Diamide Triggers Mainly S Thiolations in the Cytoplasmic Proteomes of Bacillus subtilis and Staphylococcus aureus

Glutathione constitutes a key player in the thiol redox buffer in many organisms. However, the gram-positive bacteria Bacillus subtilis and Staphylococcus aureus lack this low-molecular-weight thiol. Recently, we identified S-cysteinylated proteins in B. subtilis after treatment of cells with the disulfide-generating electrophile diamide. S cysteinylation is thought to protect protein thiols against irreversible oxidation to sulfinic and sulfonic acids. Here we show that S thiolation occurs also in S. aureus proteins after exposure to diamide. We further analyzed the formation of inter- and intramolecular disulfide bonds in cytoplasmic proteins using diagonal nonreducing/reducing sodium dodecyl sulfate gel electrophoresis. However, only a few proteins were identified that form inter- or intramolecular disulfide bonds under control and diamide stress conditions in B. subtilis and S. aureus. Depletion of the cysteine pool was concomitantly measured in B. subtilis using a metabolomics approach. Thus, the majority of reversible thiol modifications that were previously detected by two-dimensional gel fluorescence-based thiol modification assay are most likely based on S thiolations. Finally, we found that a glutathione-producing B. subtilis strain which expresses the Listeria monocytogenes gshF gene did not show enhanced oxidative stress resistance compared to the wild type.Cysteine thiols in proteins fulfill an important and diverse set of cellular functions. In particular, they participate in enzymatic catalysis; in metal coordination, such as in the generation of Fe-S-clusters; and in determining the spatial structure of proteins via disulfide bond formation (3, 22, 23, 38). Cysteines are strong nucleophiles amenable to posttranslational modifications by reactive oxygen species (ROS) and reactive nitrogen species, leading to disulfides; to sulfenic, sulfinic, or sulfonic acids; mixed disulfides with low-molecular-weight (LMW) thiols (S thiolations); and S nitrosylations (7, 16, 17, 27).The redox status of the cytoplasm is under physiological conditions in a reduced state. Thus, most cysteines are present as free thiols (6). Because aerobic organisms have to cope with oxidative stress caused by ROS, such as superoxide anions, hydrogen peroxide, or hydroxyl radicals, they need to employ effective mechanisms that maintain the reduced state. In gram-negative bacteria, the thiol-disulfide balance is accomplished by the glutathione (GSH) system, a thiol-based redox buffer. The GSH system consists of glutaredoxin (Grx), GSH (γ-glutamylcysteinyl glycine), GSH reductase, and GSH peroxidase (34). Reduction of disulfides occurs via sequential electron transfer from glutaredoxin and reduced GSH; oxidized GSH (GSSG) is reduced by the NADPH-dependent GSH reductase. GSH peroxidase enables the direct detoxification of ROS by GSH oxidation.However, many gram-positive bacteria lack genes for GSH biosynthesis. Actinomycetes instead use a thiol redox buffer based on mycothiol (50). Bacillus subtilis, Staphylococcus aureus, and other gram-positive bacteria rely on different thiol redox buffers based on cysteine, the novel 398-Da bacillithiol (BSH), or coenzyme A (CoA) (15, 52). To maintain the reduced state of the cytoplasm, most bacteria use enzymatic systems for disulfide bond reduction such as the thioredoxin (Trx) system, which is highly conserved in gram-negative bacteria (3, 10). The Trx system consists of thioredoxin (TrxA) and the NADPH-dependent thioredoxin reductase (TrxB).Any imbalance in the cellular redox state caused by ROS elicits expression of a repertoire of different proteins, commonly under the control of a redox-sensing regulator: for example, OxyR in Escherichia coli and PerR, OhrR, SarZ, and Spx in B. subtilis and S. aureus, respectively (11, 12, 41, 55, 58, 64-66). The subsequently induced proteins detoxify ROS and restore and protect the normal physiological redox state in the cell.Besides ROS and reactive nitrogen species, so-called “reactive electrophilic species” (RES) affect the thiol redox balance. RES include different chemical compounds such as aldehydes, quinones, and the azo compound diamide (2, 43, 45, 46, 53, 66). Quinones and aldehydes have electron-deficient centers that result in thiol-(S) alkylation of cysteine. Exposure of cells to diamide induces the oxidative as well as the electrophile stress response in B. subtilis (43, 45, 53). The toxicity of diamide is based on disulfide bond formation (40), which was recently visualized in B. subtilis and S. aureus by the fluorescence alkylation of oxidized thiols (FALKO) assay (32, 64). It was thought that the formation of nonnative inter- and intramolecular disulfide bonds results in damage of proteins.However, more recent findings demonstrate that diamide stress leads also to S thiolations: formation of disulfide bonds between proteins and LMW thiols (8, 13, 33). S thiolations prevent protein thiols from irreversible oxidation to sulfinic and sulfonic acids, but also affect enzyme activity (35, 47) and signal transduction (39, 42). In B. subtilis, we have identified a few cytoplasmic proteins that are S cysteinylated (33). In addition, the organic peroxide sensor OhrR was inactivated by an S bacillithiolation in B. subtilis (42).Cysteine, BSH, and CoA are also dominant LMW thiols in S. aureus (52). In this study, we have investigated in more detail the extents of S thiolations and inter- and intramolecular disulfide bond formation of B. subtilis and S. aureus in response to disulfide stress. The results showed that exposure to diamide leads to S thiolations in S. aureus. Using a nonreducing/reducing sodium dodecyl sulfate (SDS) diagonal electrophoresis approach, proteins with intermolecular disulfide bonds could be distinguished from proteins with intramolecular disulfide bonds (57). The results support that the majority of reversible thiol oxidations are based on S thiolations rather than disulfide bonds between proteins. Depletion of the free cysteine pool in B. subtilis after exposure to diamide supports this finding. To assess if GSH may have a bearing on the thiol redox buffer of B. subtilis, the gshF gene of Listeria monocytogenes (gshF_Lm) was expressed in B. subtilis, enabling GSH biosynthesis (29). Although GSH production does not enhance the resistance to oxidative stress in B. subtilis, it participates in the formation of S thiolations.     

