Pathogenic potential of environmental Vibrio cholerae strains carrying genetic variants of the toxin-coregulated pilus pathogenicity island

Dynamics of efflux pumps in antimicrobial resistance, persistence, and community living of Vibrionaceae

The marine bacteria of the Vibrionaceae family are significant from the point of view of their role in the marine geochemical cycle, as well as symbionts and opportunistic pathogens of aquatic animals and humans. The well-known pathogens of this group, Vibrio cholerae, V. parahaemolyticus, and V. vulnificus, are responsible for significant morbidity and mortality associated with a range of infections from gastroenteritis to bacteremia acquired through the consumption of raw or undercooked seafood and exposure to seawater containing these pathogens. Although generally regarded as susceptible to commonly employed antibiotics, the antimicrobial resistance of Vibrio spp. has been on the rise in the last two decades, which has raised concern about future infections by these bacteria becoming increasingly challenging to treat. Diverse mechanisms of antimicrobial resistance have been discovered in pathogenic vibrios, the most important being the membrane efflux pumps, which contribute to antimicrobial resistance and their virulence, environmental fitness, and persistence through biofilm formation and quorum sensing. In this review, we discuss the evolution of antimicrobial resistance in pathogenic vibrios and some of the well-characterized efflux pumps' contributions to the physiology of antimicrobial resistance, host and environment survival, and their pathogenicity.

Occurrence of virulence determinants in vibrio cholerae, vibrio mimicus, vibrio alginolyticus, and vibrio parahaemolyticus isolates from important water resources of Eastern Cape, South Africa

BACKGROUND

Virulence determinants are crucial to the risk assessment of pathogens in an environment. This study investigated the presence of eleven key virulence-associated genes in Vibrio cholerae (n = 111) and Vibrio mimicus (n = 22) and eight virulence determinants in Vibrio alginolyticus (n = 65) and Vibrio parahaemolyticus (n = 17) isolated from six important water resources in Eastern Cape, South Africa, using PCR techniques. The multiple virulence gene indexes (MVGI) for sampling sites and isolates as well as hotspots for potential vibriosis outbreaks among sampling sites were determined statistically based on the comparison of MVGI.

RESULT

The PCR assay showed that all the V. cholerae isolates belong to non-O1/non-O139 serogroups. Of the isolates, Vibrio Cholera (84%), V. mimicus (73%), V. alginolyticus (91%) and V. parahaemolyticus (100%) isolates harboured at least one of the virulence-associated genes investigated. The virulence gene combinations detected in isolates varied at sampling site and across sites. Typical virulence-associated determinants of V. cholerae were detected in V. mimicus while that of V. parahaemolyticus were detected in V. alginolyticus. The isolates with the highest MVGI were recovered from three estuaries (Sunday river, Swartkopps river, buffalo river) and a freshwater resource (Lashinton river). The cumulative MVGI for V. cholerae, V. mimicus, V. alginolyticus and V. parahaemolyticus isolates were 0.34, 0.20, 0.45, and 0.40 respectively. The targeted Vibrio spp. in increasing order of the public health risk posed in our study areas based on the MVGI is V. alginolyticus > V. parahaemolyticus > V. cholerae > V. mimicus. Five (sites SR, PA5, PA6, EL4 and EL6) out of the seventeen sampling sites were detected as the hotspots for potential cholera-like infection and vibriosis outbreaks.

CONCLUSIONS

Our findings suggest that humans having contact with water resources in our study areas are exposed to potential public health risks owing to the detection of virulent determinants in human pathogenic Vibrio spp. recovered from the water resources. The study affirms the relevancy of environmental Vibrio species to the epidemiology of vibriosis, cholera and cholera-like infections. Hence we suggest a monitoring program for human pathogenic Vibrio spp. in the environment most especially surface water that humans have contact with regularly.

Vibrio Phage VMJ710 Can Prevent and Treat Disease Caused by Pathogenic MDR V. cholerae O1 in an Infant Mouse Model

Cholera, a disease of antiquity, is still festering in developing countries that lack safe drinking water and sewage disposal. Vibrio cholerae, the causative agent of cholera, has developed multi-drug resistance to many antimicrobial agents. In aquatic habitats, phages are known to influence the occurrence and dispersion of pathogenic V. cholerae. We isolated Vibrio phage VMJ710 from a community sewage water sample of Manimajra, Chandigarh, in 2015 during an outbreak of cholera. It lysed 46% of multidrug-resistant V. cholerae O1 strains. It had significantly reduced the bacterial density within the first 4-6 h of treatment at the three multiplicity of infection (MOI 0.01, 0.1, and 1.0) values used. No bacterial resistance was observed against phage VMJ710 for 20 h in the time-kill assay. It is nearest to an ICP1 phage, i.e., Vibrio phage ICP1_2012 (MH310936.1), belonging to the class Caudoviricetes. ICP1 phages have been the dominant bacteriophages found in cholera patients' stools since 2001. Comparative genome analysis of phage VMJ710 and related phages indicated a high level of genetic conservation. The phage was stable over a wide range of temperatures and pH, which will be an advantage for applications in different environmental settings. The phage VMJ710 showed a reduction in biofilm mass growth, bacterial dispersal, and a clear disruption of bacterial biofilm structure. We further tested the phage VMJ710 for its potential therapeutic and prophylactic properties using infant BALB/c mice. Bacterial counts were reduced significantly when phages were administered before and after the challenge of orogastric inoculation with V. cholerae serotype O1. A comprehensive whole genome study revealed no indication of lysogenic genes, genes associated with possible virulence factors, or antibiotic resistance. Based on all these properties, phage VMJ710 can be a suitable candidate for oral phage administration and could be a viable method of combatting cholera infection caused by MDR V. cholerae pathogenic strains.

