher levels of nutrient germinants. These latter factors appear to affect the level of activated GRs/spore during nutrient germination. (v) The conclusions above lead to the simple prediction that a major factor causing heterogeneity in Bacillus spore germination is the number of functional GRs in individual spores, a number that presumably varies significantly between spores in populations.Spores of various Bacillus species are metabolically dormant and can survive for years in this state (30). However, spores constantly sense their environment, and if appropriate small molecules termed germinants are present, spores can rapidly return to life in the process of germination followed by outgrowth (25, 29, 30). The germinants that most likely trigger spore germination in the environment are low-molecular-weight nutrient molecules, the identities of which are strain and species specific, including amino acids, sugars, and purine nucleosides. Metabolism of these nutrient germinants is not needed for the triggering of spore germination. Rather, these germinants are recognized by germinant receptors (GRs) located in the spore''s inner membrane that recognize their cognate germinants in a stereospecific manner (17, 24, 25, 29). Spores have a number of such GRs, with three functional GRs in Bacillus subtilis spores and even more in Bacillus anthracis, Bacillus cereus, and Bacillus megaterium spores (6, 29, 30). Binding of nutrient germinants to some single GRs is sufficient to trigger spore germination, for example the triggering of B. subtilis spore germination by binding of l-alanine or l-valine to the GerA GR. However, many GRs cooperate such that binding of germinants by ≥2 different GRs is needed to trigger germination (2, 29): for example, the triggering of B. subtilis spore germination by the binding of components of a mixture of l-asparagine, d-glucose, d-fructose, and K⁺ ions (AGFK) to the GerB and GerK GRs. The binding of nutrient germinants to GRs triggers subsequent events in germination, although how this is accomplished is not known.The first readily measured biochemical event after addition of nutrient germinants to Bacillus spores is the rapid release of the spore''s large depot (∼10% of spore dry weight) of pyridine-2,6-dicarboxylic acid (dipicolinic acid [DPA]) plus its chelated divalent cations, predominantly Ca²⁺ (Ca-DPA), from the spore core (25, 29). Ca-DPA release then results in the activation of two redundant cortex-lytic enzymes (CLEs), CwlJ and SleB, which hydrolyze the spore''s peptidoglycan cortex layer (16, 22, 27, 29). CwlJ is activated by Ca-DPA as it is released from the spore while SleB is activated only after most DPA is released (17, 20, 22, 26, 27). Cortex hydrolysis ultimately allows the spore core to expand and take up more water, raising the core water content from the 35 to 45% of wet weight in the dormant spore to the 80% of wet weight characteristic of growing cells. Full hydration of the spore core then allows enzyme action, metabolism, and macromolecular synthesis to resume in the now fully germinated spore.Germination of spores in populations is very heterogeneous, with some spores germinating rapidly and some extremely slowly (4, 5, 9, 11, 13-15, 19, 26, 31, 32). Where it has been studied, the reason for this heterogeneity has been suggested to be due to a variable lag period (T_lag) between the time of mixing spores with a germinant and the time at which rapid DPA release begins, since once rapid DPA release begins, the time required for release of almost all DPA as well as for subsequent cortex hydrolysis is generally rather short compared to T_lag values in individual spores (5, 11, 13-15, 19, 26, 31, 32). The times required for DPA release and cortex hydrolysis are also similar in wild-type spores with both very short and long T_lag values (5, 15, 19, 27). The reasons for the variability in T_lag times between individual spores in populations are not known, although there are reports that both activation of spores for germination by a sublethal heat treatment (heat activation) as well as increasing concentrations of nutrient germinants can shorten T_lag values (12, 14, 15, 18, 32). However, there has been no detailed study of the causes of the variability in T_lag values between very large numbers of individual spores in populations.In order to study the heterogeneity in spore germination thoroughly, methods are needed to follow the germination of hundreds of individual spores over several hours. Initial studies of the germination of individual spores examined a single spore in a phase-contrast microscope and followed the germination of this spore by changes in the core''s refractive index due to DPA release and core swelling (14, 15, 32, 34). However, this method is labor-intensive for gathering data with hundreds of individual spores. More recently, confocal microscopy and then surface adsorption and optical tweezers have been used to capture single spores, and germination events have been followed by methods such as Raman spectroscopy to directly measure DPA release, as well as phase-contrast microscopy and elastic light scattering (3, 5, 9, 10, 19, 26). While the latter recent advances have allowed accumulation of much information about germination, collection of this type of data for large numbers of individual spores is still labor-intensive, although use of dual optical traps (35) and perhaps multiple traps in the future may alleviate this problem. However, phase-contrast microscopy plus appropriate computer software has recently allowed the monitoring of many hundreds of individual spores for several hours, with automated assessment of various changes in the cells during the period of observation (19). In the present work, we have used both phase-contrast and differential interference contrast (DIC) microscopy to monitor the germination of many hundreds of individual spores of three Bacillus species adhered on either an agarose pad or a glass coverslip for 1 to 2 h. This work, as well as examination of times needed for release of most DPA once rapid DPA release has begun during germination of individual spores under a variety of conditions, has allowed detailed examination of the effects of heat activation, nutrient germinant concentration, GR numbers per spore, and individual CLEs on spore germination heterogeneity and on values of T_lag for individual spores.     