Diverse Enterotoxin Gene Profiles among Clonal Complexes of Staphylococcus aureus Isolates from the Bronx,New York

Staphylococcal enterotoxins (SE) can cause toxin-mediated disease, and those that function as superantigens are implicated in the pathogenesis of allergic diseases. The prevalence of 19 enterotoxin genes was determined by PCR in clinical S. aureus strains derived from wounds (108) and blood (99). We performed spa typing and multilocus sequence typing (MLST) to determine clonal origin, and for selected strains staphylococcal enterotoxin B (SEB) production was measured by enzyme-linked immunosorbent assay. Strains carried a median of five SE genes. For most SE genes, the prevalence rates among methicillin-resistant and methicillin-sensitive S. aureus isolates, as well as wound- and blood-derived isolates, did not differ. At least one SE gene was detected in all except two S. aureus isolates (>99%). Complete egc clusters were found in only 11% of S. aureus isolates, whereas the combination of sed, sej, and ser was detected in 24% of clinical strains. S. aureus strains exhibited distinct combinations of SE genes, even if their pulsed-field gel electrophoresis and MLST patterns demonstrated clonality. USA300 strains also showed considerable variability in SE content, although they contained a lower number of SE genes (mean, 3). By contrast, SE content was unchanged in five pairs of serial isolates. SEB production by individual strains varied up to 200-fold, and even up to 15-fold in a pair of serial isolates. In conclusion, our results illustrate the genetic diversity of S. aureus strains with respect to enterotoxin genes and suggest that horizontal transfer of mobile genetic elements encoding virulence genes occurs frequently.As a commensal, Staphylococcus aureus colonizes the nasal mucosa of 20 to 40% of humans (54), and as a pathogen it causes pyogenic diseases and toxin-mediated diseases (38). S. aureus produces many different virulence factors, including enterotoxins (SEs), which can cause defined toxic shock syndromes (4). The characterization of some of these toxins led to the discovery of superantigens (41), which bind to major histocompatibility complex class II molecules and Vβ chains of T-cell receptors, resulting in the activation of large numbers of T cells (20 to 30%) and massive cytokine production (10, 18). These superantigen-induced “cytokine storms” are responsible for the toxic effects seen in staphylococcal entertoxin B (SEB)- and toxic shock syndrome toxin (TSST)-associated shock syndromes in S. aureus infections (13, 40, 47). To date, 19 SEs have been identified based on sequence homologies, and studies have reported enterotoxin genes in up to 80% of all S. aureus strains (4, 21). Although many new enterotoxins have been identified, i.e., seg ser and seu (33, 37, 44, 49), their precise functions have not been characterized yet. The majority of experimental work with SEs is still done with SEB, toxic shock syndrome toxin 1, and SEA (27, 31), because these toxins are commercially available. Most SEs are located on mobile elements in bacterial genomes such as plasmids or pathogenicity islands and can thus be easily transferred horizontally between strains (5, 34, 35). Certain SE genes are grouped together. For instance seg, sei, sem, sen, and seo are commonly found in a gene cluster (egc) on genomic island νSAβ (34), and sel and sek are often found together with seb or sec on S. aureus pathogenicity islands. Other staphylococcal superantigen genes are encoded on plasmids (sed, sej, and ser) or are linked to the antibiotic resistance cassette SCCmec (seh) (44, 55). Phage φ3 carries either sea (strain Mu50), sep (N315), or sea sek seq (MW2) (1, 29).Although a few clinical studies have attempted to correlate shock and outcome with the presence of certain SEs in patients with S. aureus infections (17, 28), the contribution of these toxins to outcome is still unclear. Recent papers have proposed the SEs are immunomodulators and that colonization with S. aureus strains that produce SEB may contribute to the pathogenesis of asthma, chronic rhinitis, and dermatitis (2, 36, 46, 48, 56). The superantigen function of SEs in supernatants of S. aureus cultures can be neutralized by serum of colonized patients (21, 23). With new data emerging implicating SEs in the pathogenesis of chronic allergic syndromes, production of monoclonal antibodies and or vaccine strategies targeting SEs may be considered (6, 24, 26, 30) in the future. It is therefore important to characterize the prevalence of SE genes in clinical S. aureus strains.In this study, we analyzed SE content in both methicillin-resistant S. aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) strains that were cultured from wounds (including USA300) and bloodstream infections of patients from a defined geographical area. In addition, SEB production was quantified by enzyme-linked immunosorbent assay (ELISA) in S. aureus strains carrying the seb gene, and spa typing confirmed clonal diversity among S. aureus isolates from different patients, as well as clonal stability in serial isolates, and multilocus sequence typing (MLST) done on a subset of less common spa types. We conclude that SE genes are abundant in S. aureus strains, albeit less abundant in USA300. SE content and combination are highly diverse and therefore more discriminatory than pulsed-field gel electrophoresis (PFGE) and MLST typing, albeit stable in serial isolates. Quantification of SEB production demonstrates that enterotoxin secretion can vary greatly among strains, even if they belong to the same S. aureus lineage. Given the complexities of SE prevalence, regulation, and possible function, we propose that the association of these toxins with chronic allergic diseases or outcome may be oversimplified at present. Precise characterizations of SE function and secretion patterns in individual S. aureus clones are warranted.     