Phage for treatment of Vibrio cholerae infection

Bacteriophages (or "phages") are ubiquitous and the amplest biological entities on our planet. It is a natural enemy of bacteria. Cholera is one of the most known diseases to cause multiple pandemics around the world, killing millions of people. The pathogen of cholera is Vibrio species. Up until the emergence of multidrug resistance, preventive therapeutics like antibiotics were the most effective means of battling bacteria. Globally, one of the most significant challenges in treating microbial infections is the development of drug-resistant strains. Based on their antibacterial properties and unique characteristics, phages are being comprehensively evaluated taxonomically. Moreover, phage-based vaccination is evolving as one of the most encouraging preventive approaches. Due to this, its related research got remarkable recognition. However, due to the rapid emergence of bacterial resistance to antibiotics, the use of phages (phage therapy) could be a major motive for research because the most promising solution lies in bacteriophages. This chapter briefly highlights the promising use of bacteriophages to combat Vibrio-related infectious diseases.

Putative antigenic proteins identified by comparative and subtractive reverse vaccinology in necrotic enteritis-causing Clostridium perfringens isolated from broiler chickens

BACKGROUND

Avian necrotic enteritis (NE) caused by Clostridium perfringens is a disease with a major economic impact, generating losses estimated to 6 billion of dollars annually for the poultry industry worldwide. The incidence of the disease is particularly on the rise in broiler chicken flocks eliminating the preventive use of antibiotics. To date, no alternative allows for the efficient prevention of NE and a control of the disease using a vaccinal strategy would be mostly prized. For this purpose, comparative and subtractive reverse vaccinology identifying putative immunogenic bacterial surface proteins is one of the most promising approaches.

RESULTS

A comparative genomic study was performed on 16 C. perfringens strains isolated from healthy broiler chickens and from broilers affected with necrotic enteritis. Results showed that the analyzed genomes were composed of 155,700 distinct proteins from which 13% were identified as extracellular, 65% as cytoplasmic and 22% as part of the bacterial membrane. The evaluation of the immunogenicity of these proteins was determined using the prediction software VaxiJen®.

CONCLUSIONS

For the most part, proteins with the highest scores were associated with an extracellular localisation. For all the proteins analyzed, the combination of both the immunogenicity score and the localisation prediction led to the selection of 12 candidate proteins that were mostly annotated as hypothetical proteins. We describe 6 potential candidates of higher interest due to their antigenic potential, their extracellular localisation, and their possible role in virulence of C. perfringens.

DksA coordinates bile-mediated regulation of virulence-associated phenotypes in type three secretion system-positive Vibrio cholerae

In order to cause disease, pathogenic strains of Vibrio cholerae rely on intricate regulatory networks to orchestrate the transition between their native aquatic environment and the human host. For example, bacteria in a nutrient-starved environment undergo a metabolic shift called the stringent response, which is mediated by the alarmone ppGpp and an RNA-polymerase binding transcriptional factor, DksA. In O1 serogroup strains of V. cholerae, which use the toxin co-regulated pilus (TCP) and cholera toxin (CT) as primary virulence factors, DksA was reported to have additional functions as a mediator of virulence gene expression. However, little is known about the regulatory networks coordinating virulence phenotypes in pathogenic strains that use TCP/CT-independent virulence mechanisms. We therefore investigated whether functions of DksA outside of the stringent response are conserved in type three secretion system (T3SS)-positive V. cholerae. In using the T3SS-positive clinically isolated O39 serogroup strain AM-19226, we observed an increase in dksA expression in the presence of bile at 37 °C. However, DksA was not required for wild-type levels of T3SS structural gene expression, or for colonization in vivo. Rather, data indicate that DksA positively regulates the expression of master regulators in the motility hierarchy. Interestingly, the ΔdksA strain forms a less robust biofilm than the WT parent strain at both 30 and 37 °C. We also found that DksA regulates the expression of hapR, encoding a major regulator of biofilm formation and protease expression. Athough DksA does not appear to modulate T3SS virulence factor expression, its activity is integrated into existing regulatory networks governing virulence-related phenotypes. Strain variations therefore may take advantage of conserved ancestral proteins to expand regulons responding to in vivo signals and thus coordinate multiple phenotypes important for infection.

Interbacterial competition and anti-predatory behaviour of environmental Vibrio cholerae strains

Vibrio cholerae isolates responsible for cholera pandemics represent only a small portion of the diverse strains belonging to this species. Indeed, most V. cholerae are encountered in aquatic environments. To better understand the emergence of pandemic lineages, it is crucial to discern what differentiates pandemic strains from their environmental relatives. Here, we studied the interaction of environmental V. cholerae with eukaryotic predators or competing bacteria and tested the contributions of the haemolysin and the type VI secretion system (T6SS) to those interactions. Both of these molecular weapons are constitutively active in environmental isolates but subject to tight regulation in the pandemic clade. We showed that several environmental isolates resist amoebal grazing and that this anti‐grazing defense relies on the strains' T6SS and its actincross‐linking domain (ACD)‐containing tip protein. Strains lacking the ACD were unable to defend themselves against grazing amoebae but maintained high levels of T6SS‐dependent interbacterial killing. We explored the latter phenotype through whole‐genome sequencing of 14 isolates, which unveiled a wide array of novel T6SS effector and (orphan) immunity proteins. By combining these in silico predictions with experimental validations, we showed that highly similar but non‐identical immunity proteins were insufficient to provide cross‐immunity among those wild strains.

Structural basis for virulence regulation in Vibrio cholerae by unsaturated fatty acid components of bile

The AraC/XylS-family transcriptional regulator ToxT is the master virulence activator of Vibrio cholerae, the gram-negative bacterial pathogen that causes the diarrheal disease cholera. Unsaturated fatty acids (UFAs) found in bile inhibit the activity of ToxT. Crystal structures of inhibited ToxT bound to UFA or synthetic inhibitors have been reported, but no structure of ToxT in an active conformation had been determined. Here we present the 2.5 Å structure of ToxT without an inhibitor. The structure suggests release of UFA or inhibitor leads to an increase in flexibility, allowing ToxT to adopt an active conformation that is able to dimerize and bind DNA. Small-angle X-ray scattering was used to validate a structural model of an open ToxT dimer bound to the cholera toxin promoter. The results presented here provide a detailed structural mechanism for virulence gene regulation in V. cholerae by the UFA components of bile and other synthetic ToxT inhibitors.

