RpoS-dependent stress response and exoenzyme production in Vibrio vulnificus

rpoS involved in immune response of Macrobrachium nipponens to Vibrio mimicus infection

Vibrio mimicus is a pathogenic bacterium that cause red body disease in Macrobrachium nipponense, leading to high mortality and financial loss. Based on previous studies, rpoS gene contribute to bacterial pathogenicity during infection, but the role of RpoS involved in the immune response of M. nipponense under V. mimicus infection remains unclear. In this study, the pathogen load and the RNA-seq of M. nipponense under wild-type and ΔrpoS strain V. mimicus infection were investigated. Over the entire infection period, the ΔrpoS strain pathogen load was always lower than that of the wild-type strain in the M. nipponense hemolymph, hepatopancreas, gill and muscle. Furthermore, the expression level of rpoS gene in the hepatopancreas was the highest at 24 hours post infection (hpi), then the samples of hepatopancreas tissue infected with the wild type and ΔrpoS strain at 24 hpi were selected for RNA-seq sequencing. The results revealed a significant change in the transcriptomes of the hepatopancreases infected with ΔrpoS strain. In contrast to the wild-type infected group, the ΔrpoS strain infected group exhibited differentially expressed genes (DEGs) enriched in 181 KEGG pathways at 24 hpi. Among these pathways, 8 immune system-related pathways were enriched, including ECM-receptor interaction, PI3K-Akt signaling pathway, Rap1 signaling pathway, Gap junction, and Focal adhesion, etc. Among these pathways, up-regulated genes related to Kazal-type serine protease inhibitors, S-antigen protein, copper zinc superoxide dismutase, tight junction protein, etc. were enriched. This study elucidates that rpoS can affect tissue bacterial load and immune-related pathways, thereby impacting the survival rate of M. nipponense under V. mimicus infection. These findings validate the potential of rpoS as a promising target for the development of a live attenuated vaccine against V. mimicus.

Cellular and physiological roles of sigma factors in Vibrio spp.: A comprehensive review

Vibrio species are motile gram-negative bacteria commonly found in aquatic environments. Vibrio species include pathogenic as well as non-pathogenic strains. Pathogenic Vibrio species have been reported in invertebrates and humans, whereas non-pathogenic strains are involved in symbiotic relationships with their eukaryotic hosts. These bacteria are also able to adapt to fluctuations in temperature, salinity, and pH, in addition to oxidative stress, and osmotic pressure in aquatic ecosystems. Moreover, they have also developed protective mechanisms against the immune systems of their hosts. Vibrio species accomplish adaptation to changing environments outside or inside the host by altering their gene expression profiles. To this end, several sigma factors specifically regulate gene expression, particularly under stressful environmental conditions. Moreover, other sigma factors are associated with biofilm formation and virulence as well. This review discusses different types of sigma and anti-sigma factors of Vibrio species involved in virulence and regulation of gene expression upon changes in environmental conditions. The evolutionary relationships between sigma factors with various physiological roles in Vibrio species are also discussed extensively.

Rifampicin-resistant RpoB S522L Vibrio vulnificus exhibits disturbed stress response and hypervirulence traits

Rifampicin resistance, which is genetically linked to mutations in the RNA polymerase β-subunit gene rpoB, has a global impact on bacterial transcription and cell physiology. Previously, we identified a substitution of serine 522 in RpoB (i.e., RpoBS522L ) conferring rifampicin resistance to Vibrio vulnificus, a human food-borne and wound-infecting pathogen associated with a high mortality rate. Transcriptional and physiological analysis of V. vulnificus expressing RpoBS522L showed increased basal transcription of stress-related genes and global virulence regulators. Phenotypically these transcriptional changes manifest as disturbed osmo-stress responses and toxin-associated hypervirulence as shown by reduced hypoosmotic-stress resistance and enhanced cytotoxicity of the RpoBS522L strain. These results suggest that RpoB-linked rifampicin resistance has a significant impact on V. vulnificus survival in the environment and during infection.

In vivo characterisation of the Vibrio vulnificus stressosome: A complex involved in reshaping glucose metabolism and motility regulation, in nutrient- and iron-limited growth conditions

Stressosomes are signal-sensing and integration hubs identified in many bacteria. At present, the role of the stressosome has only been investigated in Gram-positive bacteria. This work represents the first in vivo characterisation of the stressosome in a Gram-negative bacterium, Vibrio vulnificus. Previous in vitro characterisation of the complex has led to the hypothesis of a complex involved in iron metabolism and control of c-di-GMP levels. We demonstrate that the stressosome is probably involved in reshaping the glucose metabolism in Fe- and nutrient-limited conditions and mutations of the locus affect the activation of the glyoxylate shunt. Moreover, we show that the stressosome is needed for the transcription of fleQ and to promote motility, consistent with the hypothesis that the stressosome is involved in regulating c-di-GMP. This report highlights the potential role of the stressosome in a Gram-negative bacterium, with implications for the metabolism and motility of this pathogen.

Stress Responses in Pathogenic Vibrios and Their Role in Host and Environmental Survival

Vibrio is a genus of bacteria commonly found in estuarine, marine, and freshwater environments. Vibrio species have evolved to occupy diverse niches in the aquatic ecosystem, with some having complex lifestyles. About a dozen of the described Vibrio species have been reported to cause human disease, while many other species cause disease in other organisms. Vibrio cholerae causes epidemic cholera, a severe dehydrating diarrheal disease associated with the consumption of contaminated food or water. The human pathogenic non-cholera Vibrio species, Vibrio parahaemolyticus and Vibrio vulnificus, cause gastroenteritis, septicemia, and other extra-intestinal infections. Infections caused by V. parahaemolyticus and V. vulnificus are normally acquired through exposure to sea water or through consumption of raw or undercooked contaminated seafood. The human pathogenic Vibrios are exposed to numerous different stress-inducing agents and conditions in the aquatic environment and when colonizing a human host. Therefore, they have evolved a variety of mechanisms to survive in the presence of these stressors. Here we discuss what is known about important stress responses in pathogenic Vibrio species and their role in bacterial survival.

Transcriptomic and phenotype analysis revealed the role of rpoS in stress resistance and virulence of pathogenic Enterobacter cloacae from Macrobrachium rosenbergii

Enterobacter cloacae is widely distributed in the aquatic environment, and has been determined as a novel pathogen of various aquatic animals recently. Our previous studies have indicated E. cloacae caused repeated infections in Macrobrachium rosenbergii, suggesting a high survival ability of the bacteria, and rpoS gene has been known to regulate stress response and virulence of many bacteria. In this study, the E. cloacae-rpoS RNAi strain was constructed by RNAi technology, and the regulation role of rpoS in stress resistance and virulence of E. cloacae was explored by transcriptomic and phenotype analysis. The transcriptome analysis showed a total of 488 differentially expressed genes (DEGs) were identified between rpoS-RNAi and wild-type strains, including 30 up-regulated genes and 458 down-regulated genes, and these down-regulated DEGs were mainly related to environmental response, biofilm formation, bacterial type II secretory system, flagellin, fimbrillin, and chemotactic protein which associated with bacterial survival and virulence. The phenotype changes also showed the E. cloacae-rpoS RNAi strain exhibited significantly decreasing abilities of survival in environmental stresses (starvation, salinity, low pH, and oxidative stress), biofilm production, movement, adhesion to cells, pathogenicity, and colonization to M. rosenbergii. These results reveal that rpoS plays an important regulatory role in environmental stress adaptation and virulence of E. cloacae.

Differential expression profiling of ΔlitR and ΔrpoQ mutants reveals insight into QS regulation of motility, adhesion and biofilm formation in Aliivibrio salmonicida

BACKGROUND

The coordination of group behaviors in bacteria is achieved by a cell-cell signaling process called quorum sensing (QS). QS is an intercellular communication system, which synchronously controls expression of a vast range of genes in response to changes in cell density and is mediated by autoinducers that act as extracellular signals. Aliivibrio salmonicida, the causative agent of cold-water vibrosis in marine aquacultures, uses QS to regulate several activities such as motility, biofilm formation, adhesion and rugose colony morphology. However, little is known about either genes or detailed mechanisms involved in the regulation of these phenotypes.