Contribution of Lipoproteins and Lipoprotein Processing to Endocarditis Virulence in Streptococcus sanguinis

Streptococcus sanguinis is an important cause of infective endocarditis. Previous studies have identified lipoproteins as virulence determinants in other streptococcal species. Using a bioinformatic approach, we identified 52 putative lipoprotein genes in S. sanguinis strain SK36 as well as genes encoding the lipoprotein-processing enzymes prolipoprotein diacylglyceryl transferase (lgt) and signal peptidase II (lspA). We employed a directed signature-tagged mutagenesis approach to systematically disrupt these genes and screen each mutant for the loss of virulence in an animal model of endocarditis. All mutants were viable. In competitive index assays, mutation of a putative phosphate transporter reduced in vivo competitiveness by 14-fold but also reduced in vitro viability by more than 20-fold. Mutations in lgt, lspA, or an uncharacterized lipoprotein gene reduced competitiveness by two- to threefold in the animal model and in broth culture. Mutation of ssaB, encoding a putative metal transporter, produced a similar effect in culture but reduced in vivo competiveness by >1,000-fold. [³H]palmitate labeling and Western blot analysis confirmed that the lgt mutant failed to acylate lipoproteins, that the lspA mutant had a general defect in lipoprotein cleavage, and that SsaB was processed differently in both mutants. These results indicate that the loss of a single lipoprotein, SsaB, dramatically reduces endocarditis virulence, whereas the loss of most other lipoproteins or of normal lipoprotein processing has no more than a minor effect on virulence.Streptococcus sanguinis is a member of the viridans group of streptococci and is a primary colonizer of teeth (8). The viridans species and, in particular, S. sanguinis (15, 18) are a leading cause of infective endocarditis, a serious infection of the valves or lining of the heart (48). Damage to the heart resulting from rheumatic fever or certain congenital heart defects dramatically increases the risk of developing endocarditis (48, 71). The damage is thought to result in the formation of sterile cardiac “vegetations” composed of platelets and fibrin (48) that can be colonized by certain bacteria during periods of bacteremia. This view is supported by animal studies in which formation of sterile vegetation by cardiac catheterization is required for the efficient establishment of streptococcal endocarditis (17). Prevention of infective endocarditis currently relies upon prophylactic administration of antibiotics prior to dental or other surgical procedures that are likely to produce bacteremia. The growing realization that oral bacteria such as S. sanguinis can enter the bloodstream through routine daily activities such as eating has led the American Heart Association (71) and others (57) to question the value of using antibiotic prophylaxis for dental procedures. Clearly, a better understanding of the bacterial virulence factors that contribute to endocarditis could lead to better preventive measures, such as a vaccine that could potentially afford continuous protection to high-risk patients (71).In a previous study, we used the signature-tagged mutagenesis (STM) technique to search for endocarditis virulence factors of S. sanguinis in a rabbit model (53). This study identified a number of housekeeping enzymes that contribute to endocarditis. Because these proteins are not likely to be surface localized, they hold little promise as vaccine candidates. One class of streptococcal surface proteins that is rich in both virulence factors (4, 7, 25, 33, 38, 60) and promising vaccine candidates (6, 39, 42, 51, 70) is the lipoproteins. Lipoprotein activities that have been suggested to contribute to streptococcal virulence include adhesion (4, 7, 63), posttranslational modification (25, 29, 51), and ATP-binding cassette (ABC)-mediated transport (33, 52, 60). In the last instance, lipoproteins anchored to the cell membrane by their lipid tails appear to serve the same transport function as the periplasmic substrate-binding proteins of gram-negative bacteria (66). STM studies performed with Streptococcus pneumoniae (26, 41, 55) and Streptococcus agalactiae (34) have identified multiple lipoprotein mutants among collections of reduced virulence mutants. In an attempt to determine the cumulative contribution of streptococcal lipoproteins to virulence, some investigators have created mutations in the lgt or lspA genes, encoding lipoprotein-processing enzymes (12, 25, 27, 36). The lgt gene encodes prolipoprotein diacylglyceryl transferase, which catalyzes the transfer of a diacylglycerol lipid unit to a cysteine in the conserved N-terminal “lipobox” of lipoproteins, while lspA encodes the signal peptidase II enzyme that cleaves the signal peptide of the prolipoprotein just prior to the conserved cysteine (59, 65). While mutation of these genes has been shown to be lethal in gram-negative bacteria (21, 73), many gram-positive bacterial species have been shown to tolerate such mutations, often with only minor effects on growth (3, 12, 13, 25, 27, 36, 54). Some of these studies indicated a deleterious effect on the virulence of the lgt (25, 54) or lspA (36) mutation, but others found no effect (12) or an enhancement of virulence (27). It is clear from these and other studies (3, 13) that neither the loss of acylation due to lgt inactivation nor the loss of signal peptidase II-mediated cleavage completely eliminates lipoprotein function, necessitating alternative approaches for assessing the global contribution of lipoproteins to virulence.We have used bioinformatic approaches to identify every putative lipoprotein encoded by S. sanguinis strain SK36. To determine the contribution of these lipoproteins to the endocarditis virulence of S. sanguinis, we have systematically mutagenized each of these genes, as well as the lgt and lspA genes, and evaluated these mutants for virulence by using STM in an animal model. Selected mutants were further examined for virulence in competitive index (CI) assays. A strain with a disrupted ssaB gene, which encodes a putative metal transport protein, was found to exhibit a profound defect in virulence that was far greater than that of any other strain tested, including the lgt or lspA mutant.     

