Vibrio cholerae temperate phage O139: characteristics and role in changing expression of chromosomal virulence genes

Restriction analysis of temperate cholera phage 139 isolated from Vibrio cholerae P16064, serogroup 0139, showed its DNA to be double-stranded linear with cohesive terminals. DNA-DNA hybridization on nylon membranes revealed that many V. cholerae strains of serogroup 0139 isolated in different regions contained a temperate cholera phage 139 in their genomes. Southern blot hybridization of chromosomal DNA PST-fragments with phage probe showed that the temperate phage 139 was identical to the temperate phage of serogroup II V. eltor. The phage integrated in the chromosome near genes encoding motility (mot) and production of the capsule (cap) and purine (pur). Phage genome is apparently responsible for instability of cap, pur, and mot genes whose products are important for the development of an infectious process in cholera.     

Polymerase chain reaction amplification of naphthalene-catabolic and 16S rRNA gene sequences from indigenous sediment bacteria.

We report the amplification of bacterial genes from uninoculated surface and subsurface sediments by the polymerase chain reaction (PCR). PCR amplification of indigenous bacterial 16S ribosomal DNA genes was unsuccessful when subsurface sediment containing approximately 10(7) cells.g-1 was added directly to a PCR mixture. However, when 10 mg of sediment was inoculated with approximately 10(5) cells of Pseudomonas putida G7, the nahAc naphthalene dioxygenase gene characteristic of the P. putida G7 NAH7 plasmid was detected by PCR amplification. Southern blotting of the PCR amplification product improved sensitivity to 10(3) to 10(4) cells from samples inoculated with P. putida G7, but controls with no sediment added showed that the PCR was partially inhibited by the sediments. Lysozyme-sodium dodecyl sulfate-freeze-thaw DNA extraction was combined with gel electrophoretic partial purification in the presence of polyvinylpyrrolidone to render DNA from indigenous bacteria in surface or subsurface sediment samples amplifiable by PCR using eubacterial 16S ribosomal DNA primers. The nahAc gene could also be amplified from indigenous bacteria by using nahAc-specific primers when PCR conditions were modified by increasing Taq and primer concentrations. Restriction digests of the nahAc amplification products from surface and subsurface sediments revealed polymorphism relative to P. putida G7. The procedures for DNA extraction, purification, and PCR amplification described here demonstrate that the PCR is a potentially useful tool in studies of function- and taxon-specific DNA from indigenous microbial communities in sediment and groundwater environments.     

Overestimation of the abundance of sulfate-reducing bacteria in human feces by quantitative PCR targeting the Desulfovibrio 16S rRNA gene

The dominant genus of sulfate-reducing bacteria (SRB) in humans is Desulfovibrio, and quantitative PCR (QPCR) targeting the 16S rRNA gene is often used in assays. We show that the 16S rRNA gene assay overestimated SRB abundance in feces from 24 adults compared to QPCR assays using primers targeting two genes involved in SRB energy metabolism.     

Development and experimental validation of a predictive threshold cycle equation for quantification of virulence and marker genes by high-throughput nanoliter-volume PCR on the OpenArray platform

Development of quantitative PCR (QPCR) assays typically requires extensive screening within and across a given species to ensure specific detection and lucid identification among various pathogenic and nonpathogenic strains and to generate standard curves. To minimize screening requirements, multiple virulence and marker genes (VMGs) were targeted simultaneously to enhance reliability, and a predictive threshold cycle (C(T)) equation was developed to calculate the number of starting copies based on an experimental C(T). The empirical equation was developed with Sybr green detection in nanoliter-volume QPCR chambers (OpenArray) and tested with 220 previously unvalidated primer pairs targeting 200 VMGs from 30 pathogens. A high correlation (R(2) = 0.816) was observed between the predicted and experimental C(T)s based on the organism's genome size, guanine and cytosine (GC) content, amplicon length, and stability of the primer's 3' end. The performance of the predictive C(T) equation was tested using 36 validation samples consisting of pathogenic organisms spiked into genomic DNA extracted from three environmental waters. In addition, the primer success rate was dependent on the GC content of the target organisms and primer sequences. Targeting multiple assays per organism and using the predictive C(T) equation are expected to reduce the extent of the validation necessary when developing QPCR arrays for a large number of pathogens or other targets.     