RESULTS

Differential expression profiling allowed us to define the genes involved in controlling phenotypes related to QS in A. salmonicida LFI1238. RNA sequencing data revealed that the number of expressed genes in A. salmonicida, ΔlitR and ΔrpoQ mutants were significantly altered due to changes in cell density. These included genes that were distributed among the 21 functional groups, mainly presented in cell envelope, cell processes, extrachromosomal/foreign DNA and transport-binding proteins functional groups. The comparative transcriptome of A. salmonicida wild-type at high cell density relative to low cell density revealed 1013 genes to be either up- or downregulated. Thirty-six downregulated genes were gene clusters encoding biosynthesis of the flagellar and chemotaxis genes. Additionally we identified significant expression for genes involved in acyl homoserine lactone (AHL) synthesis, adhesion and early colonization. The transcriptome profile of ΔrpoQ compared to the wild-type revealed 384 differensially expressed genes (DEGs) that allowed us to assign genes involved in regulating motility, adhesion and colony rugosity. Indicating the importance of RpoQ in controlling several QS related activities. Furthermore, the comparison of the transcriptome profiles of ΔlitR and ΔrpoQ mutants, exposed numerous overlapping DEGs that were essential for motility, exopolysaccharide production via syp operon and genes associated with tad operon.

CONCLUSION

Our findings indicate previously unexplained functional roles for LitR and RpoQ in regulation of different phenotypes related to QS. Our transcriptome data provide a better understanding of the regulation cascade of motility, wrinkling colony morphology and biofilm formation and will offer a major source for further research and analysis on this important field.

Non-redundant expression of the two katG genes in Vibrio parahaemolyticus V110

Vibrio parahaemolyticus V110 is a marine origin pathogen infecting shrimp. Its resistance to oxidative stress is important for its survival in the complex marine ecosystems. vpa0768 (katG1) and vpa0453 (katG2) were previously found to contribute to the resistance against H₂ O₂ and isopropylbenzene. Our data showed that purified KatG2 and KatG1 possessed similar activity for hydrolyzing H₂ O₂ at 37 °C. The transcription of katG genes was induced by H₂ O₂ , cumene, and tert-butyl hydroperoxide (TBHP). The fold change of katG2 transcripts induced by isopropylbenzene was significantly higher than that of katG1. oxyR and rpoS are well-known regulatory genes which control the anti-oxidative and general stress response pathways, respectively. Deletion of rpoS resulted pathways, respectively. Deletion of rpoS resulted in abolishing the induction of katGs by the peroxides, and oxyR deletion only weakened the expression of the two genes. These results indicate that the two katGs encoding active enzymes are both inducible, but differ in their inducer preference. RpoS and oxyR are required for the full expression of katGs, but other unknown sensing regulators could be involved in the oxidative stress response besides OxyR.

Cyclo-(l-Phe-l-Pro), a Quorum-Sensing Signal of Vibrio vulnificus, Induces Expression of Hydroperoxidase through a ToxR-LeuO-HU-RpoS Signaling Pathway To Confer Resistance against Oxidative Stress

Vibrio vulnificus, an opportunistic human pathogen, produces cyclo-(l-Phe-l-Pro) (cFP), which serves as a signaling molecule controlling the ToxR-dependent expression of innate bacterial genes, and also as a virulence factor eliciting pathogenic effects on human cells by enhancing intracellular reactive oxygen species levels. We found that cFP facilitated the protection of V. vulnificus against hydrogen peroxide. At a concentration of 1 mM, cFP enhanced the level of the transcriptional regulator RpoS, which in turn induced expression of katG, encoding hydroperoxidase I, an enzyme that detoxifies H₂O₂ to overcome oxidative stress. We found that cFP upregulated the transcription of the histone-like proteins vHUα and vHUβ through the cFP-dependent regulator LeuO. LeuO binds directly to upstream regions of vhuA and vhuB to enhance transcription. vHUα and vHUβ then enhance the level of RpoS posttranscriptionally by stabilizing the mRNA. This cFP-mediated ToxR-LeuO-vHUαβ-RpoS pathway also upregulates genes known to be members of the RpoS regulon, suggesting that cFP acts as a cue for the signaling pathway responsible for both the RpoS and the LeuO regulons. Taken together, this study shows that cFP plays an important role as a virulence factor, as well as a signal for the protection of the cognate pathogen.

Vibrio vulnificus RtxA1 Toxin Expression Upon Contact With Host Cells Is RpoS-Dependent

The expression of virulence genes in bacteria is known to be regulated by various environmental and host factors. Vibrio vulnificus, an estuarine bacterium, experiences a dramatic environmental change during its infection process. We reported that V. vulnificus RtxA1 toxin caused acute cell death only when close contact to host cells was allowed. A sigma factor RpoS is a very important regulator for the maximal survival of pathogens under stress conditions. Here, we studied the role of RpoS in V. vulnificus cytotoxicity and mouse lethality. The growth of rpoS mutant strain was comparable to that of wild-type in heart infusion (HI) media and DMEM with HeLa cell lysate. An rpoS mutation resulted in decreased cytotoxicity, which was restored by in trans complementation. Interestingly, host contact increased the expression and secretion of V. vulnificus RtxA1 toxin, which was decreased and delayed by the rpoS mutation. Transcription of the cytotoxic gene rtxA1 and its transporter rtxB1 was significantly increased after host factor contact, whereas the activity was decreased by the rpoS mutation. In contrast, the rpoS mutation showed no effect on the transcriptional activity of a cytolytic heamolysin gene (vvhA). Additionally, the LD₅₀ of the rpoS mutant was 15-fold higher than that of the wild-type in specific pathogen-free CD-1 female mice. Taken together, these results show that RpoS regulates the expression of V. vulnificus RtxA1 toxin and its transporter upon host contact.

Post-Genomic Analysis of Members of the Family Vibrionaceae

Similar to other genera and species of bacteria, whole genomic sequencing has revolutionized how we think about and address questions of basic Vibrio biology. In this review we examined 36 completely sequenced and annotated members of the Vibrionaceae family, encompassing 12 different species of the genera Vibrio, Aliivibrio, and Photobacterium. We reconstructed the phylogenetic relationships among representatives of this group of bacteria by using three housekeeping genes and 16S rRNA sequences. With an evolutionary framework in place, we describe the occurrence and distribution of primary and alternative sigma factors, global regulators present in all bacteria. Among Vibrio we show that the number and function of many of these sigma factors differs from species to species. We also describe the role of the Vibrio-specific regulator ToxRS in fitness and survival. Examination of the biochemical capabilities was and still is the foundation of classifying and identifying new Vibrio species. Using comparative genomics, we examine the distribution of carbon utilization patterns among Vibrio species as a possible marker for understanding bacteria-host interactions. Finally, we discuss the significant role that horizontal gene transfer, specifically, the distribution and structure of integrons, has played in Vibrio evolution.

The ghosts of selection past reduces the probability of plastic rescue but increases the likelihood of evolutionary rescue to novel stressors in experimental populations of wild yeast

Persistence by adaptation is called evolutionary rescue. Evolutionary rescue is more likely in populations that have been previously exposed to lower doses of the same stressor. Environmental fluctuations might also reduce the possibility of rescue, but little is known about the effect of evolutionary history on the likelihood of rescue. In this study, we hypothesised that the ubiquitous operation of generalised stress responses in many organisms increases the likelihood of rescue after exposure to other stressors. We tested this hypothesis with experimental populations that had been exposed to long-term starvation and were then selected on different, unrelated stressors. We found that prior adaptation to starvation imposes contrary effects on the plastic and evolutionary responses of populations to subsequent stressors. When first exposed to new stressors, such populations become extinct more often. If they survive the initial exposure to the new stressors, however, they are more likely to undergo evolutionary rescue.