GuaA and GuaB Are Essential for Borrelia burgdorferi Survival in the Tick-Mouse Infection Cycle

Pathogens lacking the enzymatic pathways for de novo purine biosynthesis are required to salvage purines and pyrimidines from the host environment for synthesis of DNA and RNA. Two key enzymes in purine salvage pathways are IMP dehydrogenase (GuaB) and GMP synthase (GuaA), encoded by the guaB and guaA genes, respectively. While these genes are typically found on the chromosome in most bacterial pathogens, the guaAB operon of Borrelia burgdorferi is present on plasmid cp26, which also harbors a number of genes critical for B. burgdorferi viability. Using molecular genetics and an experimental model of the tick-mouse infection cycle, we demonstrate that the enzymatic activities encoded by the guaAB operon are essential for B. burgdorferi mouse infectivity and provide a growth advantage to spirochetes in the tick. These data indicate that the GuaA and GuaB proteins are critical for the survival of B. burgdorferi in the infection cycle and highlight a potential difference in the requirements for purine salvage in the disparate mammalian and tick environments.Purine metabolism is critical for the growth and virulence in mammals of many bacterial pathogens (11, 26, 29, 33, 51). Borrelia burgdorferi, the infectious agent of Lyme borreliosis, lacks the genes encoding the enzymes required for de novo nucleotide synthesis (8, 12) and therefore must rely on salvage of purines and pyrimidines from its hosts for nucleic acid biosynthesis (21, 35). Furthermore, B. burgdorferi lacks the genes encoding key enzymes required for a classic purine salvage pathway, including hpt (hypoxanthine-guanine phosphoribosyltransferase), purA (adenylosuccinate synthase), purB (adenylosuccinate lyase), and the locus encoding a ribonucleotide reductase (4, 8, 12, 35, 66). Despite the absence of a ribonucleotide reductase, an enzyme critical for the generation of deoxynucleotides through enzymatic reduction of ribonucleotides (32), a novel purine salvage pathway that involves salvage of deoxynucleosides from the host and interconversion of purine bases to deoxynucleosides by BB0426, a deoxyribosyl transferase, has recently been demonstrated for B. burgdorferi (23) (Fig. ​(Fig.11).Open in a separate windowFIG. 1.Pivotal role of the GuaAB proteins in the purine salvage pathway of B. burgdorferi. A novel pathway for purine salvage has recently been elucidated for B. burgdorferi (23). Extracellular adenine and hypoxanthine are salvaged by B. burgdorferi from mammalian and tick host environments (61). Following transport, adenine can be converted to hypoxanthine by adenine deaminase (BBK17) (21). This pathway proposes two possible fates for hypoxanthine, as follows. (i) Hypoxanthine is converted to IMP by a putative xanthine-guanine phosphoribosyl transferase (BB0103), IMP is converted to XMP by IMPDH (GuaB or BBB17), and XMP is converted to GMP by GMP synthase (GuaA or BBB18), resulting in guanine nucleotides for RNA synthesis. (ii) Direct transport of deoxynucleosides appears to provide a source of deoxyribose for interconversion of hypoxanthine to deoxyinosine by a deoxyribosyl transferase (BB0426) (23). dIMP is generated by a putative deoxynucleotide kinase (BB0239). GuaB converts dIMP to dXMP, and GuaA converts dXMP to dGMP, providing guanine deoxynucleotides for DNA synthesis (23). Salvage of free guanine nucleosides and guanine deoxynucleosides, when they are available in the host environment, may allow B. burgdorferi to circumvent the GuaAB requirement for GMP and dGMP biosynthesis. The dashed arrows indicate dephosphorylation of nucleotide monophosphate or deoxynucleotide monophosphate prior to transport by the spirochete and rephosphorylation of nucleoside and deoxynucleoside to nucleotide triphosphate and deoxynucleotide triphosphate, respectively, for RNA and DNA synthesis. NMP, nucleotide monophosphate; N, nucleoside; dN, deoxynucleoside; dNMP, deoxynucleotide monophosphate; OM, outer membrane; IM, inner membrane.In its infection cycle, B. burgdorferi passages between two disparate environments with potentially distinct purine availabilities, the tick vector and a mammalian host. Hypoxanthine is the most abundant purine in mammalian blood (17), and it is available for salvage by B. burgdorferi during the blood meal of an infected tick and during the spirochete''s transient presence in the mammalian bloodstream. Despite the absence of the hpt gene, we and others have shown that B. burgdorferi is able to transport and incorporate low levels of hypoxanthine (23, 35). During mammalian infection B. burgdorferi resides in various tissues, including the skin, heart, bladder, and joints. Adenine has been shown to be ubiquitous in mammalian tissues (61) and therefore is available for salvage by B. burgdorferi. Guanine is present at low levels in mammalian blood and tissues (17, 61); however, the amount may not be sufficient for survival of the spirochete.The limiting step in guanine nucleotide biosynthesis from adenine and hypoxanthine is the conversion of IMP to XMP, which is catalyzed by IMP dehydrogenase (IMPDH) (65). Guanine nucleotides are essential for DNA and RNA synthesis, signal transduction, and cell cycle control; thus, IMPDH activity is critical for the survival of most organisms (60). The enzymes required for the final two steps of guanine nucleotide biosynthesis, IMPDH and GMP synthase, are encoded by the guaB and guaA genes, respectively. The guaA and guaB genes and the corresponding activities of their protein products are conserved in B. burgdorferi (28, 67). These genes are typically carried on the chromosomes of bacterial species. However, in B. burgdorferi, the guaAB operon resides on a 26-kbp circular plasmid, cp26, and it shares an approximately 185-bp intergenic region with, and is transcribed divergently from, the essential virulence gene ospC (8, 12, 28, 50, 54, 62). The cp26 plasmid has been shown to harbor numerous genes important for B. burgdorferi survival in vivo and in vitro, including ospC (16, 34, 50, 53, 56) and resT (7), as well as BBB26 and BBB27 (20). Because of these critical functions, this plasmid is the only plasmid of the approximately 21 B. burgdorferi plasmids that is present in all natural isolates and has never been shown to be lost during in vitro culture (2, 7, 18, 20, 44, 52).Here we establish that the enzymatic activities of GuaA and GuaB are critical for the survival of B. burgdorferi in the infectious cycle and highlight a potential difference in this spirochete''s requirement for purine salvage in the disparate mammalian and tick environments.     