Preliminary Work Towards Finding Proteins as Potential Vaccine Candidates for Vibrio cholerae Pakistani Isolates through Reverse Vaccinology

Background and Objective: Vibrio cholerae continues to emerge as a dangerous pathogen because of increasing resistance to a number of antibiotics. This paper provides a solution to emerging antibiotic resistance by introducing novel proteins as vaccine candidates against cholera. Materials and Methods: Vibrio cholerae genome versatility is a hurdle for developing a vaccine to combat diarrhoeal infection, so its core gene information was used to determine a potential vaccine candidate. Whole genome sequence data of more than 100 Vibrio cholerae strains were used simultaneously to get core genome information. The VacSol pipeline based on reverse vaccinology was selected to address the problem of safe, cheap, temperature-stable, and effective vaccine candidates which can be used for vaccine development against Vibrio cholerae. VacSol screens vaccine candidates using integrated, well-known, and robust algorithms/tools for proteome analysis. The proteomes of the pathogens were initially screened to predict homology using BLASTp. Proteomes that are non-homologous to humans are then subjected to a predictor for localization. Helicer predicts transmembrane helices for the protein. Proteins failing to comply with the set parameters were filtered at each step, and finally, 11 proteins were filtered as vaccine candidates. Results: This selected group of vaccine candidates consists of proteins from almost all structural parts of Vibrio cholerae. Their blast results show that this filtered group includes flagellin A protein, a protein from the Zn transporter system, a lipocarrier outer membrane protein, a peptidoglycan-associated protein, a DNA-binding protein, a chemotaxis protein, a tRNA Pseuriudine synthase A, and two selected proteins, which were beta lactamases. The last two uncharacterized proteins possess 100% similarity to V. albensis and Enterobacter, respectively. Tertiary structure and active site determination show a large number of pockets on each protein. Conclusions: The most interesting finding of this study is that 10 proteins out of 11 filtered proteins are introduced as novel potential vaccine candidates. These novel vaccine candidates can result in the development of cost-effective and broad-spectrum vaccines which can be used in countries where cholera is a major contributor to diarrheal disease.

Haitian Variant Vibrio cholerae O1 Strains Manifest Higher Virulence in Animal Models

Vibrio cholerae causes fatal diarrheal disease cholera in humans due to consumption of contaminated water and food. To instigate the disease, the bacterium must evade the host intestinal innate immune system; penetrate the mucus layer of the small intestine, adhere and multiply on the surface of microvilli and produce toxin(s) through the action of virulence associated genes. V. cholerae O1 that has caused a major cholera outbreak in Haiti contained several unique genetic signatures. These novel traits are used to differentiate them from the canonical El Tor strains. Several studies reported the spread of these Haitian variant strains in different parts of the world including Asia and Africa, but there is a paucity of information on the clinical consequence of these genetic changes. To understand the impact of these changes, we undertook a study involving mice and rabbit models to evaluate the pathogenesis. The colonization ability of Haitian variant strain in comparison to canonical El Tor strain was found to be significantly more in both suckling mice and rabbit model. Adult mice also displayed the same results. Besides that, infection patterns of Haitian variant strains showed a completely different picture. Increased mucosal damaging, colonization, and inflammatory changes were observed through hematoxylin-eosin staining and transmission electron microscopy. Fluid accumulation ability was also significantly higher in rabbit model. Our study indicated that these virulence features of the Haitian variant strain may have some association with the severe clinical outcome of the cholera patients in different parts of the world.

Virulence gene profiles, biofilm formation, and antimicrobial resistance of Vibrio cholerae non-O1/non-O139 bacteria isolated from West Bengal, India

Vibrio cholerae is the causative agent of acute dehydrating diarrhoeal disease cholera. Among 71 V. cholerae non-O1/non-O139 isolates, all yielded negative results for ctxA, ctxB and tcpA genes in PCR assay. Few strains were positive for stn (28.38%), and ompU (31.08%) genes. While all isolates were negative for ace gene, only two were positive for zot gene. All strains expressed toxR and toxT genes. It was also found that all isolates were slime-producer and these were capable of forming moderate to high biofilm. Biofilm formation was controlled positively by the transcriptional regulators VpsR and VpsT and was regulated negatively by HapR, as well as CRP regulatory complex. These isolates were resistant to ampicillin, furazolidone, doxycycline, vancomycin, erythromycin, while these were susceptible to ciprofloxacin, gentamycin, kanamycin, polymixin B, norfloxacin, chloramphenicol, sulphamethoxazole-trimethoprim, tetracycline, nalidixic acid, and streptomycin. Indeed, 69.01% isolates were resistant to multiple antibiotics (MAR: resistance to 3 or more antibiotics). Treatment protocols for cholera patients should be based on local antibiogram data.

Improvement and evaluation of loop-mediated isothermal amplification combined with a chromatographic flow dipstick assay and utilization in detection of Vibrio cholerae

Loop-mediated isothermal amplification (LAMP) is a specific, sensitive, and easy-to-perform nucleic acid analytical technique with wide application for diagnosis of disease. Recently, LAMP combined with use of a lateral chromatographic flow dipstick (LFD) has been widely used in nucleic acid detection. However, the LFD mechanism has not been systematically analyzed, and the optimal combination of labeled primers has not been adequately evaluated. We analyzed the LAMP mechanism and discovered that the labeled loop primers played a significant role in the LFD assay. To verify our hypothesis, we developed two LFD assays for Vibrio cholerae to detect the ctxA gene and the 16S-23S ribosomal DNA internal transcribed spacer (ITS). We labeled the inner primers [forward inner primer (FIP) and backward inner primer (BIP)] and loop primers [forward loop primer (LF) and backward loop primer (LB)]. Then the labeled and unlabeled primers were combined to form ten different primer sets. We assessed the specificity, sensitivity, and efficiency of LFD assays with use of different primer compositions. All triple-labeled primer sets resulted in false positive results in the LFD assay, as did the FIP and BIP double-labeled primer set. Other double-labeled-primer sets used in LFD assays showed higher sensitivity than the LAMP assays. Moreover, FIP and LF double-labeled and BIP and LB double-labeled sets had the highest sensitivity. In both cases, assays could be performed in 20 min. We also applied the ITS LFD assays in food samples. The enrichment broths of 112 oyster samples were tested, and the proportion that tested positive by the LFD assays was 6.25%, which was not lower than the rate for the conventional PCR method (5.36%). Graphical abstract ᅟ.