Functions of VPA1418 and VPA0305 Catalase Genes in Growth of Vibrio parahaemolyticus under Oxidative Stress

The marine foodborne enteropathogen Vibrio parahaemolyticus has four putative catalase genes. The functions of two katE-homologous genes, katE1 (VPA1418) and katE2 (VPA0305), in the growth of this bacterium were examined using gene deletion mutants with or without complementary genes. The growth of the mutant strains in static or shaken cultures in a rich medium at 37°C or at low temperatures (12 and 4°C), with or without competition from Escherichia coli, did not differ from that of the parent strain. When 175 μM extrinsic H2O2 was added to the culture medium, bacterial growth of the ΔkatE1 strain was delayed and growth of the ΔkatE1 ΔkatE2 and ΔkatE1 ΔahpC1 double mutant strains was completely inhibited at 37°C for 8 h. The sensitivity of the ΔkatE1 strain to the inhibition of growth by H2O2 was higher at low incubation temperatures (12 and 22°C) than at 37°C. The determined gene expression of these catalase and ahpC genes revealed that katE1 was highly expressed in the wild-type strain at 22°C under H2O2 stress, while the katE2 and ahpC genes may play an alternate or compensatory role in the ΔkatE1 strain. This study demonstrated that katE1 encodes the chief functional catalase for detoxifying extrinsic H2O2 during logarithmic growth and that the function of these genes was influenced by incubation temperature.

Deep-sea hydrothermal vent bacteria related to human pathogenic Vibrio species

Vibrio species are both ubiquitous and abundant in marine coastal waters, estuaries, ocean sediment, and aquaculture settings worldwide. We report here the isolation, characterization, and genome sequence of a novel Vibrio species, Vibrio antiquarius, isolated from a mesophilic bacterial community associated with hydrothermal vents located along the East Pacific Rise, near the southwest coast of Mexico. Genomic and phenotypic analysis revealed V. antiquarius is closely related to pathogenic Vibrio species, namely Vibrio alginolyticus, Vibrio parahaemolyticus, Vibrio harveyi, and Vibrio vulnificus, but sufficiently divergent to warrant a separate species status. The V. antiquarius genome encodes genes and operons with ecological functions relevant to the environment conditions of the deep sea and also harbors factors known to be involved in human disease caused by freshwater, coastal, and brackish water vibrios. The presence of virulence factors in this deep-sea Vibrio species suggests a far more fundamental role of these factors for their bacterial host. Comparative genomics revealed a variety of genomic events that may have provided an important driving force in V. antiquarius evolution, facilitating response to environmental conditions of the deep sea.

Microbial experimental evolution as a novel research approach in the Vibrionaceae and squid-Vibrio symbiosis

The Vibrionaceae are a genetically and metabolically diverse family living in aquatic habitats with a great propensity toward developing interactions with eukaryotic microbial and multicellular hosts (as either commensals, pathogens, and mutualists). The Vibrionaceae frequently possess a life history cycle where bacteria are attached to a host in one phase and then another where they are free from their host as either part of the bacterioplankton or adhered to solid substrates such as marine sediment, riverbeds, lakebeds, or floating particulate debris. These two stages in their life history exert quite distinct and separate selection pressures. When bound to solid substrates or to host cells, the Vibrionaceae can also exist as complex biofilms. The association between bioluminescent Vibrio spp. and sepiolid squids (Cephalopoda: Sepiolidae) is an experimentally tractable model to study bacteria and animal host interactions, since the symbionts and squid hosts can be maintained in the laboratory independently of one another. The bacteria can be grown in pure culture and the squid hosts raised gnotobiotically with sterile light organs. The partnership between free-living Vibrio symbionts and axenic squid hatchlings emerging from eggs must be renewed every generation of the cephalopod host. Thus, symbiotic bacteria and animal host can each be studied alone and together in union. Despite virtues provided by the Vibrionaceae and sepiolid squid-Vibrio symbiosis, these assets to evolutionary biology have yet to be fully utilized for microbial experimental evolution. Experimental evolution studies already completed are reviewed, along with exploratory topics for future study.

Transcriptome sequencing reveals the virulence and environmental genetic programs of Vibrio vulnificus exposed to host and estuarine conditions

Vibrio vulnificus is a natural inhabitant of estuarine waters worldwide and is of medical relevance due to its ability to cause grievous wound infections and/or fatal septicemia. Genetic polymorphisms within the virulence-correlated gene (vcg) serve as a primary feature to distinguish clinical (C-) genotypes from environmental (E-) genotypes. C-genotypes demonstrate superior survival in human serum relative to E-genotypes, and genome comparisons have allowed for the identification of several putative virulence factors that could potentially aid C-genotypes in disease progression. We used RNA sequencing to analyze the transcriptome of C-genotypes exposed to human serum relative to seawater, which revealed two divergent genetic programs under these two conditions. In human serum, cells displayed a distinct "virulence profile" in which a number of putative virulence factors were upregulated, including genes involved in intracellular signaling, substrate binding and transport, toxin and exoenzyme production, and the heat shock response. Conversely, the "environmental profile" exhibited by cells in seawater revealed upregulation of transcription factors such as rpoS, rpoN, and iscR, as well as genes involved in intracellular signaling, chemotaxis, adherence, and biofilm formation. This dichotomous genetic switch appears to be largely governed by cyclic-di-GMP signaling, and remarkably resembles the dual life-style of V. cholerae as it transitions from host to environment. Furthermore, we found a "general stress response" module, known as the stressosome, to be upregulated in seawater. This signaling system has been well characterized in Gram-positive bacteria, however its role in V. vulnificus is not clear. We examined temporal gene expression patterns of the stressosome and found it to be upregulated in natural estuarine waters indicating that this system plays a role in sensing and responding to the environment. This study advances our understanding of gene regulation in V. vulnificus, and brings to the forefront a number of previously overlooked genetic networks.

Activities of Alkyl Hydroperoxide Reductase Subunits C1 and C2 of Vibrio parahaemolyticus against Different Peroxides

Alkyl hydroperoxide reductase subunit C gene (ahpC) functions were characterized in Vibrio parahaemolyticus, a commonly occurring marine food-borne enteropathogenic bacterium. Two ahpC genes, ahpC1 (VPA1683) and ahpC2 (VP0580), encoded putative two-cysteine peroxiredoxins, which are highly similar to the homologous proteins of Vibrio vulnificus. The responses of deletion mutants of ahpC genes to various peroxides were compared with and without gene complementation and at different incubation temperatures. The growth of the ahpC1 mutant and ahpC1 ahpC2 double mutant in liquid medium was significantly inhibited by organic peroxides, cumene hydroperoxide and tert-butyl hydroperoxide. However, inhibition was higher at 12°C and 22°C than at 37°C. Inhibiting effects were prevented by the complementary ahpC1 gene. Inconsistent detoxification of H2O2 by ahpC genes was demonstrated in an agar medium but not in a liquid medium. Complementation with an ahpC2 gene partially restored the peroxidase effect in the double ahpC1 ahpC2 mutant at 22°C. This investigation reveals that ahpC1 is the chief peroxidase gene that acts against organic peroxides in V. parahaemolyticus and that the function of the ahpC genes is influenced by incubation temperature.

Alternative sigma factor RpoE is important for Vibrio parahaemolyticus cell envelope stress response and intestinal colonization

Vibrio parahaemolyticus is a halophile that inhabits brackish waters and a wide range of hosts, including crustaceans, fish, mollusks, and humans. In humans, it is the leading cause of bacterial seafood-borne gastroenteritis. The focus of this work was to determine the role of alternative sigma factors in the stress response of V. parahaemolyticus RIMD2210633, an O3:K6 pandemic isolate. Bioinformatics identified five putative extracytoplasmic function (ECF) family of alternative sigma factors: VP0055, VP2210, VP2358, VP2578, and VPA1690. ECF factors typically respond to cell wall/cell envelope stress, iron levels, and the oxidation state of the cell. We have demonstrated here that one such sigma factor, VP2578, a homologue of RpoE from Escherichia coli, is important for survival under a number of cell envelope stress conditions and in gastrointestinal colonization of a streptomycin-treated adult mouse. In this study, we determined that an rpoE deletion mutant strain BHM2578 compared to the wild type (WT) was significantly more sensitive to polymyxin B, ethanol, and high-temperature stresses. We demonstrated that in in vivo competition assays between the rpoE mutant and the WT marked with the β-galactosidase gene lacZ (WBWlacZ), the mutant strain was defective in colonization compared to the WT. In contrast, deletion of the rpoS stress response regulator did not affect in vivo survival. In addition, we examined the role of the outer membrane protein, OmpU, which in V. cholerae is proposed to be the sole activator of RpoE. We found that an ompU deletion mutant was sensitive to bile salt stress but resistant to polymyxin B stress, indicating OmpU is not essential for the cell envelope stress responses or RpoE function. Overall, these data demonstrate that RpoE is a key cell envelope stress response regulator and, similar to E. coli, RpoE may have several factors that stimulate its function.