Optical Mapping Reveals a Large Genetic Inversion between Two Methicillin-Resistant Staphylococcus aureus Strains

Staphylococcus aureus is a highly versatile and evolving bacterium of great clinical importance. S. aureus can evolve by acquiring single nucleotide polymorphisms and mobile genetic elements and by recombination events. Identification and location of novel genomic elements in a bacterial genome are not straightforward, unless the whole genome is sequenced. Optical mapping is a new tool that creates a high-resolution, in situ ordered restriction map of a bacterial genome. These maps can be used to determine genomic organization and perform comparative genomics to identify genomic rearrangements, such as insertions, deletions, duplications, and inversions, compared to an in silico (virtual) restriction map of a known genome sequence. Using this technology, we report here the identification, approximate location, and characterization of a genetic inversion of ∼500 kb of a DNA element between the NRS387 (USA800) and FPR3757 (USA300) strains. The presence of the inversion and location of its junction sites were confirmed by site-specific PCR and sequencing. At both the left and right junction sites in NRS387, an IS1181 element and a 73-bp sequence were identified as inverted repeats, which could explain the possible mechanism of the inversion event.Staphylococcus aureus is a gram-positive bacterium of immense clinical importance. This opportunistic pathogen is capable of causing a wide range of diseases from skin and soft-tissue infections to life-threatening bacteremia, endocarditis, and osteomyelitis (14). Approximately 75% of the S. aureus genome is composed of a core genome that is common in all strains, and 25% of the genome is composed of variable regions which can differ between different strains (4, 16, 24-26). S. aureus evolves primarily by introducing single nucleotide polymorphisms in its core genome and by acquiring mobile genetic elements (MGEs) through horizontal gene transfer. These MGEs include pathogenicity/genomic islands, plasmids, transposons, and bacteriophages that become integrated in the chromosome (4, 11, 16, 31, 32). Despite being a heterogeneous organism, genetic recombination in S. aureus is proposed to be rather rare (20, 24, 29, 35). Its clones are more likely to evolve by point mutations than by recombination events (12). The MGEs contribute to the phenotypic and genotypic diversity seen with the S. aureus population. Acquisition of the staphylococcal cassette chromosome (SCCmec) elements through site-specific recombinases has led to the epidemic of methicillin-resistant S. aureus (MRSA) strains in hospitals and communities all over the world (6, 10, 15). In recent years, the integration of arginine catabolite mobile element in the USA300 lineage of MRSA has been proposed to give the pathogen its epidemiological advantage, including traits for surviving in low-pH conditions and oxygen tension environments (11). In addition, chromosomal replacements have been observed within lineages of sequence type 34 (ST34) and ST42 (34) and ST8 and ST30 (13).Genomic rearrangements, such as inversions, have been observed with genomes of enteric bacteria, such as Salmonella enterica, Shigella flexneri, Yersinia pestis KIM, Escherichia coli (K12 and O157:H7), and group A Streptococcus pyogenes (8, 9, 18, 27, 28, 30, 37). No genomic inversions in S. aureus have been reported to date. With the use of optical mapping, large genomic rearrangements, such as inversions, that would otherwise be missed with other comparative genotyping approaches, including microarray analysis, can be identified. Optical mapping uses high-resolution restriction maps (optical maps) of a bacterial genome to determine its genomic organization (5, 21, 23, 33, 36). These optical maps can be compared to an in silico (virtual) restriction map of a known genome sequence and can be used to identify gene rearrangements and their locations. Using optical mapping in conjunction with subsequent site-specific PCR and sequencing, we report the identification, approximate location, and partial characterization of an ∼500-kb DNA element in NRS387, a USA800 strain that was found to be inverted relative to USA300FPR3757. Identification of IS1181 elements and a novel 73-bp element at both ends of the ∼500-kb element in NRS387 could suggest the possibility of an inversion event in an ancestral strain of NRS387.     

Organoselenium Coating on Cellulose Inhibits the Formation of Biofilms by Pseudomonas aeruginosa and Staphylococcus aureus

