In vivo induced clpB1 gene of Vibrio cholerae is involved in different stress responses and affects in vivo cholera toxin production

The transcriptome response of Enterobacter sp. S-33 is modulated by low pH-stress

BACKGROUND

Acidic environments naturally occur worldwide and uncontrolled use of agricultural practices may also cause acidification of soils. The development of acidic conditions disturbs the establishment of efficient microbial populations in their natural niches. The survival of Enterobacter species under acidic stress remains poorly understood.

OBJECTIVE

This study aimed to investigate the survival of an environmental isolate Enterobacter sp. S-33 under acidic stress and to identify the various genes involved in stress protection at the global gene transcription level. The obtained results provide new targets that will allow understanding the in-depth mechanisms involved in the adaptation of bacteria to environmental pH changes.

METHODS

We used the next-generation sequencing (NGS) method to analyze the expression (up-regulation & down-regulation) of genes under varying pH conditions.

RESULTS

A total of 4214 genes were differentially expressed under acidic conditions (pH 5.0), with 294 up-regulated and 167 down-regulated. At pH 6.0, 50 genes were significantly expressed, of which 34 and 16 were identified as up-regulated and down-regulated, respectively. Many of the up-regulated genes were involved in carbohydrate metabolism, amino acid transport & metabolism, and the most down-regulated genes were related to post-translational modification, lipid transport & metabolism, etc. The observed transcriptomic regulation of genes and pathways identified that Enterobacter reduced its post-translational modification, lipid transport & metabolism, and increased carbohydrate metabolism, amino acid metabolism & transport, energy production & conversion to adapt and grow in acidic stress.

CONCLUSIONS

The present work provides in-depth information on the characterization of genes associated with tolerance or adaptation to acidic stress of Enterobacter bacterium.

Identification of ClpB, a molecular chaperone involved in the stress tolerance and virulence of Streptococcus agalactiae

Bacterial ClpB is an ATP-dependent disaggregate that belongs to the Hsp100/Clp family and facilitates bacterial survival under hostile environmental conditions. Streptococcus agalactiae, which is regarded as the major bacterial pathogen of farmed Nile tilapia (Oreochromis niloticus), is known to cause high mortality and large economic losses. Here, we report a ClpB homologue of S. agalactiae and explore its functionality. S. agalactiae with a clpB deletion mutant (∆clpB) exhibited defective tolerance against heat and acidic stress, without affecting growth or morphology under optimal conditions. Moreover, the ΔclpB mutant exhibited reduced intracellular survival in RAW264.7 cells, diminished adherence to the brain cells of tilapia, increased sensitivity to leukocytes from the head kidney of tilapia and whole blood killing, and reduced mortality and bacterial loads in a tilapia infection assay. Furthermore, the reduced virulence of the ∆clpB mutant was investigated by transcriptome analysis, which revealed that deletion of clpB altered the expression levels of multiple genes that contribute to the stress response as well as certain metabolic pathways. Collectively, our findings demonstrated that ClpB, a molecular chaperone, plays critical roles in heat and acid stress resistance and virulence in S. agalactiae. This finding provides an enhanced understanding of the functionality of this ClpB homologue in gram-positive bacteria and the survival strategy of S. agalactiae against immune clearance during infection.

The Role of ClpB in Bacterial Stress Responses and Virulence

Bacterial survival within a mammalian host is contingent upon sensing environmental perturbations and initiating an appropriate counter-response. To achieve this, sophisticated molecular machineries are used, where bacterial chaperone systems play key roles. The chaperones are a prerequisite for bacterial survival during normal physiological conditions as well as under stressful situations, e.g., infection or inflammation. Specific stress factors include, but are not limited to, high temperature, osmolarity, pH, reactive oxidative species, or bactericidal molecules. ClpB, a member of class 1 AAA⁺ proteins, is a key chaperone that via its disaggregase activity plays a crucial role for bacterial survival under various forms of stress, in particular heat shock. Recently, it has been reported that ClpB also regulates secretion of bacterial effector molecules related to type VI secretion systems. In this review, the roles of ClpB in stress responses and the mechanisms by which it promotes survival of pathogenic bacteria are discussed.