Galleria mellonella is low cost and suitable surrogate host for studying virulence of human pathogenic Vibrio cholerae

Vibrio cholerae causes a severe diarrheal disease affecting millions of people worldwide, particularly in low income countries. V. cholerae successfully persist in aquatic environment and its pathogenic strains results in sever enteric disease in humans. This dual life style contributes towards its better survival and persistence inside host gut and in the environment. Alternative animal replacement models are of great value in studying host-pathogen interaction and for quick screening of various pathogenic strains. One such model is Galleria mellonella, a wax moth which has a complex innate immune system and here we investigate its suitability as a model for clinical human isolates of O1 El TOR, Ogawa serotype belonging to two genetically distinct subclades found in Pakistan (PSC-1 and PSC-2). We demonstrate that the PSC-2 strain D59 frequently isolated from inland areas, was more virulent than PSC-1 strain K7 mainly isolated from coastal areas (p=0.0001). In addition, we compared the relative biofilm capability of the representative strains as indicators of their survival and persistence in the environment and K7 showed enhanced biofilm forming capabilities (p=0.004). Finally we present the annotated genomes of the strains D59 and K7, and compared them with the reference strain N16961.

Role of integrons, plasmids and SXT elements in multidrug resistance of Vibrio cholerae and Providencia vermicola obtained from a clinical isolate of diarrhea

The isolates of Vibrio cholerae and Providencia vermicola obtained from a diarrheal patient were investigated for genetic elements governing their drug resistance phenotypes. Out of 14 antibiotics tested, V. cholerae Vc IDH02365 isolate showed resistance to nine antibiotics, while P. vermicola Pv NBA2365 was found to be resistant to all the antibiotics except polymyxin B. Though SXT integrase was depicted in both the bacteria, class 1 integron was found to be associated only with Pv NBA2365. Integrons in Pv NBA2365 conferred resistance to β-lactams, aminoglycosides, and trimethoprim. Pv NBA2365 carried two transformable plasmids imparting distinct antibiotic resistance traits to their Escherichia coli transformants. In rabbit ileal loop assays, Pv NBA2365 did not show any fluid accumulation (FA) in contrast with Vc IDH02365 that showed high FA. To the best of our knowledge, this is the first report of a highly drug resistant P. vermicola and additionally co-existence of multidrug resistant V. cholerae and P. vermicola. Both the microbes appeared to possess a wide array of mobile genetic elements for a large spectrum of antimicrobial agents, some of which are being used in the treatment of acute diarrhea.

High prevalence and diversity of pre-CTXΦ alleles in the environmental Vibrio cholerae O1 and O139 strains in the Zhujiang River estuary

Toxigenic conversion of environmental Vibrio cholerae strains through lysogenic infection by the phage CTXΦ is an important step in the emergence of new pathogenic clones. The precursor form of the CTXΦ phage, pre-CTXΦ, does not carry the cholera toxin gene. During our investigation, we frequently found pre-CTXΦ prophages in non-toxigenic isolates in the serogroups of O1 and O139 strains in the Zhujiang estuary. We observed high amounts of sequence variation of rstR and gIII(CTX) in the pre-CTXΦ alleles as well as in the tcpA sequences within the strains. In addition, a new pre-CTXΦ allele, with a novel rstR sequence type and hybrid RS2, was identified. Our findings show that active, complicated gene recombination and horizontal transfer of pre-CTXΦs occurs within V. cholerae environmental strains, which creates a complex intermediate pool for the generation of toxigenic clones in the estuarine environment.

Synergistic effect of various virulence factors leading to high toxicity of environmental V. cholerae non-O1/ non-O139 isolates lacking ctx gene : comparative study with clinical strains

Background

Vibrio cholerae non-O1/ non-O139 serogroups have been reported to cause sporadic diarrhoea in humans. Cholera toxins have been mostly implicated for hypersecretion of ions and water into the small intestine. Though most of the V. cholerae non-O1/ non-O139 strains lack these cholera toxins, several other innate virulence factors contribute towards their pathogenicity. The environmental isolates may thus act as reservoirs for potential spreading of these virulence genes in the natural environment which may cause the emergence of epidemic-causing organisms.

Results

The environmental isolates of vibrios were obtained from water samples, zooplanktons and phytoplanktons, from a village pond in Gandhinagar, Gujarat, India. They were confirmed as Vibrio cholerae non-O1/ non-O139 using standard biochemical and serotyping tests. PCR experiments revealed that the isolates lacked ctxA, ctxB, tcpA, zot and ace genes whereas other pathogenicity genes like toxR, rtxC, hlyA, hapA and prtV were detected in these isolates. Compared with epidemic strain V. cholerae O1 El Tor N16961, culture supernatants from most of these isolates caused higher cytotoxicity to HT29 cells and higher hemolytic, hemagglutinin and protease activities. In rabbit ileal loop assays, the environmental isolates showed only 2-4 folds lesser fluid accumulation in comparison to N16961 and a V. cholerae clinical isolate IDH02365 of 2009. Pulsed Field Gel electrophoresis and Random amplification of Polymorphic DNA indicated that these isolates showed considerable diversity and did not share the same clonal lineage even though they were derived from the same water source. All the isolates showed resistance to one or more antibiotics.

Conclusion

Though these environmental isolates lacked the cholera toxins, they seem to have adopted other survival strategies by optimally utilising a diverse array of several other toxins. The current findings indicate the possibility that these isolates could cause some gastroenteric inflammation when ingested and may serve as progenitors for overt disease-causing organisms.