Among the most difficult bacterial infections encountered in treating patients are wound infections, which may occur in burn victims, patients with traumatic wounds, necrotic lesions in people with diabetes, and patients with surgical wounds. Within a wound, infecting bacteria frequently develop biofilms. Many current wound dressings are impregnated with antimicrobial agents, such as silver or antibiotics. Diffusion of the agent(s) from the dressing may damage or destroy nearby healthy tissue as well as compromise the effectiveness of the dressing. In contrast, the antimicrobial agent selenium can be covalently attached to the surfaces of a dressing, prolonging its effectiveness. We examined the effectiveness of an organoselenium coating on cellulose discs in inhibiting Pseudomonas aeruginosa and Staphylococcus aureus biofilm formation. Colony biofilm assays revealed that cellulose discs coated with organoselenium completely inhibited P. aeruginosa and S. aureus biofilm formation. Scanning electron microscopy of the cellulose discs confirmed these results. Additionally, the coating on the cellulose discs was stable and effective after a week of incubation in phosphate-buffered saline. These results demonstrate that 0.2% selenium in a coating on cellulose discs effectively inhibits bacterial attachment and biofilm formation and that, unlike other antimicrobial agents, longer periods of exposure to an aqueous environment do not compromise the effectiveness of the coating.Among the most difficult bacterial infections encountered in treating patients are wound infections, which may occur in burn victims (10), patients with traumatic wounds (33), people with diabetes (27), and patients with surgical wounds (29, 31). Two of the more common causative agents of wound infections are Staphylococcus aureus and Pseudomonas aeruginosa (10, 27, 29, 31, 33). Such infections often lead to fatality; the mortality rate among patients infected with P. aeruginosa ranges from 26% to 55% (9, 49), while mortality from S. aureus infection ranges from 19% to 38% (28, 46, 50). As opportunistic pathogens, S. aureus and P. aeruginosa cause few infections in healthy individuals but readily cause infection once host defenses are compromised, such as with the removal of skin from burns (10). S. aureus infection originates from the normal flora of either the patient or health care workers (48), while P. aeruginosa is acquired from the environment surrounding the patient (41). Once established on the skin, S. aureus and P. aeruginosa are then able to colonize the wound. Infection results if the organisms proliferate in the wound environment.Both P. aeruginosa and S. aureus often exist within burn wounds as biofilms (43, 47). A biofilm is presently defined as a sessile microbial community characterized by cells that are irreversibly attached either to a substratum or to each other (16). Biofilms, which can attain over 100 μm in thickness, are made up of multiple layers of bacteria in an exopolysaccharide matrix (12, 16, 42). Sauer et al. showed that P. aeruginosa biofilms form in distinct developmental stages: reversible attachment, irreversible attachment, two stages of maturation, and a dispersion phase (42). Clinically, biofilms present serious medical management problems through their association with different chronic infections (37). During vascular catheter-related infections and sepsis, biofilms serve as a reservoir of bacteria from which planktonic cells detach and spread throughout the tissue and/or enter the circulatory system, resulting in bacteremia or septicemia (15). Factors specific to the bacterium may influence the formation of bacterial biofilms at different infection sites or surfaces. For example, during the initial attachment stage the flagellum, lipopolysaccharide, and possibly outer membrane proteins play a major role in bringing P. aeruginosa into proximity with the surface as well as mediating the interaction with the substratum (12). Using the murine model of thermal injury, we recently showed that P. aeruginosa forms a biofilm within the thermally injured tissues (43). Clinically, the surgeons debride the infected or dead tissues; however, a few microorganisms may remain on the tissue surface and reinitiate biofilm formation.Antibiotics, silver, or chitosan, attached to or embedded in gauze, have been shown to be efficacious in preventing wound infection (21, 24, 26, 36). However, the resistance of P. aeruginosa and S. aureus to available antibiotics severely limits the choices for antibiotic treatment (13, 40). Additionally, silver compounds, such as silver nitrate and silver sulfadiazine, leaching from dressings are toxic to human fibroblasts even at low concentrations (20, 25). Thus, effective alternative antimicrobial agents that contact the thermally injured/infected tissues and prevent the development of bacterial biofilms are required. Previous studies have shown that selenium (Se) can be covalently bound to a solid matrix and retain its ability to catalyze the formation of superoxide radicals (O₂^·−) (30). These superoxide radicals inhibit bacterial attachment to the solid surface (30). In this study, we examined the ability of a newly synthesized organoselenium-methacrylate polymer (Se-MAP) to block biofilm formation by both S. aureus and P. aeruginosa. These bacteria were chosen since they cause a major share of wound infections and because drug-resistant forms of these bacteria have become a serious problem in the treatment and management of these wound infections (6, 13, 17, 18, 38). Results of the study show that 0.2% (wt/wt) Se in Se-MAP covalently attached to cellulose discs inhibited P. aeruginosa and S. aureus biofilm formation. This could lead to the development of a selenium-based antimicrobial coating for cotton materials that will prevent the bacterial attachment and colonization that can ultimately lead to bacterial biofilm formation during chronic infections.     

Thioredoxin 1 Participates in the Activity of the Salmonella enterica Serovar Typhimurium Pathogenicity Island 2 Type III Secretion System

The facultative intracellular pathogen Salmonella enterica serovar Typhimurium relies on its Salmonella pathogenicity island 2 (SPI2) type III secretion system (T3SS) for intracellular replication and virulence. We report that the oxidoreductase thioredoxin 1 (TrxA) and SPI2 are coinduced for expression under in vitro conditions that mimic an intravacuolar environment, that TrxA is needed for proper SPI2 activity under these conditions, and that TrxA is indispensable for SPI2 activity in both phagocytic and epithelial cells. Infection experiments in mice demonstrated that SPI2 strongly contributed to virulence in a TrxA-proficient background whereas SPI2 did not affect virulence in a trxA mutant. Complementation analyses using wild-type trxA or a genetically engineered trxA coding for noncatalytic TrxA showed that the catalytic activity of TrxA is essential for SPI2 activity in phagocytic cells whereas a noncatalytic variant of TrxA partially sustained SPI2 activity in epithelial cells and virulence in mice. These results show that TrxA is needed for the intracellular induction of SPI2 and provide new insights into the functional integration between catalytic and noncatalytic activities of TrxA and a bacterial T3SS in different settings of intracellular infections.In Escherichia coli, thioredoxin 1 (TrxA, encoded by trxA) is an evolutionary conserved 11-kDa cytosolic highly potent reductase that supports the activities of various oxidoreductases and ribonucleotide reductases (1, 29) and interacts with a number of additional cytoplasmic proteins through the formation of temporary covalent intermolecular disulphide bonds (32). Consequently, as trxA mutants of E. coli (51), Helicobacter pylori (13), and Rhodobacter sphaeroides (34) show increased sensitivity to hydrogen peroxide, TrxA has been defined as a significant oxidoprotectant. In addition, TrxA possess a protein chaperone function that is disconnected from cysteine interactions (30, 32).Salmonella enterica serovar Typhimurium is closely related to E. coli. During divergent evolution, the Salmonella genome acquired a number of virulence-associated genes (20). Many of these genes are clustered on genetic regions termed Salmonella pathogenicity islands (or SPIs). Of these, SPI1 and SPI2 code for separate type III secretion systems (T3SSs). T3SSs are supramolecular virulence-associated machineries that, in several pathogenic gram-negative bacterial species, enable injection of effector proteins from the bacteria into host cells (22, 57). The effector proteins, in turn, manipulate intrinsic host cell functions to facilitate the infection.The SPI1 T3SS of S. serovar Typhimurium is activated for expression in the intestine in response to increased osmolarity and decreased oxygen tension (22, 57). SPI1 effector proteins are primarily secreted into cells that constitute the epithelial layer and interfere with host cell Cdc42 and Rac-1 signaling and actin polymerization. This enables the bacteria to orchestrate their own actin-dependent uptake into nonphagocytic cells (57). SPI1 effector proteins also induce inflammatory signaling and release of interleukin-1β from infected cells (25, 26).Subsequent systemic progression of S. serovar Typhimurium from the intestinal tissue relies heavily on an ability to survive and replicate in phagocytic cells (18, 46, 53, 54). S. serovar Typhimurium uses an additional set of effector proteins secreted by the SPI2 T3SS for replication inside host cells and for coping with phagocyte innate responses to the infection (10, 11, 54). The functions of SPI2 effectors include diversion of vesicular trafficking, induction of apoptotic responses, and manipulation of ubiquitination of host proteins (28, 40, 45, 53). Hence, SPI2 effector proteins create a vacuolar environment that sustains intracellular replication of S. serovar Typhimurium (28).In addition to pathogenicity islands, the in vivo fitness of Salmonella spp. relies on selected functions shared with other enterobacteria. Thus, many virulence genes are integrated into “housekeeping” gene regulatory networks, coded for by a core genome, which steer bacterial stress responses (12, 17, 27, 55). Selected anabolic pathways also contribute to virulence of S. serovar Typhimurium (18, 27), evidently by providing biochemical building blocks for bacterial replication (36).In S. serovar Typhimurium, TrxA is a housekeeping protein that strongly contributes to virulence in cell culture and mouse infection models (8). However, the mechanism by which TrxA activity adds to virulence has not been defined. Here we show that the contribution of TrxA to virulence of S. serovar Typhimurium associates with its functional integration with the SPI2 T3SS under conditions that prevail in the intracellular vacuolar compartment of the host cell. These findings ascribe a novel role to TrxA in bridging environmental adaptations with virulence gene expression and illuminate a new aspect of the interaction between evolutionary conserved and horizontally acquired gene functions in bacteria.     