Starvation induces phenotypic diversification and convergent evolution in Vibrio vulnificus

Starvation is a common stress experienced by bacteria living in natural environments and the ability to adapt to and survive intense stress is of paramount importance for any bacterial population. A series of starvation experiments were conducted using V. vulnificus 93U204 in phosphate-buffered saline and seawater. The starved population entered the death phase during the first week and approximately 1% of cells survived. After that the population entered a long-term stationary phase, and could survive for years. Starvation-induced diversification (SID) of phenotypes was observed in starved populations and phenotypic variants (PVs) appeared in less than 8 days. The cell density, rather than the population size, had a major effect on the extent of SID. SID was also observed in strain YJ016, where it evolved at a faster pace. PVs appeared to emerge in a fixed order: PV with reduced motility, PV with reduced proteolytic activity, and PV with reduced hemolytic activity. All of the tested PVs had growth advantages in the stationary phase phenotypes and increased fitness compared with 93U204 cells in co-culture competition experiments, which indicates that they had adapted to starvation. We also found that SID occurred in natural seawater with a salinity of 1%–3%, so this mechanism may facilitate bacterial adaptation in natural environments.

Interspecific quorum sensing mediates the resuscitation of viable but nonculturable vibrios

Entry and exit from dormancy are essential survival mechanisms utilized by microorganisms to cope with harsh environments. Many bacteria, including the opportunistic human pathogen Vibrio vulnificus, enter a form of dormancy known as the viable but nonculturable (VBNC) state. VBNC cells can resuscitate when suitable conditions arise, yet the molecular mechanisms facilitating resuscitation in most bacteria are not well understood. We discovered that bacterial cell-free supernatants (CFS) can awaken preexisting dormant vibrio populations within oysters and seawater, while CFS from a quorum sensing mutant was unable to produce the same resuscitative effect. Furthermore, the quorum sensing autoinducer AI-2 could induce resuscitation of VBNC V. vulnificus in vitro, and VBNC cells of a mutant unable to produce AI-2 were unable to resuscitate unless the cultures were supplemented with exogenous AI-2. The quorum sensing inhibitor cinnamaldehyde delayed the resuscitation of wild-type VBNC cells, confirming the importance of quorum sensing in resuscitation. By monitoring AI-2 production by VBNC cultures over time, we found quorum sensing signaling to be critical for the natural resuscitation process. This study provides new insights into the molecular mechanisms stimulating VBNC cell exit from dormancy, which has significant implications for microbial ecology and public health.

Roles of alkyl hydroperoxide reductase subunit C (AhpC) in viable but nonculturable Vibrio parahaemolyticus

Alkyl hydroperoxide reductase subunit C (AhpC) is the catalytic subunit responsible for the detoxification of reactive oxygen species that form in bacterial cells or are derived from the host; thus, AhpC facilitates the survival of pathogenic bacteria under environmental stresses or during infection. This study investigates the role of AhpC in the induction and maintenance of a viable but nonculturable (VBNC) state in Vibrio parahaemolyticus. In this investigation, ahpC1 (VPA1683) and ahpC2 (VP0580) were identified in chromosomes II and I of this pathogen, respectively. Mutants with deletions of these two ahpC genes and their complementary strains were constructed from the parent strain KX-V231. The growth of these strains was monitored on tryptic soy agar-3% NaCl in the presence of the extrinsic peroxides H(2)O(2) and tert-butyl hydroperoxide (t-BOOH) at different incubation temperatures. The results revealed that both ahpC genes were protective against t-BOOH, while ahpC1 was protective against H(2)O(2). The protective function of ahpC2 at 4°C was higher than that of ahpC1. The times required to induce the VBNC state (4.7 weeks) at 4°C in a modified Morita mineral salt solution with 0.5% NaCl and then to maintain the VBNC state (4.7 weeks) in an ahpC2 mutant and an ahpC1 ahpC2 double mutant were significantly shorter than those for the parent strain (for induction, 6.2 weeks; for maintenance, 7.8 weeks) and the ahpC1 mutant (for induction, 6.0 weeks; for maintenance, 8.0 weeks) (P < 0.03). Complementation with an ahpC2 gene reversed the effects of the ahpC2 mutation in shortening the times for induction and maintenance of the VBNC state. This investigation identified the different functions of the two ahpC genes and confirmed the particular role of ahpC2 in the VBNC state of V. parahaemolyticus.

Comparative analysis of induction of osmotic-stress-dependent genes in Vibrio vulnificus exposed to hyper- and hypo-osmotic stress

Vibrio vulnificus, a halophilic pathogenic bacterium of marine environments, encounters changes in salinity in its natural habitat and in the food-processing environment. The comparative response of V. vulnificus to hyperosmotic and hypoosmotic stress in terms of gene expression was investigated. Genes belonging to the proU operon for transport of compatible solutes and compatible solute synthesis were significantly upregulated (3- to 4.7-fold) under hyperosmotic stress. Under hypoosmotic stress, upregulation of genes coding for mechanosensitive channels of small conductance (mscS) was not observed. In hyperosmotic conditions a 2.3-fold decrease in the expression of aqpZ was observed. A 2-fold induction in gyrA was observed in V. vulnificus cells on exposure to hyperosmotic stress. groEL genes, VVA1659 (1.6-fold), and VV3106 (1-fold) were induced in hypoosmotic condition. Results of this study indicate that to manage hyperosmotic stress, V. vulnificus accumulated osmoprotectants through uptake or through endogenous synthesis of compatible solutes. Expression of mscS may not be necessary for immediate protection in cells exposed to hyper- and hypo-osmotic stress. Comparative analysis of important osmotic-stress-related genes showed up- or down-regulation of 14 genes in hyperosmotic stress as compared with up- or down-regulation of only 7 genes in hypoosmotic stress, indicating that the cells respond asymmetrically to hyper- and hypo-osmotic stress.

Pyrosequencing-based comparative genome analysis of Vibrio vulnificus environmental isolates

Between 1996 and 2006, the US Centers for Disease Control reported that the only category of food-borne infections increasing in frequency were those caused by members of the genus Vibrio. The Gram-negative bacterium Vibrio vulnificus is a ubiquitous inhabitant of estuarine waters, and is the number one cause of seafood-related deaths in the US. Many V. vulnificus isolates have been studied, and it has been shown that two genetically distinct subtypes, distinguished by 16S rDNA and other gene polymorphisms, are associated predominantly with either environmental or clinical isolation. While local genetic differences between the subtypes have been probed, only the genomes of clinical isolates have so far been completely sequenced. In order to better understand V. vulnificus as an agent of disease and to identify the molecular components of its virulence mechanisms, we have completed whole genome shotgun sequencing of three diverse environmental genotypes using a pyrosequencing approach. V. vulnificus strain JY1305 was sequenced to a depth of 33×, and strains E64MW and JY1701 were sequenced to lesser depth, covering approximately 99.9% of each genome. We have performed a comparative analysis of these sequences against the previously published sequences of three V. vulnificus clinical isolates. We find that the genome of V. vulnificus is dynamic, with 1.27% of genes in the C-genotype genomes not found in the E- genotype genomes. We identified key genes that differentiate between the genomes of the clinical and environmental genotypes. 167 genes were found to be specifically associated with environmental genotypes and 278 genes with clinical genotypes. Genes specific to the clinical strains include components of sialic acid catabolism, mannitol fermentation, and a component of a Type IV secretory pathway VirB4, as well as several other genes with potential significance for human virulence. Genes specific to environmental strains included several that may have implications for the balance between self-preservation under stress and nutritional competence.