Genome-Derived Criteria for Assigning Environmental narG and nosZ Sequences to Operational Taxonomic Units of Nitrate Reducers

Ninety percent of cultured bacterial nitrate reducers with a 16S rRNA gene similarity of ≥97% had a narG or nosZ similarity of ≥67% or ≥80%, respectively, suggesting that 67% and 80% could be used as standardized, conservative threshold similarity values for narG and nosZ, respectively (i.e., any two sequences that are less similar than the threshold similarity value have a very high probability of belonging to different species), for estimating species-level operational taxonomic units. Genus-level tree topologies of narG and nosZ were generally similar to those of the corresponding 16S rRNA genes. Although some genomes contained multiple copies of narG, recent horizontal gene transfer of narG was not apparent.Nitrate reducers (i.e., both dissimilatory nitrate reducers and denitrifiers) reduce nitrate to nitrite, which can then be reduced to ammonium by dissimilatory nitrate reducers or sequentially reduced to nitric oxide, nitrous oxide, and dinitrogen by denitrifiers (29). narG codes for the alpha subunit of the dissimilatory nitrate reductase, which reduces nitrate to nitrite and is thus common to both dissimilatory nitrate reducers and denitrifiers (29). nosZ codes for nitrous oxide reductase, which reduces nitrous oxide to dinitrogen and is common to denitrifiers but not dissimilatory nitrate reducers (29). Both narG and nosZ are commonly used as gene markers for community level analysis of nitrate reducers (2, 8, 9, 16, 18, 19, 20, 25). However, standardized criteria for assigning environmental narG and nosZ sequences to operational taxonomic units (OTUs) are required so that diverse data sets on nitrate-reducing communities can be normalized. The widespread ability of bacteria and archaea to denitrify (29) complicates the development of such criteria for genes involved in denitrification. Some closely related narG and closely related nosZ genes occur in distantly related taxa, and narG or nosZ phylogenies do not always reflect 16S rRNA phylogenies (17). However, nosZ-based phylogenies in general have a high degree of congruency with 16S rRNA gene-based phylogenies (3, 10, 30), and recent horizontal gene transfer of nosZ seems unlikely (10), indicating that denitrifier structural genes might be used for estimating the species-level novelty, as well as species-level diversity, of denitrifiers in environmental samples. The limited amount of data on horizontal gene transfer of narG (4, 24) identifies a need to extend such an approach to this gene. The limited number of studies that have compared 16S rRNA with narG or nosZ phylogenies accentuates the need for a more thorough analysis of the phylogenetic relatedness of these three genes (3, 4, 7). Thus, the main objectives of this study were to (i) resolve criteria for standardizing OTU assignment of environmental narG and nosZ sequences, (ii) determine whether those criteria can be used as indicators of novel species, and (iii) investigate the impact of horizontal gene transfer on narG.     