Detection of Staphylococcus hyicus exfoliative toxin genes by dot blot hybridization and multiplex polymerase chain reaction

We designed a novel DNA probe and novel PCR primer sets for detecting the genes coding for Staphylococcus hyicus (S. hyicus) exfoliative toxin (ET). In dot blot hybridization, the novel DNA probe hybridized with chromosomal DNA of ExhA-, ExhB-, ExhC-, ExhD-, and SHETA-producing strains. This probe also hybridized with the plasmid DNA of a SHETB-producing strain. In Southern blot hybridization, the probe hybridized with a 1.5 kb HindIII fragment of chromosomal DNA from a SHETA-producing strain. The above fragment was cloned into E. coli and the nucleotide sequence of the SHETA gene determined, this gene proved to have almost the same homology (99.6%) as the ExhB gene. It was therefore thought that SHETA is a subtype of ExhB. In multiplex PCR using five primer sets, each gene gave a band distinguishable from the others. This multiplex PCR system has high specificity among the well-known S. hyicus ET genes. Of the 69 known ET-producing S. hyicus strains, 38, 19, 10, 2 and 1 strains have exhB, exhD exhA, shetb and exhC genes, respectively.     

Real-time PCR provides improved detection and titer determination of bacteriophage

The plaque assay is the traditional method for the quantification of bacteriophage, particularly for lambda cloning vectors. Unfortunately, this technique is fraught with procedural difficulties, and the quality of the data obtained from this "gold standard" assay may be inaccurate due to the subjective interpretation of the results. The application of quantitative real-time PCR (QPCR) technology can address these issues and be a more accurate platform to evaluate phage growth conditions and quantify viral titers in phage preparations. QPCR, with an improved primer set specific for lambda phage and coupled with fluorescent dye detection of PCR products, was used to detect and quantify phages in lysates with no prior DNA purification. Phages were detected below one plaque-forming unit, and at least 89 viral copies were detected from a purified DNA sample. When unknown concentrations of various phage preparations were assessed using QPCR, they were attained more efficiently, with greater sensitivity and precision, and the method produced more accurate quantitative data spanning a wider linear range than those obtained by the plaque assay (six logs vs. one log, respectively). Finally, QPCR for the detection of phage has multiple applications, including conventional cloning and in alternative fields of study such as environmental sciences.     

Bacteriophages and genetic mobilization in sewage and faecally polluted environments

Bacteriophages are one of the most abundant entities on the planet and are present in high concentrations within humans and animals, mostly in the gut. Phages that infect intestinal bacteria are released by defecation and remain free in extra‐intestinal environments, where they usually persist for longer than their bacterial hosts. Recent studies indicate that a large amount of the genetic information in bacterial genomes and in natural environments is of phage origin. In addition, metagenomic analysis reveals that a substantial number of bacterial genes are present in viral DNA in different environments. These facts support the belief that phages can play a significant role in horizontal gene transfer between bacteria. Bacteriophages are known to transfer genes by generalized and specialized transduction and indeed there are some examples of phages found in the environment carrying and transducing genes of bacterial origin. A successful transduction in the environment requires certain conditions, e.g. phage and bacterial numbers need to exceed certain threshold concentrations, the bacteria need to exist in an infection‐competent physiological state, and lastly, the physical conditions in the environment (pH, temperature, etc. of the supporting matrix) have to be suitable for phage infection. All three factors are reviewed here, and the available information suggests: (i) that the number of intestinal bacteria and phages in faecally contaminated environments guarantees bacteria–phage encounters, (ii) that transduction to intestinal bacteria in the environment is probable, and (iii) that transduction is more frequent than previously thought. Therefore, we suggest that phage‐mediated horizontal transfer between intestinal bacteria, or between intestinal and autochthonous bacteria in extra‐intestinal environments, might take place and that its relevance for the emergence of new bacterial strains and potential pathogens should not be ignored.     

Unstable lysogeny and pseudolysogeny in Vibrio harveyi siphovirus-like phage 1

Exposure of Vibrio harveyi (strain VH1114) to V. harveyi siphovirus-like phage 1 (VHS1) resulted in the production of a low percentage of lysogenized clones of variable stability. These were retrieved most easily as small colonies within dot plaques. Analysis revealed that VHS1 prophage was most likely carried by VH1114 as an episome rather than integrated into the host chromosome. In the late exponential growth phase, lysogenized VH1114 continuously produced VHS1 but also gave rise to a large number of cured progeny. The absence of phage DNA in the cured progeny was confirmed by the absence of VHS1 DNA in Southern blot and PCR assays. Curiously, these very stable, cured subclones did not show the parental phenotype of clear plaques with VHS1 but instead showed turbid plaques, both in overlaid lawns and in dot plaque assays. This phenotypic difference from the original parental isolate suggested that transient lysogeny by VHS1 had resulted in a stable genetic change in the cured clones. Such clones may be called pseudolysogens (i.e., false lysogens), since they have undergone transient lysogeny and have retained some resistance to full lytic phage development, despite the loss of viable or detectable prophage.     