Stationary-Phase Gene Regulation in Escherichia coli §

In their stressful natural environments, bacteria often are in stationary phase and use their limited resources for maintenance and stress survival. Underlying this activity is the general stress response, which in Escherichia coli depends on the σS (RpoS) subunit of RNA polymerase. σS is closely related to the vegetative sigma factor σ70 (RpoD), and these two sigmas recognize similar but not identical promoter sequences. During the postexponential phase and entry into stationary phase, σS is induced by a fine-tuned combination of transcriptional, translational, and proteolytic control. In addition, regulatory "short-cuts" to high cellular σS levels, which mainly rely on the rapid inhibition of σS proteolysis, are triggered by sudden starvation for various nutrients and other stressful shift conditons. σS directly or indirectly activates more than 500 genes. Additional signal input is integrated by σS cooperating with various transcription factors in complex cascades and feedforward loops. Target gene products have stress-protective functions, redirect metabolism, affect cell envelope and cell shape, are involved in biofilm formation or pathogenesis, or can increased stationary phase and stress-induced mutagenesis. This review summarizes these diverse functions and the amazingly complex regulation of σS. At the molecular level, these processes are integrated with the partitioning of global transcription space by sigma factor competition for RNA polymerase core enzyme and signaling by nucleotide second messengers that include cAMP, (p)ppGpp, and c-di-GMP. Physiologically, σS is the key player in choosing between a lifestyle associated with postexponential growth based on nutrient scavenging and motility and a lifestyle focused on maintenance, strong stress resistance, and increased adhesiveness. Finally, research with other proteobacteria is beginning to reveal how evolution has further adapted function and regulation of σS to specific environmental niches.

Susceptibility of a Vibrio alginolyticus rpoS mutant to environmental stresses and its expression of OMPs

Vibrio alginolyticus, one of the most important opportunistic pathogens, can be detected in human being and marine animals. Like other bacteria, V. alginolyticus is able to adapt to a variety of stressful environmental changes. The alternate sigma factor RpoS, which is a regulator during stationary phase, plays an important role in surviving under these stressful situations in many bacteria. Sequence analysis reveals a 990 bp open reading frame which is predicted to encode a 330-amino-acid protein with 68% to 96% overall identity to other reported sequences. To study the function of rpoS, the rpoS gene of V. alginolyticus VIB283 was cloned and an rpoS mutant was constructed by homologous recombination. Comparison of the study result of the wild type and the mutant showed that the mutant was more sensitive to stress conditions such as high osmolarity, oxidative stress, heat shock, and long-term starvation and that the LD(50) of the mutant strain to the zebra fish was about 2.8 times as that of the control strain. In addition, the SDS-PAGE analysis indicated that the outer membrane proteins (OMPs) existed great differences.

Vibrio caribbeanicus sp. nov., isolated from the marine sponge Scleritoderma cyanea

A Gram-negative, oxidase-positive, catalase-negative, facultatively anaerobic, motile, curved rod-shaped bacterium, strain N384(T), was isolated from a marine sponge (Scleritoderma cyanea; phylum Porifera) collected from a depth of 795 feet (242 m) off the west coast of Curaçao. On the basis of 16S rRNA gene sequencing, strain N384(T) was shown to belong to the genus Vibrio, most closely related to Vibrio brasiliensis LMG 20546(T) (98.8% similarity), Vibrio nigripulchritudo ATCC 27043(T) (98.5%), Vibrio tubiashii ATCC 19109(T) (98.6%) and V. sinaloensis DSM 21326(T) (98.2%). The DNA G+C content of strain N384(T) was 41.6 mol%. An analysis of concatenated sequences of five genes (gyrB, rpoA, pyrH, mreB and ftsZ; 4068 bp) demonstrated a clear separation between strain N384(T) and its closest neighbours and clustered strain N384(T) into the 'Orientalis' clade of vibrios. Phenotypically, the novel species belonged to the arginine dihydrolase-positive, lysine decarboxylase- and ornithine decarboxylase-negative (A+/L-/O-) cluster. The novel species was also differentiated on the basis of fatty acid composition, specifically that the proportions of iso-C(13:0), iso-C(15:0), C(15:0), iso-C(16:0), C(16:0), iso-C(17:0), C(17:1)ω8c and C(17:0) were significantly different from those found in V. brasiliensis and V. sinaloensis. The results of DNA-DNA hybridization, average nucleotide identity and physiological and biochemical tests further allowed differentiation of this strain from other described species of the genus Vibrio. Collectively, these findings confirm that strain N384(T) represents a novel Vibrio species, for which the name Vibrio caribbeanicus sp. nov. is proposed, with the type strain N384(T) ( = ATCC BAA-2122(T) = DSM 23640(T)).

Role of GacA in virulence of Vibrio vulnificus

The GacS/GacA two-component signal transduction system regulates virulence, biofilm formation and symbiosis in Vibrio species. The present study investigated this regulatory pathway in Vibrio vulnificus, a human pathogen that causes life-threatening disease associated with the consumption of raw oysters and wound infections. Small non-coding RNAs (csrB1, csrB2, csrB3 and csrC) commonly regulated by the GacS/GacA pathway were decreased (P<0.0003) in a V. vulnificus CMCP6 ΔgacA : : aph mutant compared with the wild-type parent, and expression was restored by complementation of the gacA deletion mutation in trans. Of the 20 genes examined by RT-PCR, significant reductions in the transcript levels of the mutant in comparison with the wild-type strain were observed only for genes related to motility (flaA), stationary phase (rpoS) and protease (vvpE) (P=0.04, 0.01 and 0.002, respectively). Swimming motility, flagellation and opaque colony morphology indicative of capsular polysaccharide (CPS) were unchanged in the mutant, while cytotoxicity, protease activity, CPS phase variation and the ability to acquire iron were decreased compared with the wild-type (P<0.01). The role of gacA in virulence of V. vulnificus was also demonstrated by significant impairment in the ability of the mutant strain to cause either skin (P<0.0005) or systemic infections (P<0.02) in subcutaneously inoculated, non-iron-treated mice. However, the virulence of the mutant was equivalent to that of the wild-type in iron-treated mice, demonstrating that the GacA pathway in V. vulnificus regulates the virulence of this organism in an iron-dependent manner.

Regulation of the Vibrio vulnificus vvpE expression by cyclic AMP-receptor protein and quorum-sensing regulator SmcR

In Vibrio vulnificus, cAMP-receptor protein (CRP) and the quorum-sensing regulator SmcR are simultaneously and cooperatively required for the metalloprotease vvpE gene expression, rather than sequentially in a regulatory cascade. However, this study shows a new temporal and functional sequence between the two factors in regulating vvpE expression. A crp mutation inhibited vvpE expression with growth impairment from early stage. In contrast, a smcR mutation inhibited vvpE expression only at the late stage with no effect on growth. A crp-smcR double mutation severely inhibited vvpE expression with growth impairment from early stage. The inhibited vvpE expression was restored only at the early stage by a crp single complementation, but not at all by a smcR complementation. These results indicate that CRP functions as an essential activator, whereas SmcR functions in the presence of CRP for full vvpE expression.