Growth arrest and a persister state enable resistance to osmotic shock and facilitate dissemination of Vibrio cholerae

Vibrio cholerae is a water-borne bacterial pathogen and causative agent of cholera. Although V. cholerae is a halophile, it can survive in fresh water, and this has a major role in cholera epidemics through consumption of contaminated water and subsequent fecal-oral spread. After dissemination from humans back into fresh water, V. cholerae encounters limited nutrient availability and an abrupt drop in conductivity but little is known about how V. cholerae adapts to, and survives in this environment. In this work, by abolishing or altering the expression of V. cholerae genes in a high-throughput manner, we observed that many osmotic shock tolerant mutants exhibited slowed or arrested growth, and/or generated a higher proportion of persister cells. In addition, we show that growth-arrested V. cholerae, including a persister subpopulation, are generated during infection of the intestinal tract and together allow for the successful dissemination to fresh water. Our results suggest that growth-arrested and persister subpopulations enable survival of V. cholerae upon shedding to the aquatic environment.

AcfA is an essential regulator for pathogenesis of fish pathogen Vibrio alginolyticus

V. alginolyticus is an important opportunistic pathogen which causes vibriosis in aquatic animals. AcfA, as an accessory colonization factor, is hypothesized to be involved in the pathogenesis of infection. In this study, a mutant strain with an in-frame deletion removed nucleotides 86 to 561 of the acfA gene was constructed to reveal the role of AcfA in the physiology and virulence from V. alginolyticus. An acfA mutant showed a similar growth level, an obvious decrease in swarming motility and the activity of ECPase, a higher LD₅₀ value by intraperitoneal injection of grouper fish compared to that of the wild-type. Furthermore, the deletion of acfA could enhance the level of biofilm formation and suppress the polar flagellum forming. The comparative proteomic analysis demonstrated the deletion mutation of acfA could up-regulate the expression of 4 proteins of p4alcd, deoD, phb and DctP, and down-regulate the expression of 8 proteins of Clp, hpV36980, ABCtp, pepD, arA, aggp, fla and ompA compared to that of the wild-type. The analysis of RT-qPCR showed the mRNA levels of DctP and deoD were significantly induced, and the mRNA levels of pepD, arA, fla and ompA were significantly reduced in acfA mutant compared with the wild-type. The results suggest that acfA may contribute to the overall success in the pathogenesis of V. alginolyticus by regulating the expression of some relevant genes.

The Symbiotic Performance of Chickpea Rhizobia Can Be Improved by Additional Copies of the clpB Chaperone Gene

The ClpB chaperone is known to be involved in bacterial stress response. Moreover, recent studies suggest that this protein has also a role in the chickpea-rhizobia symbiosis. In order to improve both stress tolerance and symbiotic performance of a chickpea microsymbiont, the Mesorhizobium mediterraneum UPM-Ca36T strain was genetically transformed with pPHU231 containing an extra-copy of the clpB gene. To investigate if the clpB-transformed strain displays an improved stress tolerance, bacterial growth was evaluated under heat and acid stress conditions. In addition, the effect of the extra-copies of the clpB gene in the symbiotic performance was evaluated using plant growth assays (hydroponic and pot trials). The clpB-transformed strain is more tolerant to heat shock than the strain transformed with pPHU231, supporting the involvement of ClpB in rhizobia heat shock tolerance. Both plant growth assays showed that ClpB has an important role in chickpea-rhizobia symbiosis. The nodulation kinetics analysis showed a higher rate of nodule appearance with the clpB-transformed strain. This strain also induced a greater number of nodules and, more notably, its symbiotic effectiveness increased ~60% at pH5 and 83% at pH7, compared to the wild-type strain. Furthermore, a higher frequency of root hair curling was also observed in plants inoculated with the clpB-transformed strain, compared to the wild-type strain. The superior root hair curling induction, nodulation ability and symbiotic effectiveness of the clpB-transformed strain may be explained by an increased expression of symbiosis genes. Indeed, higher transcript levels of the nodulation genes nodA and nodC (~3 folds) were detected in the clpB-transformed strain. The improvement of rhizobia by addition of extra-copies of the clpB gene may be a promising strategy to obtain strains with enhanced stress tolerance and symbiotic effectiveness, thus contributing to their success as crop inoculants, particularly under environmental stresses. This is the first report on the successful improvement of a rhizobium with a chaperone gene.