Antibiotic resistance genes in the bacteriophage DNA fraction of environmental samples

Antibiotic resistance is an increasing global problem resulting from the pressure of antibiotic usage, greater mobility of the population, and industrialization. Many antibiotic resistance genes are believed to have originated in microorganisms in the environment, and to have been transferred to other bacteria through mobile genetic elements. Among others, β-lactam antibiotics show clinical efficacy and low toxicity, and they are thus widely used as antimicrobials. Resistance to β-lactam antibiotics is conferred by β-lactamase genes and penicillin-binding proteins, which are chromosomal- or plasmid-encoded, although there is little information available on the contribution of other mobile genetic elements, such as phages. This study is focused on three genes that confer resistance to β-lactam antibiotics, namely two β-lactamase genes (blaTEM and blaCTX-M9) and one encoding a penicillin-binding protein (mecA) in bacteriophage DNA isolated from environmental water samples. The three genes were quantified in the DNA isolated from bacteriophages collected from 30 urban sewage and river water samples, using quantitative PCR amplification. All three genes were detected in the DNA of phages from all the samples tested, in some cases reaching 104 gene copies (GC) of blaTEM or 102 GC of blaCTX-M and mecA. These values are consistent with the amount of fecal pollution in the sample, except for mecA, which showed a higher number of copies in river water samples than in urban sewage. The bla genes from phage DNA were transferred by electroporation to sensitive host bacteria, which became resistant to ampicillin. blaTEM and blaCTX were detected in the DNA of the resistant clones after transfection. This study indicates that phages are reservoirs of resistance genes in the environment.     

Structure and arrangement of the cholera toxin genes in Vibrio cholerae O139

Abstract The sequence of the ctxB gene encoding the B subunit of cholera toxin has been determined for a strain of Vibrio cholerae of the novel O139 serotype associated with recent outbreaks of severe cholera throughout South-East Asia and found to be identical to the ctxB gene in V. cholerae O1 of the E1 Tor biotype. Analyses by Southern hybridization and PCR showed that all strains of the O139 serotype V. cholerae tested carried cholera toxin genes and other gene associated with a virulence cassette DNA region at two loci identical or homologous to those identified in the Classical rather than the E1 Tor biotype of V. cholerae serotype O1 although these loci in O139 could reside on restriction fragments of variable size.     

Occurrence and persistence of bacterial pathogens and indicator organisms in beach sand along the California coast

This report documents the presence of fecal indicators and bacterial pathogens in sand at 53 California marine beaches using both culture-dependent and -independent (PCR and quantitative PCR [QPCR]) methods. Fecal indicator bacteria were widespread in California beach sand, with Escherichia coli and enterococci detected at 68% and 94% of the beaches surveyed, respectively. Somatic coliphages and a Bacteroidales human-specific fecal marker were detected at 43% and 13% of the beaches, respectively. Dry sand samples from almost 30% of the beaches contained at least one of the following pathogens: Salmonella spp., Campylobacter spp., Staphylococcus aureus, and methicillin-resistant Staphylococcus aureus (MRSA), which were detected at 15%, 13%, 14%, and 3% of tested beaches, respectively. Fecal indicators and pathogens were poorly correlated to one another and to land cover. Sands were dry at the time of collection, and those with relatively high moisture tended to have higher concentrations or a more frequent occurrence of both indicators and pathogens. Using culture-dependent assays, fecal indicators decayed faster than pathogens in microcosm experiments using unaltered beach sand seeded with sewage and assessed by culture-dependent assays. The following order of persistence was observed (listed from most to least persistent): Campylobacter > Salmonella > somatic coliphages > enterococci > E. coli > F(+) phages. In contrast, pathogens decayed faster than fecal indicators in culture-independent assays: enterococci > Bacteroidales human-specific marker > Salmonella > Campylobacter. Microcosm experiments demonstrated that both indicators and pathogens were mobilized by wetting with seawater. Decay rates measured by QPCR were lower than those measured with culture-dependent methods. Enterococcal persistence and possible growth were observed for wetted microcosms relative to unwetted controls.     

Detection of sub-picogram quantities of specific DNA sequences on blot hybridization with biotinylated probes.