Staphylococcus aureus NrdH Redoxin Is a Reductant of the Class Ib Ribonucleotide Reductase

Staphylococci contain a class Ib NrdEF ribonucleotide reductase (RNR) that is responsible, under aerobic conditions, for the synthesis of deoxyribonucleotide precursors for DNA synthesis and repair. The genes encoding that RNR are contained in an operon consisting of three genes, nrdIEF, whereas many other class Ib RNR operons contain a fourth gene, nrdH, that determines a thiol redoxin protein, NrdH. We identified a 77-amino-acid open reading frame in Staphylococcus aureus that resembles NrdH proteins. However, S. aureus NrdH differs significantly from the canonical NrdH both in its redox-active site, C-P-P-C instead of C-M/V-Q-C, and in the absence of the C-terminal [WF]SGFRP[DE] structural motif. We show that S. aureus NrdH is a thiol redox protein. It is not essential for aerobic or anaerobic growth and appears to have a marginal role in protection against oxidative stress. In vitro, S. aureus NrdH was found to be an efficient reductant of disulfide bonds in low-molecular-weight substrates and proteins using dithiothreitol as the source of reducing power and an effective reductant for the homologous class Ib RNR employing thioredoxin reductase and NADPH as the source of the reducing power. Its ability to reduce NrdEF is comparable to that of thioredoxin-thioredoxin reductase. Hence, S. aureus contains two alternative thiol redox proteins, NrdH and thioredoxin, with both proteins being able to function in vitro with thioredoxin reductase as the immediate hydrogen donors for the class Ib RNR. It remains to be clarified under which in vivo physiological conditions the two systems are used.Ribonucleotide reductases (RNRs) are essential enzymes in all living cells, providing the only known de novo pathway for the biosynthesis of deoxyribonucleotides, the immediate precursors of DNA synthesis and repair. RNRs catalyze the controlled reduction of all four ribonucleotides to maintain a balanced pool of deoxyribonucleotides during the cell cycle (29). Three main classes of RNRs are known. Class I RNRs are oxygen-dependent enzymes, class II RNRs are oxygen-independent enzymes, and class III RNRs are oxygen-sensitive enzymes. Class I RNRs are divided into two subclasses, subclasses Ia and Ib.Staphylococcus aureus is a Gram-positive facultative aerobe and a major human pathogen (24). S. aureus contains class Ib and class III RNRs, which are essential for aerobic and anaerobic growth, respectively (26). The class Ib NrdEF RNR is encoded by the nrdE and nrdF genes: NrdE contains the substrate binding and allosteric binding sites, and NrdF contains the catalytic site for ribonucleotide reduction. The S. aureus nrdEF genes form an operon containing a third gene, nrdI, the product of which, NrdI, is a flavodoxin (5, 33). Many other bacteria such as Escherichia coli (16), Lactobacillus lactis (17), and Mycobacterium and Corynebacterium spp. possess class Ib RNR operons that contain a fourth gene, nrdH (30, 44, 50), whose product, NrdH, is a thiol-disulfide redoxin (16, 17, 40, 43, 49). More-complex situations are found for some bacteria, where the class Ib RNR operon may be duplicated and one or more of the nrdI and nrdH genes may be missing or located in another part of the chromosome (29).NrdH proteins are glutaredoxin-like protein disulfide oxidoreductases that alter the redox state of target proteins via the reversible oxidation of their active-site dithiol proteins. NrdH proteins function with high specificity as electron donors for class I RNRs (9, 16-18). They are widespread in bacteria, particularly in those bacteria that lack glutathione (GSH), where they function as a hydrogen donor for the class Ib RNR (12, 16, 17). In E. coli, which possesses class Ia and class Ib RNRs, NrdH functions in vivo as the primary electron donor for the class Ib RNR, whereas thioredoxin or glutaredoxin is used by the class Ia NrdAB RNR (12, 17). NrdH redoxins contain a conserved CXXC motif, have a low redox potential, and can reduce insulin disulfides. NrdH proteins possess an amino acid sequence similar to that of glutaredoxins but behave functionally more like thioredoxins. NrdH proteins are reduced by thioredoxin reductase but not by GSH and lack those residues in glutaredoxin that bind GSH and the GSH binding cleft (39, 40). The structures of the E. coli and Corynebacterium ammoniagenes NrdH redoxins reveal the presence of a wide hydrophobic pocket at the surface, like that in thioredoxin, that is presumed to be involved in binding to thioredoxin reductase (39, 40). NrdI proteins are flavodoxin proteins that function as electron donors for class Ib RNRs and are involved in the maintenance of the NrdF diferric tyrosyl radical (5, 33). In Streptococcus pyogenes, NrdI is essential for the activity of the NrdHEF system in a heterologous E. coli in vivo complementation assay (33). Class Ib RNRs are proposed to depend on two specific electron donors, NrdH, which provides reducing power to the NrdE subunit, and NrdI, which supplies electrons to the NrdF subunit (33).In this report we identify an open reading frame (ORF) in S. aureus encoding an NrdH-like protein with partial sequence relatedness to the E. coli, Salmonella enterica serovar Typhimurium, L. lactis, and C. ammoniagenes NrdH proteins. In contrast to these bacteria, the S. aureus nrdH gene does not form part of the class Ib RNR operon. The S. aureus NrdH protein differs in its structure from the canonical NrdH in its redox-active site, C-P-P-C instead of C-M/V-Q-C, and in the absence of the C-terminal [WF]SGFRP[DE] structural motif. We show that in vitro, S. aureus NrdH reduces protein disulfides and is an electron donor for the homologous class Ib NrdEF ribonucleotide reductase.     