Population structure and evolution of non-O1/non-O139 Vibrio cholerae by multilocus sequence typing

Pathogenic non-O1/non-O139 Vibrio cholerae strains can cause sporadic outbreaks of cholera worldwide. In this study, multilocus sequence typing (MLST) of seven housekeeping genes was applied to 55 non-O1/non-O139 isolates from clinical and environmental sources. Data from five published O1 isolates and 17 genomes were also included, giving a total of 77 isolates available for analysis. There were 66 sequence types (STs), with the majority being unique, and only three clonal complexes. The V. cholerae strains can be divided into four subpopulations with evidence of recombination among the subpopulations. Subpopulations I and III contained predominantly clinical strains. PCR screening for virulence factors including Vibrio pathogenicity island (VPI), cholera toxin prophage (CTXΦ), type III secretion system (T3SS), and enterotoxin genes (rtxA and sto/stn) showed that combinations of these factors were present in the clinical isolates with 85.7% having rtxA, 51.4% T3SS, 31.4% VPI, 31.4% sto/stn (NAG-ST) and 11.4% CTXΦ. These factors were also present in environmental isolates but at a lower frequency. Five strains previously mis-identified as V. cholerae serogroups O114 to O117 were also analysed and formed a separate population with V. mimicus. The MLST scheme developed in this study provides a framework to identify sporadic cholera isolates by genetic identity.

Fitness factors in vibrios: a mini-review

Vibrios are Gram-negative curved bacilli that occur naturally in marine, estuarine, and freshwater systems. Some species include human and animal pathogens, and some vibrios are necessary for natural systems, including the carbon cycle and osmoregulation. Countless in vivo and in vitro studies have examined the interactions between vibrios and their environment, including molecules, cells, whole animals, and abiotic substrates. Many studies have characterized virulence factors, attachment factors, regulatory factors, and antimicrobial resistance factors, and most of these factors impact the organism's fitness regardless of its external environment. This review aims to identify common attributes among factors that increase fitness in various environments, regardless of whether the environment is an oyster, a rabbit, a flask of immortalized mammalian cells, or a planktonic chitin particle. This review aims to summarize findings published thus far to encapsulate some of the basic similarities among the many vibrio fitness factors and how they frame our understanding of vibrio ecology. Factors representing these similarities include hemolysins, capsular polysaccharides, flagella, proteases, attachment factors, type III secretion systems, chitin binding proteins, iron acquisition systems, and colonization factors.

Generalized reproduction numbers and the prediction of patterns in waterborne disease

Understanding, predicting, and controlling outbreaks of waterborne diseases are crucial goals of public health policies, but pose challenging problems because infection patterns are influenced by spatial structure and temporal asynchrony. Although explicit spatial modeling is made possible by widespread data mapping of hydrology, transportation infrastructure, population distribution, and sanitation, the precise condition under which a waterborne disease epidemic can start in a spatially explicit setting is still lacking. Here we show that the requirement that all the local reproduction numbers R0 be larger than unity is neither necessary nor sufficient for outbreaks to occur when local settlements are connected by networks of primary and secondary infection mechanisms. To determine onset conditions, we derive general analytical expressions for a reproduction matrix G0, explicitly accounting for spatial distributions of human settlements and pathogen transmission via hydrological and human mobility networks. At disease onset, a generalized reproduction number Λ0 (the dominant eigenvalue of G0) must be larger than unity. We also show that geographical outbreak patterns in complex environments are linked to the dominant eigenvector and to spectral properties of G0. Tests against data and computations for the 2010 Haiti and 2000 KwaZulu-Natal cholera outbreaks, as well as against computations for metapopulation networks, demonstrate that eigenvectors of G0 provide a synthetic and effective tool for predicting the disease course in space and time. Networked connectivity models, describing the interplay between hydrology, epidemiology, and social behavior sustaining human mobility, thus prove to be key tools for emergency management of waterborne infections.

Sequence analyses of type IV pili from Vibrio cholerae, Vibrio parahaemolyticus, and Vibrio vulnificus

Bacterial surface structures called pili have been studied extensively for their role as possible colonization factors. Most sequenced Vibrio genomes predict a variety of pili genes in these organisms, including several types of type IV pili. In particular, the mannose-sensitive hemagglutinin (MSHA) and the PilA pili, also known as the chitin-regulated pilus (ChiRP), are type IVa pili commonly found in Vibrio genomes and have been shown to play a role in the colonization of Vibrio species in the environment and/or host tissue. Here, we report sequence comparisons of two type IVa pilin subunit genes, mshA and pilA, and their corresponding amino acid sequences, for several strains from the three main human pathogenic Vibrio species, V. cholerae, V. parahaemolyticus, and V. vulnificus. We identified specific groupings of these two genes in V. cholerae, whereas V. parahaemolyticus and V. vulnificus strains had no apparent allelic clusters, and these genes were strikingly divergent. These results were compared with other genes from the MSHA and PilA operons as well as another Vibrio pili from the type IVb group, the toxin co-regulated pilus (TCP) from V. cholerae. Our data suggest that a selective pressure exists to cause these strains to vary their MSHA and PilA pilin subunits. Interestingly, V. cholerae strains possessing TCP have the same allele for both mshA and pilA. In contrast, V. cholerae isolates without TCP have polymorphisms in their mshA and pilA sequences similar to what was observed for both V. parahaemolyticus and V. vulnificus. This data suggests a possible linkage between host interactions and maintaining a highly conserved type IV pili sequence in V. cholerae. Although the mechanism underlying this intriguing diversity has yet to be elucidated, our analyses are an important first step towards gaining insights into the various aspects of Vibrio ecology.

Genome sequencing reveals unique mutations in characteristic metabolic pathways and the transfer of virulence genes between V. mimicus and V. cholerae

Vibrio mimicus, the species most similar to V. cholerae, is a microbe present in the natural environmental and sometimes causes diarrhea and internal infections in humans. It shows similar phenotypes to V. cholerae but differs in some biochemical characteristics. The molecular mechanisms underlying the differences in biochemical metabolism between V. mimicus and V. cholerae are currently unclear. Several V. mimicus isolates have been found that carry cholera toxin genes (ctxAB) and cause cholera-like diarrhea in humans. Here, the genome of the V. mimicus isolate SX-4, which carries an intact CTX element, was sequenced and annotated. Analysis of its genome, together with those of other Vibrio species, revealed extensive differences within the Vibrionaceae. Common mutations in gene clusters involved in three biochemical metabolism pathways that are used for discrimination between V. mimicus and V. cholerae were found in V. mimicus strains. We also constructed detailed genomic structures and evolution maps for the general types of genomic drift associated with pathogenic characters in polysaccharides, CTX elements and toxin co-regulated pilus (TCP) gene clusters. Overall, the whole-genome sequencing of the V. mimicus strain carrying the cholera toxin gene provides detailed information for understanding genomic differences among Vibrio spp. V. mimicus has a large number of diverse gene and nucleotide differences from its nearest neighbor, V. cholerae. The observed mutations in the characteristic metabolism pathways may indicate different adaptations to different niches for these species and may be caused by ancient events in evolution before the divergence of V. cholerae and V. mimicus. Horizontal transfers of virulence-related genes from an uncommon clone of V. cholerae, rather than the seventh pandemic strains, have generated the pathogenic V. mimicus strain carrying cholera toxin genes.