Adaptation of Vibrio vulnificus and an rpoS mutant to bile salts

Vibrio vulnificus is an opportunistic pathogen commonly found in oyster and marine environments, which frequently encounters different stresses in its natural habitat, food processing environment and during infection. In this paper, the adaptation of V. vulnificus to bile and the role of RpoS in this process were examined using a wild-type strain and an rpoS isogenic mutant. Adaptation to bile was readily induced in the exponential phase cells in phosphate-buffered saline with 2% bile salts, and the adapted cells exhibited enhanced tolerance against 10% bile. Addition of 1% Brain Heart Infusion medium to the adaptation medium significantly increased the survival of V. vulnificus against bile. The bile-adapted cells were cross-protected against alkaline treatment but sensitized against acid, heat, high salinity and detergents (sodium dodecyl sulfate, Triton X-100, 3-[(3-cholamidopropyl) dimethylammonio]- 1- propanesulfonate, and cetylpyridinium bromide). Addition of efflux pump inhibitor (carbonyl cyanide m-chlorophenylhydrazone) or protein synthesis inhibitor (chloramphenicol) completely eliminated or down-graded the enhanced bile tolerance of the adapted cells, respectively. Production of GroEL was not markedly influenced but DnaK was inhibited in the bile-adapted cells. The bile-adapted parent strain exhibited significantly higher survival than the rpoS mutant against the challenge of high pH, heat, high salinity and detergents. The induction of bile-adaptation in the rpoS mutant occurred at a significantly slower rate than for the parent strain. Results indicate that RpoS plays a significant role in the response of V. vulnificus to bile.

Role of RpoS in the susceptibility of low salinity-adapted Vibrio vulnificus to environmental stresses

Vibrio vulnificus is an opportunistic pathogen commonly found in oyster and marine environments, which frequently encounters low salinity stress in its natural and food processing environment. In this study, the responses of a V. vulnificus wild-type strain C78140o and its rpoS isogenic mutant AH1 to sublethal low salinity were examined to investigate the role of rpoS in this response. Both strains, adapted in low salinity (0.4% NaCl), were protected against the lethal low salinity (0.1% NaCl), but were not protected against heat (45 degrees C) or acid stress (pH 3.5), and were sensitized against 5% bile. Protection of the adapted cells against the lethal low salinity was not inhibited by the addition of chloramphenicol. Hemolysis was detected only in the adapted C78140o cells and its spent medium, and was inhibited by chloramphenicol. Transcription of the mechanosensitive channels (VVl_1542 and VVl_2579) and an aquaporin gene (VVl_2010) was markedly increased in the wild-type cells adapted in low salinity medium, while transcription of these genes was slightly enhanced or inhibited in AH1 cells. Results of this study support the active role of rpoS in the low salinity adaptation of V. vulnificus by regulating the expression of virulence and low salinity-associated factors, although rpoS is not related to the immediate protection of the adapted cells against lethal low salinity.

Role of RpoS in virulence of pathogens

Understanding mechanisms of bacterial pathogenesis is critical for infectious disease control and treatment. Infection is a sophisticated process that requires the participation of global regulators to coordinate expression of not only genes coding for virulence factors but also those involved in other physiological processes, such as stress response and metabolic flux, to adapt to host environments. RpoS is a key response regulator to stress conditions in Escherichia coli and many other proteobacteria. In contrast to its conserved well-understood role in stress response, effects of RpoS on pathogenesis are highly variable and dependent on species. RpoS contributes to virulence through either enhancing survival against host defense systems or directly regulating expression of virulence factors in some pathogens, while RpoS is dispensable, or even inhibitory, to virulence in others. In this review, we focus on the distinct and niche-dependent role of RpoS in virulence by surveying recent findings in many pathogens.

Global effect of RpoS on gene expression in pathogenic Escherichia coli O157:H7 strain EDL933

BACKGROUND

RpoS is a conserved stress regulator that plays a critical role in survival under stress conditions in Escherichia coli and other gamma-proteobacteria. RpoS is also involved in virulence of many pathogens including Salmonella and Vibrio species. Though well characterized in non-pathogenic E. coli K12 strains, the effect of RpoS on transcriptome expression has not been examined in pathogenic isolates. E. coli O157:H7 is a serious human enteropathogen, possessing a genome 20% larger than that of E. coli K12, and many of the additional genes are required for virulence. The genomic difference may result in substantial changes in RpoS-regulated gene expression. To test this, we compared the transcriptional profile of wild type and rpoS mutants of the E. coli O157:H7 EDL933 type strain.

RESULTS

The rpoS mutation had a pronounced effect on gene expression in stationary phase, and more than 1,000 genes were differentially expressed (twofold, P<0.05). By contrast, we found 11 genes expressed differently in exponential phase. Western blot analysis revealed that, as expected, RpoS level was low in exponential phase and substantially increased in stationary phase. The defect in rpoS resulted in impaired expression of genes responsible for stress response (e.g., gadA, katE and osmY), arginine degradation (astCADBE), putrescine degradation (puuABCD), fatty acid oxidation (fadBA and fadE), and virulence (ler, espI and cesF). For EDL933-specific genes on O-islands, we found 50 genes expressed higher in wild type EDL933 and 49 genes expressed higher in the rpoS mutants. The protein levels of Tir and EspA, two LEE-encoded virulence factors, were elevated in the rpoS mutants under LEE induction conditions.

CONCLUSION

Our results show that RpoS has a profound effect on global gene expression in the pathogenic strain O157:H7 EDL933, and the identified RpoS regulon, including many EDL933-specific genes, differs substantially from that of laboratory K12 strains.

Application of the rpoS gene for the detection of Vibrio anguillarum in flounder and prawn by polymerase chain reaction

Vibrio anguillarum, an opportunistic fish pathogen, is the main species responsible for vibriosis, a disease that affects feral and farmed fish and shellfish, and causes considerable economic losses in marine aquaculture. In this study, we used polymerase chain reaction (PCR) to detect V. anguillarum. PCR specificity was evaluated by amplifying the rpoS gene, a general stress regulator, in six strains of V. anguillarum and 36 other bacterial species. PCR amplified a species-specific fragment (689 bp) from V. anguillarum. Furthermore, the PCR assay was sensitive enough to detect rpoS expression from 3 pg of genomic DNA, or from six colony-forming units (CFU) mL(-1) of cultured V. anguillarum. However, the assay was less sensitive when genomic DNA from the infected flounder and prawn was used (limit of detection, 50 ng and 10 ng g(-1) tissue, respectively). These data demonstrate that PCR amplification of the rpoS gene is a sensitive and species-specific method to detect V. anguillarum in practical situations.

Vibrio vulnificus rpoS expression is repressed by direct binding of cAMP-cAMP receptor protein complex to its two promoter regions

Vibrio vulnificus, a septicemia-causing pathogenic bacterium, acquires resistance against various stresses and expresses virulence factors via an rpoS gene product. In this study, we investigated the transcriptional characteristics of this global regulator. Two distinct transcriptional initiation sites for the rpoS gene, the proximal promoter (P(p)) and the distal promoter (P(d)), were defined by primer extension experiments. Various rpoS::luxAB transcriptional fusions indicated that P(d) is a major promoter of rpoS expression. Western blot analysis showed that RpoS levels were inversely correlated with intracellular levels of 3',5'-cyclic AMP (cAMP). The expressions of both P(d) and P(p) were increased in cya and crp mutants. The exogenous addition of cAMP to the cya mutant resulted in repressed expression of rpoS. In addition, rpoS expression was significantly lowered in the cpdA mutant, in which the level of cAMP was elevated because of the absence of 3',5'-cAMP phosphodiesterase. In vitro transcription assays using the V. vulnificus RNA polymerase showed that the transcripts from both promoters were reduced by addition of the cAMP-cAMP receptor protein (CRP). The cAMP-CRP was shown to bind to two rpoS promoters by electrophoretic mobility shift assays. The alteration of the putative CRP-binding site on each rpoS promoter, via site-directed mutagenesis, abolished the binding of cAMP-CRP as well as regulation by cAMP-CRP. Therefore, this study shows a relationship between the level of intracellular cAMP and the degree of rpoS expression and further demonstrates, for the first time, the direct binding of the cAMP-CRP complex to rpoS upstream regions, which results in repression of rpoS gene expression.