A sensitive method for detecting biotinylated DNA probes on dot and Southern blots is described which is based on the principle outlined by Leary et al (1). This system has two main components: detection of biotinylated DNA by a two-step procedure with streptavidin and poly(alkaline phosphatase); and blocking background with Tween 20. 32fg and 80fg of lambda phage DNA was detected on dot and Southern blot hybridizations respectively. 150fg of beta-globin was detected on Southern blots of genomic DNA. This method is fast, reproducible and can detect single copy genes in 0.25 micrograms genomic DNA on Southern blots.     

Unstable Lysogeny and Pseudolysogeny in Vibrio harveyi Siphovirus-Like Phage 1

Exposure of Vibrio harveyi (strain VH1114) to V. harveyi siphovirus-like phage 1 (VHS1) resulted in the production of a low percentage of lysogenized clones of variable stability. These were retrieved most easily as small colonies within dot plaques. Analysis revealed that VHS1 prophage was most likely carried by VH1114 as an episome rather than integrated into the host chromosome. In the late exponential growth phase, lysogenized VH1114 continuously produced VHS1 but also gave rise to a large number of cured progeny. The absence of phage DNA in the cured progeny was confirmed by the absence of VHS1 DNA in Southern blot and PCR assays. Curiously, these very stable, cured subclones did not show the parental phenotype of clear plaques with VHS1 but instead showed turbid plaques, both in overlaid lawns and in dot plaque assays. This phenotypic difference from the original parental isolate suggested that transient lysogeny by VHS1 had resulted in a stable genetic change in the cured clones. Such clones may be called pseudolysogens (i.e., false lysogens), since they have undergone transient lysogeny and have retained some resistance to full lytic phage development, despite the loss of viable or detectable prophage.     

Phages and the evolution of bacterial pathogens: from genomic rearrangements to lysogenic conversion.

Comparative genomics demonstrated that the chromosomes from bacteria and their viruses (bacteriophages) are coevolving. This process is most evident for bacterial pathogens where the majority contain prophages or phage remnants integrated into the bacterial DNA. Many prophages from bacterial pathogens encode virulence factors. Two situations can be distinguished: Vibrio cholerae, Shiga toxin-producing Escherichia coli, Corynebacterium diphtheriae, and Clostridium botulinum depend on a specific prophage-encoded toxin for causing a specific disease, whereas Staphylococcus aureus, Streptococcus pyogenes, and Salmonella enterica serovar Typhimurium harbor a multitude of prophages and each phage-encoded virulence or fitness factor makes an incremental contribution to the fitness of the lysogen. These prophages behave like "swarms" of related prophages. Prophage diversification seems to be fueled by the frequent transfer of phage material by recombination with superinfecting phages, resident prophages, or occasional acquisition of other mobile DNA elements or bacterial chromosomal genes. Prophages also contribute to the diversification of the bacterial genome architecture. In many cases, they actually represent a large fraction of the strain-specific DNA sequences. In addition, they can serve as anchoring points for genome inversions. The current review presents the available genomics and biological data on prophages from bacterial pathogens in an evolutionary framework.     

Filamentous phages linked to virulence of Vibrio cholerae

The pathogenicity of Vibrio cholerae depends upon its production of two key virulence factors: the toxin co-regulated pilus (TCP), a colonization factor, and cholera toxin, an exotoxin. Genes encoding both virulence factors were introduced into V. cholerae by horizontal gene transfer. The toxin genes are contained within the genome of CTXphi, an integrated filamentous phage identified in 1996. In the past few years, it has been shown that CTXphi relies on novel processes for phage DNA integration, replication and secretion. In addition, expression of CTXphi genes--including the toxin genes--and transmission of CTXphi were recently found to be promoted by the antirepressor RstC, which is encoded within RS1, a newly described satellite phage of CTXphi. The genetic island that encodes TCP has also been described as a filamentous phage; however, these sequences are unlike the genome of any previously characterized filamentous phage.     

Gram-positive merA gene in gram-negative oral and urine bacteria

Clinical mercury resistant (Hg(r)) Gram-negative bacteria carrying Gram-positive mercury reductase (merA)-like genes were characterized using DNA-DNA hybridization, PCR and sequencing. A PCR assay was developed which discriminated between the merA genes related to Staphylococcus and those related to the Bacillus/Streptococcus merA genes by the difference in size of the PCR product. DNA sequence analysis correlated with the PCR assay. The merA genes from Acinetobacter junii, Enterobacter cloacae and Escherichia coli were sequenced and shared 98-99% identical nucleotide (nt) and 99.6-100% amino acid identity with the Staphylococcus aureus MerA protein. A fourth merA gene, from Pantoeae agglomerans, was partially sequenced (60%) and had 99% identical nt and 100% amino acid identity with the Streptococcus oralis MerA protein. All the Hg(r) Gram-negative bacteria transferred their Gram-positive merA genes to a Gram-positive Enterococcus faecalis recipient with the resulting transconjugants expressing mercury resistance. These Gram-positive merA genes join Gram-positive tetracycline resistance and Gram-positive macrolide resistance genes in their association with mobile elements which are able to transfer and express in Gram-negative bacteria.     