Crystal structure of an integron gene cassette-associated protein from Vibrio cholerae identifies a cationic drug-binding module

BACKGROUND

The direct isolation of integron gene cassettes from cultivated and environmental microbial sources allows an assessment of the impact of the integron/gene cassette system on the emergence of new phenotypes, such as drug resistance or virulence. A structural approach is being exploited to investigate the modularity and function of novel integron gene cassettes.

METHODOLOGY/PRINCIPAL FINDINGS

We report the 1.8 Å crystal structure of Cass2, an integron-associated protein derived from an environmental V. cholerae. The structure defines a monomeric beta-barrel protein with a fold related to the effector-binding portion of AraC/XylS transcription activators. The closest homologs of Cass2 are multi-drug binding proteins, such as BmrR. Consistent with this, a binding pocket made up of hydrophobic residues and a single glutamate side chain is evident in Cass2, occupied in the crystal form by polyethylene glycol. Fluorescence assays demonstrate that Cass2 is capable of binding cationic drug compounds with submicromolar affinity. The Cass2 module possesses a protein interaction surface proximal to its drug-binding cavity with features homologous to those seen in multi-domain transcriptional regulators.

CONCLUSIONS/SIGNIFICANCE

Genetic analysis identifies Cass2 to be representative of a larger family of independent effector-binding proteins associated with lateral gene transfer within Vibrio and closely-related species. We propose that the Cass2 family not only has capacity to form functional transcription regulator complexes, but represents possible evolutionary precursors to multi-domain regulators associated with cationic drug compounds.

Functional analysis of VopF activity required for colonization in Vibrio cholerae

Vibrio cholerae, a Gram-negative facultative pathogen, is the etiologic agent for the diarrheal disease cholera. We previously characterized a clinical isolate, AM-19226, that translocates a type III secretion system (T3SS) effector protein with actin-nucleating activity, VopF, into the host cells. From comparative genomic studies, we identified a divergent T3SS island in additional isolates which possess a VopF homolog, VopN. Unlike the VopF-mediated protrusion formation, VopN localizes to stress fiber in host cells similarly to VopL, which is present in the pandemic strain of Vibrio parahaemolyticus. Chimera and yeast two-hybrid studies indicated that the amino-terminal regions of VopF and VopN proteins interact with distinct host cell factors. We determined that AM-19226-infected cells are arrested at S phase of the cell cycle and that VopF/VopN are antiapoptotic factors. To understand how VopF may contribute to the pathogenesis of AM-19226, we examined the effect of VopF in an in vitro polarized-epithelial model and an in vivo adult rabbit diarrheal model. Within the T3SS pathogenicity island is VopE, a homolog of YopE from Yersinia, which has been shown to loosen tight junctions. In polarized intestinal epithelia, VopF and VopE compromised the integrity of tight junctions by inducing cortical actin depolymerization and aberrant localization of the tight-junction protein ZO-1. An assay for pathogenicity in the adult rabbit diarrhea model suggested that these effectors are involved in eliciting the diarrheal response in infected rabbits.

Vibrio cholerae is a bacterial pathogen that causes the diarrheal disease cholera, which remains a major public health problem in many developing countries. While the major virulence factors of the pandemic V. cholerae strains have been characterized, new clinical strains of V. cholerae have arisen, causing sporadic cholera-like diseases using unknown pathogenic mechanisms. Previously, we discovered the type III secretion system in a new clinical strain of V. cholerae and also identified an effector protein, VopF, which is injected into the host cells and induces changes in the actin cytoskeleton. In this work, we identified a homolog of VopF that causes a distinct cellular phenotype and interactions between the effectors and host proteins. We also discovered that both effectors prevent bacterium-induced cell death in infected cells. In our tissue culture and animal models, we showed that VopF contributes to the disruption of epithelial integrity and the diarrheal response.

Characterization of potentially virulent non-O1/non-O139 Vibrio cholerae strains isolated from human patients

Traditional methods of typing Vibrio cholerae define virulent strains according to their recognition by sera directed against the known epidemic serogroups O1 and O139, overlooking potentially virulent non-O1/non-O139 strains. Here, we have undertaken the characterization of eight clinical isolates of non-O1/non-O139 V. cholerae, collected during cholera outbreaks in Brazil. Seven of these were typed as O26 and one, 17155, was defined as non-typable. A PCR-based approach has previously detected in these strains several virulence genes derived from the CTXvarphi prophage and generally associated with pathogenic strains. Here, the presence of the O1-specific wbeN gene was investigated through PCR and found to be restricted to strain 17155, as well as one of the O26 strains, 4756, although neither strain was recognized by O1-specific antisera. The same two isolates were the only strains able to express the cholera toxin in culture, assayed by western blotting. They also possessed four repeats of the heptanucleotide TTTTGAT upstream of the ctxAB genes encoding the cholera toxin. The remaining strains possessed only two intact repeats, whereas pathogenic O1 possessed four to six repeats. To define their evolutionary relationships, selected 16S-23S intergenic rRNA spacer regions were sequenced from the various strains and the resulting sequences used to build phylogenetic trees. Strains 4756 and 17155 always clustered with control O1 strains, whereas the remaining O26 strains clustered separately. These results confirm that, despite their serological phenotype, these two strains are genotypically related to O1 strains and potentially able to produce epidemic cholera.