Proteomic and metabolomic profiles demonstrate variation among free-living and symbiotic vibrio fischeri biofilms

Background

A number of bacterial species are capable of growing in various life history modes that enable their survival and persistence in both planktonic free-living stages as well as in biofilm communities. Mechanisms contributing to either planktonic cell or biofilm persistence and survival can be carefully delineated using multiple differential techniques (e.g., genomics and transcriptomics). In this study, we present both proteomic and metabolomic analyses of Vibrio fischeri biofilms, demonstrating the potential for combined differential studies for elucidating life-history switches important for establishing the mutualism through biofilm formation and host colonization.

Methods

The study used a metabolomics/proteomics or “meta-proteomics” approach, referring to the combined protein and metabolic data analysis that bridges the gap between phenotypic changes (planktonic cell to biofilm formation) with genotypic changes (reflected in protein/metabolic profiles). Our methods used protein shotgun construction, followed by liquid chromatography coupled with mass spectrometry (LC-MS) detection and quantification for both free-living and biofilm forming V. fischeri.

Results

We present a time-resolved picture of approximately 100 proteins (2D-PAGE and shotgun proteomics) and 200 metabolites that are present during the transition from planktonic growth to community biofilm formation. Proteins involved in stress response, DNA repair damage, and transport appeared to be highly expressed during the biofilm state. In addition, metabolites detected in biofilms correspond to components of the exopolysaccharide (EPS) matrix (sugars and glycerol-derived). Alterations in metabolic enzymes were paralleled by more pronounced changes in concentration of intermediates from the glycolysis pathway as well as several amino acids.

Conclusions

This combined analysis of both types of information (proteins, metabolites) has provided a more complete picture of the biochemical processes of biofilm formation and what determines the switch between the two life history strategies. The reported findings have broad implications for Vibrio biofilm ecology, and mechanisms for successful survival in the host and environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0560-z) contains supplementary material, which is available to authorized users.

Gene Expression of Type VI Secretion System Associated with Environmental Survival in Acidovorax avenae subsp. avenae by Principle Component Analysis

Valine glycine repeat G (VgrG) proteins are regarded as one of two effectors of Type VI secretion system (T6SS) which is a complex multi-component secretion system. In this study, potential biological roles of T6SS structural and VgrG genes in a rice bacterial pathogen, Acidovorax avenae subsp. avenae (Aaa) RS-1, were evaluated under seven stress conditions using principle component analysis of gene expression. The results showed that growth of the pathogen was reduced by H₂O₂ and paraquat-induced oxidative stress, high salt, low temperature, and vgrG mutation, compared to the control. However, pathogen growth was unaffected by co-culture with a rice rhizobacterium Burkholderia seminalis R456. In addition, expression of 14 T6SS structural and eight vgrG genes was significantly changed under seven conditions. Among different stress conditions, high salt, and low temperature showed a higher effect on the expression of T6SS gene compared with host infection and other environmental conditions. As a first report, this study revealed an association of T6SS gene expression of the pathogen with the host infection, gene mutation, and some common environmental stresses. The results of this research can increase understanding of the biological function of T6SS in this economically-important pathogen of rice.