Role of RpoS in stress survival, synthesis of extracellular autoinducer 2, and virulence in Vibrio alginolyticus

Vibrio alginolyticus, a marine bacterium, is an opportunistic pathogen capable of causing vibriosis with high mortality to fishes in the South China Sea. Stress resistance is very important for its survival in the natural environment and upon infection of the host. RpoS, an alternative sigma factor, is considered as an important regulator involved in stress response and virulence in many pathogens. In this study, the rpoS gene was cloned and characterized to evaluate the role of RpoS in V. alginolyticus. The predicted protein showed high identity with other reported rpoS gene products. The in-frame deleted mutation of rpoS in V. alginolyticus led to sensitivity of the strain to ethanol, hyperosmolarity, heat, and hydrogen peroxide challenges. Further studies showed that extracellular autoinducer 2 level, four of seven detected protease activities, and cytotoxicity of extracellular products were markedly decreased in the rpoS mutant compared with that in the wild-type strain. The results indicated that the global regulator RpoS was part of the regulatory networks of virulence and LuxS quorum sensing system.

Gene expression of cold shock and other stress-related genes in Vibrio vulnificus grown in pure culture under shellstock temperature control conditions

Shellstock refrigeration after harvesting is recommended to prevent further increases in Vibrio vulnificus numbers in oysters, but it could potentially induce a cold shock response in this bacterium. V. vulnificus was incubated at 35, 25, 20, and 15 degrees C and then subjected to 7.2 and 4 degrees C for 1 week. A cold-adaptation response that enhanced cell culturability was observed when cells were incubated at 15 degrees C prior to cold shock at 7.2 degrees C. In vitro cold shock gene expression was analyzed by reverse transcriptase PCR (RT-PCR). The expression of cold shock genes csp1 and csp5 (homologous genes to cspA and cspV) remained constant, despite cold shock. However, the transcript of csp3 was constitutively expressed before and after cold shock, with a few exceptions. The synthesis of csp3 mRNA in V. vulnificus C7184Tr (an avirulent strain) was induced only after 15 degrees C incubation and cold shock at 4 degrees C. The expression of csp4 was repressed after cold shock. Our data showed that the csp(s) tested in this study are not cold inducible. The transcripts of two oxidative stress-related genes, oxyR and katG, showed different induction patterns among strains after cold shock, suggesting that V. vulnificus cells encountered oxidative stress during cold shock.

Survival of and in situ gene expression by Vibrio vulnificus at varying salinities in estuarine environments

The opportunistic human pathogen Vibrio vulnificus survives in a wide range of ecological environments, which demonstrates its ability to adapt to highly variable conditions. Survival and gene expression under various conditions have been extensively studied in vitro; however, little work has been done to evaluate this bacterium in its natural habitat. Therefore, this study monitored the long-term survival of V. vulnificus in situ and simultaneously evaluated the expression of stress (rpoS, relA, hfq, and groEL) and putative virulence (vvpE, smcR, viuB, and trkA) genes at estuarine sites of varying salinity. Additionally, the survival and gene expression of an rpoS and an oxyR mutant were examined under the same conditions. Differences between the sampling sites in the long-term survival of any strain were not seen. However, differences were seen in the expression of viuB, trkA, and relA but our findings differed from what has been previously shown in vitro. These results also routinely demonstrated that genes required for survival under in vitro stress or host conditions are not necessarily required for survival in the water column. Overall, this study highlights the need for further in situ evaluation of this bacterium in order to gain a true understanding of its ecology and how it relates to its natural habitat.

Production of Vibrio vulnificus metalloprotease VvpE begins during the early growth phase: Usefulness of gelatin-zymography

Recent studies have demonstrated that expression of the vvpE gene begins during the early growth phase albeit at low levels. However, we found that the traditional protease assay method that is used to measure caseinolytic activity in culture supernatants is not suitable for the measurement of extracellular VvpE that is produced at low levels during the early growth phase. By using gelatin-zymography in place of the protease assay, we could specifically detect only VvpE of several proteases produced by Vibrio vulnificus. Moreover, we could sensitively measure VvpE produced at low levels during the early growth phase, which was consistent with transcription of the vvpE gene. The extracellular production of VvpE was reduced or delayed by mutation of the pilD gene which encodes for the type IV leader peptidase/N-methyltransferase associated with the type II general secretion system; the delayed production of VvpE was recovered by in trans complementation of the wild-type pilD gene. These results indicate that VvpE begins to be produced during the early growth phase via the PilD-mediated type II general secretion system, and that the use of gelatin-zymography is recommended as a simple method for the sensitive and specific detection of VvpE production.

Identification of OmpU of Vibrio vulnificus as a fibronectin-binding protein and its role in bacterial pathogenesis

Vibrio vulnificus is a pathogenic bacterium that causes gastroenteritis and primary septicemia. To identify factors involved in microbial adherence to the host cells, we investigated bacterial proteins capable of binding to fibronectin, one of the main components comprised of the extracellular matrix of mammalian cells. A protein of approximately 35 kDa was purified from the extracts of V. vulnificus by its property to bind to immobilized fibronectin. This protein was identified as OmpU, one of the major outer membrane proteins of V. vulnificus. In binding assays using immobilized fibronectin, the number of ompU mutant cells bound to fibronectin was only 4% of that of wild-type cells bound to fibronectin. In addition, the exogenous addition of antibodies against OmpU resulted in a decreased ability of wild-type V. vulnificus to adhere to fibronectin. The ompU mutant was also defective in its adherence to RGD tripeptide (5% of the adherence of the wild type to RGD), cytoadherence to HEp-2 cells (7% of the adherence of the wild type to HEp-2), cytotoxicity to cell cultures (39% of the cytotoxicity of the wild type), and mortality in mice (10-fold increase in the 50% lethal dose). The ompU mutant complemented with the intact ompU gene restored its abilities for adherence to fibronectin, RGD tripeptide, and HEp-2 cells; cytotoxicity to HEp-2 cells; and mouse lethality. This study indicates that OmpU is an important virulence factor involved in the adherence of V. vulnificus to the host cells.

Polyphosphate stores enhance the ability of Vibrio cholerae to overcome environmental stresses in a low-phosphate environment

Vibrio cholerae, the causative agent of Asiatic cholera, has been reported to make large quantities of polyphosphate. Inorganic polyphosphate is a ubiquitous molecule with a variety of functions in prokaryotic and eukaryotic cells. We constructed a V. cholerae mutant with a deletion in the polyphosphate kinase (ppk) gene. The mutant was defective in polyphosphate biosynthesis. Deletion of ppk had no significant effect on production of cholera toxin, hemagglutinin/protease, motility, biofilm formation, and colonization of the suckling mouse intestine. The wild type and mutant had similar growth rates in rich and minimal medium and exhibited similar phosphate uptake and alkaline phosphatase induction. In contrast to ppk mutants from other gram-negative bacteria, the V. cholerae mutant survived prolonged starvation in LB medium and artificial seawater basal salts. The ppk mutant was significantly more sensitive to low pH, high salinity, and oxidative stress when it was cultured in low-phosphate minimal medium. The ppk mutant failed to induce catalase when it was downshifted to phosphorus-limiting conditions. Furthermore, the increased sensitivity of the ppk mutant to environmental stressors in phosphate-limited medium correlated with a diminished capacity to synthesize ATP from intracellular reservoirs. We concluded that polyphosphate protects V. cholerae from environmental stresses under phosphate limitation conditions. It has been proposed that toxigenic V. cholerae can survive in estuaries and brackish waters in which phosphorus and/or nitrogen can be a limiting nutrient. Thus, synthesis of large polyphosphate stores could enhance the ability of V. cholerae to survive in the aquatic environment.

Capsular polysaccharide phase variation in Vibrio vulnificus

Commonly found in raw oysters, Vibrio vulnificus poses a serious health threat to immunocompromised individuals and those with serum iron overload, with a fatality rate of approximately 50%. An essential virulence factor is its capsular polysaccharide (CPS), which is responsible for a significant increase in virulence compared to nonencapsulated strains. However, this bacterium is known to vary the amount of CPS expressed on the cell surface, converting from an opaque (Op) colony phenotype to a translucent (Tr) colony phenotype. In this study, the consistency of CPS conversion was determined for four strains of V. vulnificus. Environmental conditions including variations in aeration, temperature, incubation time, oxidative stress, and media (heart infusion or modified maintenance medium agar) were investigated to determine their influence on CPS conversion. All conditions, with the exception of variations in media and oxidative stress, significantly affected the conversion of the population, with high ranges of CPS expression found even within cells from a single colony. The global quorum-sensing regulators RpoS and AI-2 were also examined. While RpoS was found to significantly mediate phenotypic conversion, quorum sensing was not. Finally, 12 strains that comprise the recently found clinical (C) and environmental (E) genotypes of V. vulnificus were examined to determine their rates of population conversion. C-genotype strains, which are most often associated with infection, had a significantly lower rate of population conversion from Op to Tr phenotypes than did E-genotype strains (ca. 38% versus ca. 14%, respectively). Biofilm capabilities of these strains, however, were not correlated with increased population conversion.