Rapid detection of virulence-associated genes in environmental strains of Vibrio cholerae by multiplex PCR

Vibrio cholerae, the causative agent of cholera is ubiquitously distributed in aquatic environment particularly in coastal waters, estuaries, and rivers. In the present investigation, a multiplex PCR assay was developed for the detection of virulence-associated genes (rtxA, tcpA, ctxA, hlyA, and sto) in environmental isolates of V. cholerae. A total of 90 strains isolated from different environmental sources were screened for the presence of virulence-associated genes. Our results showed that this method represents a simple, cost effective, and robust tool for rapid detection of virulence-associated genes. This multiplex PCR can be used for examining prevalence of virulence-associated genes and hence will be useful for better understanding of epidemiology of environmental V. cholerae.

Genetic diversity of toxigenic and nontoxigenic Vibrio cholerae serogroups O1 and O139 revealed by array-based comparative genomic hybridization

Toxigenic serogroups O1 and O139 of Vibrio cholerae may cause cholera epidemics or pandemics. Nontoxigenic strains within these serogroups also exist in the environment, and also some may cause sporadic cases of disease. Herein, we investigate the genomic diversity among toxigenic and nontoxigenic O1 and O139 strains by comparative genomic microarray hybridization with the genome of El Tor strain N16961 as a base. Conservation of the toxigenic O1 El Tor and O139 strains is found as previously reported, whereas accumulation of genome changes was documented in toxigenic El Tor strains isolated within the 40 years of the seventh pandemic. High phylogenetic diversity in nontoxigenic O1 and O139 strains is observed, and most of the genes absent from nontoxigenic strains are clustered together in the N16961 genome. By comparing these toxigenic and nontoxigenic strains, we observed that the small chromosome of V. cholerae is quite conservative and stable, outside of the superintegron region. In contrast to the general stability of the genome, the superintegron demonstrates pronounced divergence among toxigenic and nontoxigenic strains. Additionally, sequence variation in virulence-related genes is found in nontoxigenic El Tor strains, and we speculate that these intermediate strains may have pathogenic potential should they acquire CTX prophage alleles and other gene clusters. This genome-wide comparison of toxigenic and nontoxigenic V. cholerae strains may promote understanding of clonal differentiation of V. cholerae and contribute to an understanding of the origins and clonal selection of epidemic strains.

Molecular-beacon multiplex real-time PCR assay for detection of Vibrio cholerae

A multiplex real-time PCR assay was developed using molecular beacons for the detection of Vibrio cholerae by targeting four important virulence and regulatory genes. The specificity and sensitivity of this assay, when tested with pure culture and spiked environmental water samples, were high, surpassing those of currently published PCR assays for the detection of this organism.

Identification and characterization of an antigen I/II family protein produced by group A Streptococcus

Group A Streptococcus (GAS) is a gram-positive human bacterial pathogen that causes infections ranging in severity from pharyngitis to life-threatening invasive disease, such as necrotizing fasciitis. Serotype M28 strains are consistently isolated from invasive infections, particularly puerperal sepsis, a severe infection that occurs during or after childbirth. We recently sequenced the genome of a serotype M28 GAS strain and discovered a novel 37.4-kb foreign genetic element designated region of difference 2 (RD2). RD2 is similar in gene content and organization to genomic islands found in group B streptococci (GBS), the major cause of neonatal infections. RD2 encodes seven proteins with conventional gram-positive secretion signal sequences, six of which have not been characterized. Herein, we report that one of these six proteins (M28_Spy1325; Spy1325) is a member of the antigen I/II family of cell surface-anchored molecules produced by oral streptococci. PCR and DNA sequence analysis found that Spy1325 is very well conserved in GAS strains of distinct M protein serotypes. As assessed by real-time TaqMan quantitative PCR, the Spy1325 gene was expressed in vitro, and Spy1325 protein was present in culture supernatants and on the GAS cell surface. Western immunoblotting and enzyme-linked immunosorbent assays indicated that Spy1325 was produced by GAS in infected mice and humans. Importantly, the immunization of mice with recombinant Spy1325 fragments conferred protection against GAS-mediated mortality. Similar to other antigen I/II proteins, recombinant Spy1325 bound purified human salivary agglutinin glycoprotein. Spy1325 may represent a shared virulence factor among GAS, GBS, and oral streptococci.

Characterization of functional domains of the Vibrio cholerae virulence regulator ToxT

The toxT gene encodes an AraC family transcriptional activator that is responsible for regulating virulence gene expression in Vibrio cholerae. Analysis of ToxT by dominant/negative assays and a LexA-based reporter system demonstrated that the N-terminus of the protein contains dimerization determinants, indicating that ToxT likely functions as a dimer. Additionally, a natural variant of ToxT with only 60% identity in the N-terminus, as well as a mutant form of ToxT with an altered amino acid in the N-terminus (L107F), exhibited altered transcriptional responses to bile, suggesting that the N-terminus is involved in environmental sensing. The C-terminus of ToxT functions to bind DNA and requires dimerization for stable binding in vitro, as demonstrated by gel shift analysis. Interestingly, a dimerized form of the ToxT C-terminus is able to bind DNA in vitro but is transcriptionally inactive in vivo, indicating that the N-terminus contains determinants that are required for transcriptional activation. These results provide a model for a two-domain structure for ToxT, with an N-terminal dimerization and environmental sensing domain and a C-terminal DNA-binding domain; unlike other well-studied AraC family proteins, both domains of ToxT appear to be required for transcriptional activation.

Multiplex real-time PCR detection of Vibrio cholerae

Cholera is an important enteric disease, which is endemic to different regions of the world and has historically been the cause of severe pandemics. Vibrio cholerae is a natural inhabitant of the aquatic environment and the toxigenic strains are causative agents of potentially life-threatening diarrhoea. A multiplex, real-time detection assay was developed targeting four genes characteristic of potentially toxigenic strains of V. cholerae, encoding: repeat in toxin (rtxA), extracellular secretory protein (epsM), mannose-sensitive pili (mshA) and the toxin coregulated pilus (tcpA). The assay was developed on the Cepheid Smart Cycler using SYBR Green I for detection and the products were differentiated based on melting temperature (Tm) analysis. Validation of the assay was achieved by testing against a range of Vibrio and non-Vibrio species. The detection limit of the assay was determined to be 10(3) CFU using cells from pure culture. This assay was also successful at detecting V. cholerae directly from spiked environmental water samples in the order of 10(4) CFU, except from sea water which inhibited the assay. The incorporation of a simple DNA purification step prior to the addition to the PCR increased the sensitivity 10 fold to 10(3) CFU. This multiplex real-time PCR assay allows for a more reliable, rapid detection and identification of V. cholerae which is considerably faster than current conventional detection assays.