IFN-γ, but not IL-17A, is required for survival during secondary pulmonary Francisella tularensis Live Vaccine Stain infection

IL-17 and IFN-γ production by Th17 and Th1 cells, respectively, is critical for survival during primary respiratory infection with the pathogenic bacterium, Francisella tularensis Live Vaccine Strain (LVS). The importance, however, of these T cell subsets and their soluble mediators is not well understood during a secondary or memory response. We measured the number of CD4(+) T cells producing IFN-γ or IL-17 in the spleen and lungs of vaccinated mice on day four of the secondary response using intracellular cytokine staining in order to identify protective T cell subsets participating in the memory response. Few bacteria were present in spleens of vaccinated mice on day four and a T cell response was not observed. In the lung, where more bacteria were present, there was a robust Th1 response in vaccinated mice but Th17 cells were not present at higher numbers in vaccinated mice compared to unvaccinated mice. These data show that the lung is the dominant site of the secondary immune response and suggest that Th17 cells are not required for survival during secondary challenge. To further investigate the importance of IFN-γ and IL-17 during the secondary response to F. tularensis, we neutralized either IFN-γ or IL-17 in vivo using monoclonal antibody treatment. Vaccinated mice treated with anti-IFN-γ lost more weight and had higher bacterial burdens compared to vaccinated mice treated with isotype control antibody. In contrast, treatment with anti-IL-17A antibody did not alter weight loss profiles or bacterial burdens compared to mice treated with isotype control antibody. Together, these results suggested that IFN-γ is required during both primary and secondary respiratory F. tularensis infection. IL-17, on the other hand, is only critical during the primary response to respiratory F. tularensis but dispensable during the secondary response.

Role of 6-gingerol in reduction of cholera toxin activity in vitro and in vivo

Vibrio cholerae is one of the major bacterial pathogens responsible for the devastating diarrheal disease called cholera. Chemotherapy is often used against V. cholerae infections; however, the emergence of V. cholerae with multidrug resistance (MDR) toward the chemotherapeutic agents is a serious clinical problem. This scenario has provided us with the impetus to look into herbal remediation, especially toward blocking the action of cholera toxin (CT). Our studies were undertaken to determine the antidiarrheal potential of 6-gingerol (6G) on the basis of its effect on CT, the virulence factor secreted by V. cholerae. We report here that 6G binds to CT, hindering its interaction with the GM1 receptor present on the intestinal epithelial cells. The 50% inhibitory concentration (IC50) was determined to be 10 μg/ml. The detailed mechanistic study was conducted by enzyme-linked immunosorbent assay (ELISA), fluorescence spectroscopy, and isoelectric focusing. These results were validated with in vitro studies performed with the CHO, HeLa, and HT-29 cell lines, whereas a rabbit ileal loop assay was done to estimate the in vivo action, which confirms the efficacy of 6G in remediation of the choleragenic effects of CT. Thus, 6G can be an effective adjunctive therapy with oral rehydration solution for severe CT-mediated diarrhea.

A ClpB chaperone knockout mutant of Mesorhizobium ciceri shows a delay in the root nodulation of chickpea plants

Several molecular chaperones are known to be involved in bacteria stress response. To investigate the role of chaperone ClpB in rhizobia stress tolerance as well as in the rhizobia-plant symbiosis process, the clpB gene from a chickpea microsymbiont, strain Mesorhizobium ciceri LMS-1, was identified and a knockout mutant was obtained. The ClpB knockout mutant was tested to several abiotic stresses, showing that it was unable to grow after a heat shock and it was more sensitive to acid shock than the wild-type strain. A plant-growth assay performed to evaluate the symbiotic performance of the clpB mutant showed a higher proportion of ineffective root nodules obtained with the mutant than with the wild-type strain. Nodulation kinetics analysis showed a 6- to 8-day delay in nodule appearance in plants inoculated with the ΔclpB mutant. Analysis of nodC gene expression showed lower levels of transcript in the ΔclpB mutant strain. Analysis of histological sections of nodules formed by the clpB mutant showed that most of the nodules presented a low number of bacteroids. No differences in the root infection abilities of green fluorescent protein-tagged clpB mutant and wild-type strains were detected. To our knowledge, this is the first study that presents evidence of the involvement of the chaperone ClpB from rhizobia in the symbiotic nodulation process.