Reliability of quantitative real-time PCR for bacterial detection in cystic fibrosis airway specimens

The cystic fibrosis (CF) airway microbiome is complex; polymicrobial infections are common, and the presence of fastidious bacteria including anaerobes make culture-based diagnosis challenging. Quantitative real-time PCR (qPCR) offers a culture-independent method for bacterial quantification that may improve diagnosis of CF airway infections; however, the reliability of qPCR applied to CF airway specimens is unknown. We sought to determine the reliability of nine specific bacterial qPCR assays (total bacteria, three typical CF pathogens, and five anaerobes) applied to CF airway specimens. Airway and salivary specimens from clinically stable pediatric CF subjects were collected. Quantitative PCR assay repeatability was determined using triplicate reactions. Split-sample measurements were performed to measure variability introduced by DNA extraction. Results from qPCR were compared to standard microbial culture for Pseudomonas aeruginosa, Staphylococcus aureus, and Haemophilus influenzae, common pathogens in CF. We obtained 84 sputa, 47 oropharyngeal and 27 salivary specimens from 16 pediatric subjects with CF. Quantitative PCR detected bacterial DNA in over 97% of specimens. All qPCR assays were highly reproducible at quantities≥10(2) rRNA gene copies/reaction with coefficient of variation less than 20% for over 99% of samples. There was also excellent agreement between samples processed in duplicate. Anaerobic bacteria were highly prevalent and were detected in mean quantities similar to that of typical CF pathogens. Compared to a composite gold standard, qPCR and culture had variable sensitivities for detection of P. aeruginosa, S. aureus and H. influenzae from CF airway samples. By reliably quantifying fastidious airway bacteria, qPCR may improve our understanding of polymicrobial CF lung infections, progression of lung disease and ultimately improve antimicrobial treatments.     

Application of real-time PCR for simultaneous identification and quantification of larval abalone

A paucity of direct studies of marine invertebrate larval dispersal motivated the development of a high-throughput method for identification and quantification of pinto abalone (Haliotis kamtschatkana) larvae in seawater. DNA extracted from sample retentate provided template to screen for species-specific cytochrome oxidase I (COI) mitochondrial DNA sequence via quantitative PCR (QPCR) technology. Primers and a dual-labeled probe were designed and used to identify and quantify DNA from the target species in blind tests of unknown samples alongside a standard template quantity series. Quantity estimates derived from QPCR standard curves were verified via direct enumeration of larvae using light microscopy. Multiplex reactions containing an internal positive control minimized underestimation of quantity and false negatives via partial or full PCR inhibition, respectively. Planned controlled field release and collection experiments to examine larval dispersion patterns via sampling over short and long postrelease times anticipate similar QPCR assays for other marine invertebrate species to aid investigations of larval dispersal in the marine environment.     

Class 1 integrons potentially predating the association with tn402-like transposition genes are present in a sediment microbial community

Integrons are genetic elements that contribute to lateral gene transfer in bacteria as a consequence of possessing a site-specific recombination system. This system facilitates the spread of genes when they are part of mobile cassettes. Most integrons are contained within chromosomes and are confined to specific bacterial lineages. However, this is not the case for class 1 integrons, which were the first to be identified and are one of the single biggest contributors to multidrug-resistant nosocomial infections, carrying resistance to many antibiotics in diverse pathogens on a global scale. The rapid spread of class 1 integrons in the last 60 years is partly a result of their association with a specific suite of transposition functions, which has facilitated their recruitment by plasmids and other transposons. The widespread use of antibiotics has acted as a positive selection pressure for bacteria, especially pathogens, which harbor class 1 integrons and their associated antibiotic resistance genes. Here, we have isolated bacteria from soil and sediment in the absence of antibiotic selection. Class 1 integrons were recovered from four different bacterial species not known to be human pathogens or commensals. All four integrons lacked the transposition genes previously considered to be a characteristic of this class. At least two of these integrons were located on a chromosome, and none of them possessed antibiotic resistance genes. We conclude that novel class 1 integrons are present in a sediment environment in various bacteria of the beta-proteobacterial class. These data suggest that the dispersal of this class may have begun before the "antibiotic era."