Identification of ORF636 in Phage φSLT Carrying Panton-Valentine Leukocidin Genes,Acting as an Adhesion Protein for a Poly(Glycerophosphate) Chain of Lipoteichoic Acid on the Cell Surface of Staphylococcus aureus

The temperate phage φSLT of Staphylococcus aureus carries genes for Panton-Valentine leukocidin. Here, we identify ORF636, a constituent of the phage tail tip structure, as a recognition/adhesion protein for a poly(glycerophosphate) chain of lipoteichoic acid on the cell surface of S. aureus. ORF636 bound specifically to S. aureus; it did not bind to any other staphylococcal species or to several gram-positive bacteria.Staphylococcus aureus, a ubiquitous and harmful human pathogen, produces three types of bicomponent pore-forming cytotoxins, namely, γ-hemolysin (LukF and Hlg2), leukocidin (LukF and LukS), and Panton-Valentine leukocidin (PVL) (LukF-Pv and LukS-Pv) (16). Of these, PVL has been investigated as a virulence-related factor of some S. aureus infectious diseases (7, 11, 23, 24, 31, 37). PVL shows high cytolytic specificity against human polymorphonuclear leukocytes and macrophages, and it is closely associated with most cutaneous necrotic lesions, such as furuncles or primary abscesses, and severe necrotic skin infection (24, 31), as well as with severe necrotic hemorrhagic pneumonia (11, 23). LukF-Pv and LukS-Pv are expressed by the PVL locus (pvl), which is distinct from the γ-hemolysin locus (hlg) (16, 32). In previous research, we found that pvl genes are located in the genome of the lysogenic bacteriophage φPVL (17, 18). We also found another PVL-carrying temperate elongated-head Siphoviridae phage, φSLT, which has the ability to convert S. aureus to the PVL-producing strain from a clinical isolate (29). These findings indicated that at least two types of staphylococcal temperate phages are involved in the horizontal transfer of pvl genes among S. aureus strains (16, 29). Recently, the emergence of a single clonal community-acquired methicillin-resistant S. aureus (CA-MRSA), which produces PVL, was reported (7). Most CA-MRSA strains isolated in the United States and Australia carry the staphylococcal cassette chromosome mec (SCCmec) IV, and they were divided into five clonal complexes by multilocus sequence typing (30). The analysis of the CA-MRSA clones confirmed the presence of PVL genes and SCCmec IV in CA-MRSA and suggested that various CA-MRSA strains have arisen from the diverse genetic backgrounds associated with each geographic origin, rather than from the worldwide spread of a single clone (30, 37). Although there is great debate as to whether PVL is an important virulence factor, numerous studies support the hypothesis that PVL plays an important role in the pathogenesis of CA-MRSA necrotizing pneumonia (3, 6). In regard to the acquisition of PVL gene clusters and the proliferation of PVL-carrying CA-MRSA, the horizontal transfer of PVL via PVL-carrying phages, as well as that of SCCmec, has become the focus of intense research interest. To understand the horizontal transfer of PVL, the analysis of the infection ability of a PVL-carrying phage is important. If the phage has a wide host range, the PVL-carrying phage might threaten to become a source of emerging PVL-positive bacteria. Phage infection starts from an interaction between a phage virion and its host cell surface receptor. Nevertheless, little is known about phage receptors on the surface of S. aureus, and the mechanism of host cell-specific binding of staphylococcal phages has been poorly characterized. In addition, there is no information about staphylococcal phage proteins involved in host cell recognition and/or binding. Here, we identify ORF636, with a mass of 66 kDa, as a structural protein of the φSLT tail and determine that it acts as a protein for recognition/adhesion of a poly(glycerophosphate) moiety of lipoteichoic acid (LTA) on the cell surface of the host S. aureus in the first stage of infection by φSLT.