Transcriptional analysis of major chaperone genes in salt-tolerant and salt-sensitive mesorhizobia

Salinity is an important abiotic stress that limits rhizobia-legume symbiosis, affecting plant growth, thus reducing crop productivity. Our aims were to evaluate the tolerance to salinity of native chickpea rhizobia as well as to investigate the expression of chaperone genes groEL, dnaKJ and clpB in both tolerant and sensitive isolates. One hundred and six native chickpea mesorhizobia were screened for salinity tolerance by measuring their growth with 1.5% and 3% NaCl. Most isolates were salt-sensitive, showing a growth below 20% compared to control. An association between salt tolerance and province of origin of the isolates was found. The transcriptional analysis by northern hybridization of chaperone genes was performed using tolerant and sensitive isolates belonging to different Mesorhizobium species. Upon salt shock, most isolates revealed a slight increase in the expression of the dnaK gene, whereas the groESL and clpB expression was unchanged or slightly repressed. No clear relationship was found between the chaperone genes induction and the level of salt tolerance of the isolates. This is the first report on transcriptional analysis of the major chaperones genes in chickpea mesorhizobia under salinity, which may contribute to a better understanding of the mechanisms that influence rhizobia salt tolerance.

Termination of Vibrio cholerae virulence gene expression is mediated by proteolysis of the major virulence activator, ToxT

Vibrio cholerae is the causative agent of cholera, a severe diarrhoeal illness. V. cholerae produces two major virulence factors: the cholera toxin, which directly causes diarrhoea, and the toxin-coregulated pilus, which is required for intestinal colonization. Production of these virulence factors is dependent on the major virulence regulator, ToxT. Under virulence-inducing growth conditions, transcription factors ToxR and TcpP initially activate transcription of toxT. However, once ToxT has been expressed, it produces more of itself independent of ToxR and TcpP by activating transcription of the long tcpA operon, within which toxT is located. It is known that V. cholerae terminates virulence gene expression prior to escape from the host, but it is unknown how this ToxT-positive feedback loop is broken, an essential step in terminating virulence gene expression. To better understand how ToxT protein activity is regulated, we monitored ToxT accumulation and activity under virulence-inducing and -repressing growth conditions. Our results suggest that ToxT protein undergoes proteolytic degradation to terminate virulence gene expression. This directed degradation of ToxT supports a model for terminating V. cholerae virulence gene expression late in infection, with both ToxT and TcpP undergoing proteolysis prior to escape from the host.

Acid stress response in enteropathogenic gammaproteobacteria: an aptitude for survivalThis paper is one of a selection of papers published in this special issue entitled “Canadian Society of Biochemistry, Molecular & Cellular Biology 52nd Annual Meeting — Protein Folding: Principles and Diseases” and has undergone the Journal's usual peer review process

Enteric bacteria such as Escherichia coli have acquired a wide array of acid stress response systems to counteract the extreme acidity encountered when invading the host’s digestive or urinary tracts. These acid stress response systems are both enzyme and chaperone based. The 3 main enzyme-based acid resistance pathways are glutamate-, arginine-, and lysine-decarboxylase pathways. They are under a complex regulatory network allowing the bacteria to fine tune its response to the external environment. HdeA and HdeB are the main chaperones involved in acid stress response. The decarboxylase systems are also found in Vibrio cholera, Vibrio vulnifus, Shigella flexneri, and Salmonella typhimurium, although some differences exist in their functional mechanism and regulation.