Spatiotemporal Homogeneity of Campylobacter Subtypes from Cattle and Sheep across Northeastern and Southwestern Scotland

Source attribution using molecular subtypes has implicated cattle and sheep as sources of human Campylobacter infection. Whether the Campylobacter subtypes associated with cattle and sheep vary spatiotemporally remains poorly known, especially at national levels. Here we describe spatiotemporal patterns of prevalence, bacterial enumeration, and subtype composition in Campylobacter isolates from cattle and sheep feces from northeastern (63 farms, 414 samples) and southwestern (71 farms, 449 samples) Scotland during 2005 to 2006. Isolates (201) were categorized as sequence type (ST), as clonal complex (CC), and as Campylobacter jejuni or Campylobacter coli using multilocus sequence typing (MLST). No significant difference in average prevalence (cattle, 22%; sheep, 25%) or average enumeration (cattle, 2.7 × 10⁴ CFU/g; sheep, 2.0 × 10⁵ CFU/g) was found between hosts or regions. The four most common STs (C. jejuni ST-19, ST-42, and ST-61 and C. coli ST-827) occurred in both hosts, whereas STs of the C. coli ST-828 clonal complex were more common in sheep. Neither host yielded evidence for regional differences in ST, CC, or MLST allele composition. Isolates from the two hosts combined, categorized as ST or CC, were more similar within than between farms but showed no further spatiotemporal trends up to 330 km and 50 weeks between farm samples. In contrast, both regions yielded evidence for significant differences in ST, CC, and allele composition between hosts, such that 65% of isolates could be attributed to a known host. These results suggest that cattle and sheep within the spatiotemporal scales analyzed are each capable of contributing homogeneous Campylobacter strains to human infections.Campylobacter species are the largest cause of bacterial intestinal infection in the developed and developing world (3). Almost all reported human Campylobacter infections in the United Kingdom are caused by Campylobacter jejuni, which accounts for approximately 92% of cases, and Campylobacter coli, which accounts for most of the rest (8). Campylobacter species are carried asymptomatically in a wide range of host animals and excreted into the environment in feces (23). Humans can be infected by several routes including consumption of contaminated water (14) or food (23); indeed, case control studies indicate that consumption of poultry meat is a risk factor (11, 12, 28), but other foods including unpasteurized milk (33) and meat from cattle and sheep contaminated at the abattoir might be important (30).Cattle and sheep on farms are major hosts of Campylobacter, with up to 89% of cattle herds (31) and up to 55% of sheep flocks (26) being infected. The prevalence of C. jejuni and C. coli combined, estimated at the level of individual animals from fecal specimens, is 23 to 54% in cattle (22, 25) and up to 20% in sheep (37). Campylobacter enumeration in feces shed from individual animals ranges from <10² to 10⁷ CFU/g in both hosts (31), and the concentration shed varies with time. Meat products of cattle and sheep, by contrast, have generally lower levels of Campylobacter contamination. Prevalence values are 0.5 to 4.9% in surveys of retail beef (11a, 17, 36) and 6.9 to 12.6% in surveys of retail lamb and mutton (17, 35).Clinical Campylobacter strains can be attributed to infection sources in animals by comparing subtype frequencies in clinical cases with those in different candidate sources, including cattle, sheep, pigs, and the physical environment. Campylobacter subtype data sets are most transferable when subtypes are defined as sequence type (ST) using multilocus sequence typing (MLST). Three recent MLST-based studies based in northwestern England (34), mainland Scotland (29), northeastern Scotland (32), and New Zealand (24) have used source attribution models to infer the source of human clinical infection. The results suggest that retail chicken is the source with the highest (55 to 80%) attribution while cattle and sheep combined are ranked second (20 to 40%). These attribution models require further empirical validation but appear to be showing broadly similar results.Attribution of human Campylobacter infections to cattle and sheep raises the question of whether Campylobacter subtypes infecting farm cattle and sheep are generally homogeneous or tend to have spatiotemporal structure. Regarding spatial differences, isolates of C. jejuni from a 100-km² study of farmland area with dairy cattle and sheep in northwestern England displayed increased genetic similarity up to 1 km apart but no further trend over distances of 1 to 14 km apart (7), and isolates from three dairy cattle farms 2 or 5 km apart in the same area demonstrated differences in the frequencies of strains of clonal complexes (CCs) ST-42 and ST-61 (15). Regarding temporal differences, isolates of C. jejuni from five dairy cattle farms in the same area demonstrated differences in the frequency of strains of CC ST-61 between the spring and summer of 2003 (15). Lastly, regarding host-associated strains, STs of CCs ST-21, ST-42, and ST-61 are associated with cattle, and the more limited data for STs from sheep also show the presence of ST-21 and ST-61 (7, 15).The larger-scale spatiotemporal structure of Campylobacter strains from cattle and sheep is poorly known. The main aims of the present study were (i) to characterize C. jejuni and C. coli from cattle and sheep from two distinct geographical Scottish regions in terms of Campylobacter prevalence and enumeration and C. jejuni and C. coli ST composition and (ii) to estimate the extent of host association of C. jejuni and C. coli STs from cattle versus sheep.