Genetics of stress adaptation and virulence in toxigenic Vibrio cholerae

Vibrio cholerae, a Gram-negative bacterium belonging to the gamma-subdivision of the family Proteobacteriaceae is the etiologic agent of cholera, a devastating diarrheal disease which occurs frequently as epidemics. Any bacterial species encountering a broad spectrum of environments during the course of its life cycle is likely to develop complex regulatory systems and stress adaptation mechanisms to best survive in each environment encountered. Toxigenic V. cholerae, which has evolved from environmental nonpathogenic V. cholerae by acquisition of virulence genes, represents a paradigm for this process in that this organism naturally exists in an aquatic environment but infects human beings and cause cholera. The V. cholerae genome, which is comprised of two independent circular mega-replicons, carries the genetic determinants for the bacterium to survive both in an aquatic environment as well as in the human intestinal environment. Pathogenesis of V. cholerae involves coordinated expression of different sets of virulence associated genes, and the synergistic action of their gene products. Although the acquisition of major virulence genes and association between V. cholerae and its human host appears to be recent, and reflects a simple pathogenic strategy, the establishment of a productive infection involves the expression of many more genes that are crucial for survival and adaptation of the bacterium in the host, as well as for its onward transmission and epidemic spread. While a few of the virulence gene clusters involved directly with cholera pathogenesis have been characterized, the potential exists for identification of yet new genes which may influence the stress adaptation, pathogenesis, and epidemiological characteristics of V. cholerae. Coevolution of bacteria and mobile genetic elements (plasmids, transposons, pathogenicity islands, and phages) can determine environmental survival and pathogenic interactions between bacteria and their hosts. Besides horizontal gene transfer mediated by genetic elements and phages, the evolution of pathogenic V. cholerae involves a combination of selection mechanisms both in the host and in the environment. The occurrence of periodic epidemics of cholera in endemic areas appear to enhance this process.

Genomic characterization of non-O1, non-O139 Vibrio cholerae reveals genes for a type III secretion system

Non-O1, non-O139 Vibrio cholerae can cause gastroenteritis and extraintestinal infections, but, unlike O1 and O139 strains of V. cholerae, little is known about the virulence gene content of non-O1, non-O139 strains and their phylogenetic relationship to other pathogenic V. cholerae. Comparative genomic microarray analysis of four pathogenic non-O1, non-O139 strains indicates that these strains are quite divergent from O1 and O139 strains. Genomic sequence analysis of a non-O1, non-O139 strain (AM-19226) that appeared particularly pathogenic in experimental animals suggests that this strain carries a type III secretion system (TTSS) that is related to the TTSS2 gene cluster found in a pandemic clone of Vibrio parahaemolyticus. The genes for this V. cholerae TTSS system appear to be present in many clinical and environmental non-O1, non-O139 strains, including at least one clone that is globally distributed. We hypothesize that the TTSS present in some pathogenic strains of non-O1, non-O139 V. cholerae may be involved in the virulence and environmental fitness of these strains.

Genetic diversity and virulence potential of environmental Vibrio cholerae population in a cholera-endemic area

To understand the evolutionary events and possible selection mechanisms involved in the emergence of pathogenic Vibrio cholerae, we analyzed diverse strains of V. cholerae isolated from environmental waters in Bangladesh by direct enrichment in the intestines of adult rabbits and by conventional laboratory culture. Strains isolated by conventional culture were mostly (99.2%) negative for the major virulence gene clusters encoding toxin-coregulated pilus (TCP) and cholera toxin (CT) and were nonpathogenic in animal models. In contrast, all strains selected in rabbits were competent for colonizing infant mice, and 56.8% of these strains carried genes encoding TCP alone or both TCP and CT. Ribotypes of toxigenic O1 and O139 strains from the environment were similar to pandemic strains, whereas ribotypes of non-O1 non-O139 strains and TCP(-) nontoxigenic O1 strains diverged widely from the seventh pandemic O1 and the O139 strains. Results of this study suggest that (i) the environmental V. cholerae population in a cholera-endemic area is highly heterogeneous, (ii) selection in the mammalian intestine can cause enrichment of environmental strains with virulence potential, (iii) pathogenicity of V. cholerae involves more virulence genes than currently appreciated, and (iv) most environmental V. cholerae strains are unlikely to attain a pandemic potential by acquisition of TCP and CT genes alone. Because most of the recorded cholera pandemics originated in the Ganges Delta region, this ecological setting presumably favors extensive genetic exchange among V. cholerae strains and thus promotes the rare, multiple-gene transfer events needed to assemble the critical combination of genes required for pandemic spread.

Pathogenicity islands and phages in Vibrio cholerae evolution

The identification of accessory genetic elements (plasmids, phages and chromosomal 'pathogenicity islands') encoding virulence-associated genes has facilitated our efforts to understand the origination of pathogenic microorganisms. Toxigenic Vibrio cholerae, the etiologic agent of cholera, represents a paradigm for this process in that this organism evolved from environmental nonpathogenic V. cholerae by acquisition of virulence genes. The major virulence genes in V. cholerae, which are clustered in several chromosomal regions, appear to have been recently acquired from phages or through undefined horizontal gene transfer events. Evidence is accumulating that the interactions of phages with each other can also influence the emergence of pathogenic clones of V. cholerae. Therefore, to track the evolution of pathogens from their nonpathogenic progenitors, it is also crucial to identify and characterize secondary genetic elements that mediate lateral transfer of virulence genes in trans. Understanding the evolutionary events that lead to the emergence of pathogenic clones might provide new approaches to the control of cholera and other infectious diseases.