Differentially expressed genes after hyper- and hypo-salt stress in the halophilic archaeonMethanohalophilus portucalensis

Methanohalophilus portucalensis FDF1 can grow over a range of external NaCl concentrations, from 1.2 to 2.9 mol/L. Differential gene expression in response to long-term hyper-salt stress (3.1 mol/L of NaCl) and hypo-salt stress (0.9 mol/L of NaCl) were compared by differential display RT-PCR. Fourteen differentially expressed genes responding to long-term hyper- or hypo-salt stress were detected, cloned, and sequenced. Several of the differentially expressed genes were related to the unique energy-acquiring methanogenesis pathway in this organism, including the transmembrane protein MttP, cobalamin biosynthesis protein, methenyl-H4MPT cyclohydrolase and monomethylamine methyltransferase. One signal transduction histidine kinase was identified from the hyper-salt stress cultures. Moreover, 3 known stress-response gene homologues — the DNA mismatch repair protein, MutS, the universal stress protein, UspA, and a member of the protein-disaggregating multichaperone system, ClpB — were also detected. The transcriptional analysis of these long-term salt stress response and adaptation-related genes for cells immediately after salt stress indicated that the expression of the energy metabolism genes was arrested during hyper-salt shock, while the chaperone clpB gene was stimulated by both hypo- and hyper-salt shock.

Differential modulation of NF-κB-mediated pro-inflammatory response in human intestinal epithelial cells by cheY homologues of Vibrio cholerae

Vibrio cholerae, the etiological agent of cholera, colonizes the small intestine, produces an enterotoxin and causes acute inflammatory response at intestinal epithelial surface. Chemotaxis and motility greatly influence the infectivity of V. cholerae although the role of chemotaxis genes in V. cholerae pathogenesis is less well understood. Four cheY genes are present in three clusters in the complete genome sequence of V. cholerae. A less motile and less adherent mutant was generated by inactivation of cheY-3 (O395Y3N) or cheY-4 (O395Y4N) whereas alterations in motility or adherence were not observed for cheY-1 (O395Y1N) or cheY-2 (O395Y2N) insertional mutants. In contrast to O395Y1N and O395Y2N, O395Y3N and O395Y4N showed reduced cholera toxin production compared to wild-type in vitro. Infection of the human intestinal epithelial cell line Int407 with O395Y3N and O395Y4N caused reduced secretion of interleukin (IL)-1a, IL-6, tumor necrosis factor (TNF-a) and monocyte chemotactic protein-1 (MCP-1) compared to wild-type and was associated with delayed activation of nuclear factor kappa B (NF-κB) p65 and its co-activator cAMP response element binding protein (CREB). Further, the absence of nuclear translocation of NF-κB p50 subunit upon infection with O395Y3N or O395Y4N and its reversal upon complementation indicates the involvement of cheY-3 and cheY-4 in V. cholerae-induced pro-inflammatory response in the INT407 cell line.

Role of melanin pigment in expression of Vibrio cholerae virulence factors

We identified the mutated gene locus in a pigment-overproducing Vibrio cholerae mutant of strain A1552. The deduced gene product is suggested to be an oxidoreductase based on partial homology to putative homogentisate 1,2-dioxygenase in Pseudomonas aeruginosa and Mesorhizobium loti, and we propose that the gene VC1345 in the V. cholerae genome be denoted hmgA in accordance with the nomenclature for other species. The hmgA::mini-Tn5 mutant showed a nonpigmented phenotype after complementation with a plasmid clone carrying the WT hmgA(+) locus. Microarray transcription analysis revealed that expression of hmgA and the neighboring genes encoding a postulated two-component sensor system was growth phase dependent. Results from quantitative reverse transcription-PCR analysis showed that hmgA operon expression was reduced in the rpoS mutant, but pigment production by the WT V. cholerae or the hmgA mutant was not detectably influenced by the stationary-phase regulator RpoS. The pigmented mutant showed increased UV resistance in comparison with the WT strain. Interestingly, the pigment-producing mutant expressed more toxin-coregulated pilus and cholera toxin than WT V. cholerae. Moreover, the hmgA mutant showed a fivefold increase in the ability to colonize the intestines of infant mice. A possible mechanism by which pigment production might cause induction of the ToxR regulon due to generation of hydrogen peroxide was supported by results from tests showing that externally supplied H(2)O(2) led to higher TcpA levels. Taken together, our findings suggest that melanin pigment formation may play a role in V. cholerae virulence factor expression.