Vibrio vulnificus vulnibactin, but not metalloprotease VvpE, is essentially required for iron-uptake from human holotransferrin

The roles of metalloprotease (VvpE) and catechol-siderophore (vulnibactin) in the uptake of iron from human transferrins by Vibrio vulnificus have been determined using different experimental conditions and methods. Therefore, in this study, we attempted to elucidate the roles of VvpE and vulnibactin using the same methods and experimental conditions, in an in vitro and a human ex vivo system, and in accordance with the molecular version of Koch's postulates. Neither vvpE mutation nor in trans vvpE complementation affected vulnibactin production, iron-assimilation from human holotransferrin (HT), and bacterial growth in a HT-containing deferrated Heart-Infusion medium (HT-DF-HI) or a HT-containing cirrhotic ascites (HT-CA). In contrast, the mutation of fur gene encoding Fur, a repressor regulating expression of the vulnibactin-mediated iron-uptake system, derepressed vulnibactin production, and facilitated iron-assimilation from HT and bacterial growth in HT-DF-HI or HT-CA. The mutation of vis gene encoding isochorismate synthase required for vulnibactin synthesis abolished vulnibactin production, iron-assimilation from HT and bacterial growth in HT-DF-HI or HT-CA. These results demonstrate that vulnibactin is essentially required for iron-assimilation from transferrin, and that VvpE has no direct effect on facilitating vulnibactin-mediated iron-assimilation from transferrin in vitro or in a human ex vivo system.

RpoS involvement and requirement for exogenous nutrient for osmotically induced cross protection in Vibrio vulnificus

Vibrio vulnificus is an opportunistic human pathogen which is the causative agent of food-borne disease and wound infections. V. vulnificus is able to adapt to a variety of potentially stressful environmental changes, such as osmotic, nutrient, and temperature variations in estuarine environments, as well as oxidative, osmotic, and acidity differences following infection of a human host. After exposure to sub-lethal levels of a particular environmental stress, many bacteria become resistant to unrelated stresses, a phenomenon termed cross protection. In this study, we examined the ability of osmotic shock to cross protect V. vulnificus to high temperature as well as oxidative stress. Log phase cells of V. vulnificus strain C7184o were cross protected by prior osmotic shock to both heat and oxidative challenge, but only when exogenous nutrient was present during the osmotic upshift. Further, and unlike other bacteria, nutrient starvation alone did not result in cross protection against either stress. When small amounts of nutrient were present during osmotic shock, cross protection to an otherwise lethal heat challenge developed extremely rapidly, with significant protection seen within 10 min. Cross protection to oxidative stress was slower to develop, requiring several hours. Although stationary phase alone conferred some cross protection to heat and oxidative stress, the alternate sigma factor RpoS was required for complete cross protection of log phase cells to oxidative stress but not for resistance to heat challenge. Together these findings suggest that the cross protective response in V. vulnificus is complex and appears to involve multiple mechanisms.

Role of the rpoS gene in the survival of Vibrio parahaemolyticus in artificial seawater and fish homogenate

Vibrio parahaemolyticus is a foodborne pathogen isolated from coastal waters of the United States and from a variety of seafood, including fish. Seawater represents a nutrient-limiting environment for V. parahaemolyticus. During its persistence in seawater, V. parahaemolyticus is exposed to a variety of environmental stresses, including hyperosmolarity, fluctuations in temperature, and cold stress. The alternate sigma factor of RNA polymerase, designated as (RpoS), encoded by the gene rpoS has been shown to play a major role in bacterial adaptive responses to adverse environmental conditions. The present study was undertaken to investigate the role of rpoS in the survival of V. parahaemolyticus in seawater and fish. A V. parahaemolyticus rpoS mutant was constructed by the insertion of a chloramphenicol acetyltransferase gene cassette within the rpoS gene, and the wild and mutant strains were assayed for their ability to survive in artificial seawater (ASW) at 6 and 1 degrees C and in fish homogenate at 4 and 8 degrees C. The survival of the rpoS mutant of V. parahaemolyticus both in ASW and fish homogenate at either storage temperature was significantly (P < 0.05) lower than that of the wild strain. Further, the viability of V. parahaemolyticus, especially the mutant, was significantly reduced at lower storage temperatures of ASW and fish homogenate. Results of this study indicate that rpoS potentially plays an important role in the survival of V. parahaemolyticus under conditions of cold stress and hyperosmolarity.

The global regulatory proteins LetA and RpoS control phospholipase A, lysophospholipase A, acyltransferase, and other hydrolytic activities of Legionella pneumophila JR32

Legionella pneumophila possesses a variety of secreted and cell-associated hydrolytic activities that could be involved in pathogenesis. The activities include phospholipase A, lysophospholipase A, glycerophospholipid:cholesterol acyltransferase, lipase, protease, phosphatase, RNase, and p-nitrophenylphosphorylcholine (p-NPPC) hydrolase. Up to now, there have been no data available on the regulation of the enzymes in L. pneumophila and no data at all concerning the regulation of bacterial phospholipases A. Therefore, we used L. pneumophila mutants in the genes coding for the global regulatory proteins RpoS and LetA to investigate the dependency of hydrolytic activities on a global regulatory network proposed to control important virulence traits in L. pneumophila. Our results show that both L. pneumophila rpoS and letA mutants exhibit on the one hand a dramatic reduction of secreted phospholipase A and glycerophospholipid:cholesterol acyltransferase activities, while on the other hand secreted lysophospholipase A and lipase activities were significantly increased during late logarithmic growth phase. The cell-associated phospholipase A, lysophospholipase A, and p-NPPC hydrolase activities, as well as the secreted protease, phosphatase, and p-NPPC hydrolase activities were significantly decreased in both of the mutant strains. Only cell-associated phosphatase activity was slightly increased. In contrast, RNase activity was not affected. The expression of plaC, coding for a secreted acyltransferase, phospholipase A, and lysophospholipase A, was found to be regulated by LetA and RpoS. In conclusion, our results show that RpoS and LetA affect phospholipase A, lysophospholipase A, acyltransferase, and other hydrolytic activities of L. pneumophila in a similar way, thereby corroborating the existence of the LetA/RpoS regulation cascade.

In situ and in vitro gene expression by Vibrio vulnificus during entry into, persistence within, and resuscitation from the viable but nonculturable state

Isolation of Vibrio vulnificus during winter months is difficult due to the entrance of these cells into the viable but nonculturable (VBNC) state. While several studies have investigated in vitro gene expression upon entrance into and persistence within the VBNC state, to our knowledge, no in situ studies have been reported. We incubated clinical and environmental isolates of V. vulnificus in estuarine waters during winter months to monitor the expression of several genes during the VBNC state and compared these to results from in vitro studies. katG (periplasmic catalase) was down-regulated during the VBNC state in vitro and in situ compared to the constitutively expressed gene tufA. Our results indicate that the loss of catalase activity we previously reported is a direct result of katG repression, which likely accounts for the VBNC response of this pathogen. While expression of vvhA (hemolysin) was detectable in environmental strains during in situ incubation, it ceased in all cases by ca. 1 h. These results suggest that the natural role of hemolysin in V. vulnificus may be in osmoprotection and/or the cold shock response. Differences in expression of the capsular genes wza and wzb were observed in the two recently reported genotypes of this species. Expression of rpoS, encoding the stress sigma factor RpoS, was continuous upon entry into the VBNC state during both in situ and in vitro studies. We found the half-life of mRNA to be less than 60 minutes, confirming that mRNA detection in these VBNC cells is a result of de novo RNA synthesis.