Identification of a new RTX-like gene cluster in Vibrio cholerae

A gene cluster containing two genes in tandem has been identified in Vibrio cholerae ElTor N16961. Each has more than one cadherin domain and is homologous to the RTX toxin family and was common in various V. cholerae strains. Insertional mutagenesis demonstrated that each gene has a role in Hep-2 cell rounding, hemolytic activity towards human and sheep RBCs and biofilm formation. The mutants showed reduced adherence to intestinal epithelial cells as well as reduction of in vivo colonization in suckling mice. These two genes thus code for RTX-like toxins in V. cholerae and are associated with the pathogenecity of this organism.

The HcaR regulatory protein of Photorhabdus luminescens affects the production of proteins involved in oxidative stress and toxemia

Comparison of the proteomes of wild-type Photorhabdus luminescens and its hcaR derivative, grown in insect hemolymph, showed that hcaR disruption decreased the production of toxins (tcdA1, mcf, and pirAB) and proteins involved in oxidative stress response (SodA, AhpC, Gor). The disruption of hcaR did not affect growth rate in insects, but did delay the virulence of P. luminescens in Bombyx mori and Spodoptera littoralis larvae. This delayed virulence was associated with a lower toxemia rather than delay in bacteremia. The disruption of hcaR also increased bacterial sensitivity to hydrogen peroxide. A sodA mutant and an hcaR mutant had similar phenotypes in terms of sensitivity to hydrogen peroxide, virulence, toxin gene expression, and growth rate in insects. Thus, the two processes affected by hcaR disruption - toxemia and oxidative stress response - appear to be related. Besides, expression of toxin genes tcdA1, mcf, and pirAB was decreased by paraquat challenge. We provide here the first demonstration of the importance of toxemia for P. luminescens virulence. Our results also highlight the power of proteomic analysis for detecting unexpected links between different, concomitant processes in bacteria.

The heat-shock protein ClpB of Francisella tularensis is involved in stress tolerance and is required for multiplication in target organs of infected mice

Intracellular bacterial pathogens generally express chaperones such as Hsp100s during multiplication in host cells, allowing them to survive potentially hostile conditions. Francisella tularensis is a highly infectious bacterium causing the zoonotic disease tularaemia. The ability of F. tularensis to multiply and survive in macrophages is considered essential for its virulence. Although previous mutant screens in Francisella have identified the Hsp100 chaperone ClpB as important for intracellular survival, no detailed study has been performed. We demonstrate here that ClpB of F. tularensis live vaccine strain (LVS) is important for resistance to cellular stress. Promoter analysis shows that the transcriptional start is preceded by a sigma32-like promoter sequence and we demonstrate that expression of clpB is induced by heat shock. This indicates that expression of clpB is dependent on the heat-shock response mediated by sigma32, the only alternative sigma-factor present in Francisella. Our studies demonstrate that ClpB contributes to intracellular multiplication in vitro, but is not essential. However, ClpB is absolutely required for Francisella to replicate in target organs and induce disease in mice. Proteomic analysis of membrane-enriched fractions shows that five proteins are recovered at lower levels in the mutant strain. The crucial role of ClpB for in vivo persistence of Francisella may be linked to its assumed function in reactivation of aggregated proteins under in vivo stress conditions.

Type VI secretion system translocates a phage tail spike-like protein into target cells where it cross-links actin

Genes encoding type VI secretion systems (T6SS) are widely distributed in pathogenic Gram-negative bacterial species. In Vibrio cholerae, T6SS have been found to secrete three related proteins extracellularly, VgrG-1, VgrG-2, and VgrG-3. VgrG-1 can covalently cross-link actin in vitro, and this activity was used to demonstrate that V. cholerae can translocate VgrG-1 into macrophages by a T6SS-dependent mechanism. Protein structure search algorithms predict that VgrG-related proteins likely assemble into a trimeric complex that is analogous to that formed by the two trimeric proteins gp27 and gp5 that make up the baseplate "tail spike" of Escherichia coli bacteriophage T4. VgrG-1 was shown to interact with itself, VgrG-2, and VgrG-3, suggesting that such a complex does form. Because the phage tail spike protein complex acts as a membrane-penetrating structure as well as a conduit for the passage of DNA into phage-infected cells, we propose that the VgrG components of the T6SS apparatus may assemble a "cell-puncturing device" analogous to phage tail spikes to deliver effector protein domains through membranes of target host cells.

Transcriptional upregulation of inflammatory cytokines in human intestinal epithelial cells following Vibrio cholerae infection

Coordinated expression and upregulation of interleukin-1alpha, interleukin-1beta, tumor necrosis factor-alpha, interleukin-6, granulocyte-macrophage colony-stimulating factor, interleukin-8, monocyte chemotactic protein-1 (MCP-1) and epithelial cell derived neutrophil activator-78, with chemoattractant and proinflammatory properties of various cytokine families, were obtained in the intestinal epithelial cell line Int407 upon Vibrio cholerae infection. These proinflammatory cytokines also showed increased expression in T84 cells, except for interleukin-6, whereas a striking dissimilarity in cytokine expression was observed in Caco-2 cells. Gene expression studies of MCP-1, granulocyte-macrophage colony-stimulating factor, interleukin-1alpha, interleukin-6 and the anti-inflammatory cytokine transforming growth factor-beta in Int407 cells with V. cholerae culture supernatant, cholera toxin, lipopolysaccharide and ctxA mutant demonstrated that, apart from cholera toxin and lipopolysaccharide, V. cholerae culture supernatant harbors strong inducer(s) of interleukin-6 and MCP-1 and moderate inducer(s) of interleukin-1alpha and granulocyte-macrophage colony-stimulating factor. Cholera toxin- or lipopolysaccharide-induced cytokine expression is facilitated by activation of nuclear factor-kappaB (p65 and p50) and cAMP response element-binding protein in Int407 cells. Studies with ctxA mutants of V. cholerae revealed that the mutant activates the p65 subunit of nuclear factor-kappaB and cAMP response element-binding protein, and as such the activation is mediated by cholera toxin-independent factors as well. We conclude that V. cholerae elicits a proinflammatory response in Int407 cells that is mediated by activation of nuclear factor-kappaB and cAMP response element-binding protein by cholera toxin, lipopolysaccharide and/or other secreted products of V. cholerae.

Regulation of Helicobacter pylori cagA expression in response to salt

Helicobacter pylori infection and a high dietary salt intake are risk factors for the development of gastric adenocarcinoma. In this study, we tested the hypothesis that high salt concentrations might alter gene expression in H. pylori. Transcriptional profiling experiments indicated that the expression of multiple H. pylori genes, including cagA, was regulated in response to the concentrations of sodium chloride present in the bacterial culture medium. Increased expression of cagA in response to high salt conditions was confirmed by the use of transcriptional reporter strains and by immunoblotting. H. pylori CagA is translocated into gastric epithelial cells via a type IV secretion pathway, and on entry into target cells, CagA undergoes tyrosine phosphorylation and causes multiple cellular alterations. Coculture of gastric epithelial cells with H. pylori grown under high salt conditions resulted in increased tyrosine-phosphorylated CagA and increased secretion of interleukin-8 by the epithelial cells compared with coculture of the cells with H. pylori grown under low salt conditions. Up-regulation of H. pylori cagA expression in response to high salt concentrations may be a factor that contributes to the development of gastric adenocarcinoma.