Umeå University's logo

Vibrio cholerae, the causal agent of cholera typically encodes two critical virulence factors: cholera toxin (CT), which is primarily responsible for the diarrhoeal purge, and toxin-co-regulated pilus (TCP), an essential colonisation factor. Nontoxigenic strains expressing TCP can efficiently acquire the CT gene through lysogenic conversion with CTXΦ, a filamentous phage that encodes CT and uses TCP as a receptor.  V. cholerae is a Gram-negative bacterium and a natural inhabitant of estuarine and coastal waters throughout both temperate and tropical regions of the world. In the aquatic environment, V. cholerae encounters several environmental stresses, such as change in salinity, UV stress, nutrient limitation, temperature fluctuations, viral infections and protozoan predation. To fully understand the pathogenic and virulence potential of V. cholerae, knowledge is required of its interactions with, not only human, but also environmental factors. By using the nematode Caenorhabditis elegans as host model, we were able to identify a previously uncharacterised protein, the extracellular protease PrtV. PrtV was shown to be required for the killing of. elegans and also necessary for survival from grazing by the ciliate Tetrahymena pyriformis and the flagellate Cafeteria roenbergensis. The PrtV protein, which belongs to a M6 family of metallopeptidases was cloned and purified for further characterisations. The purified PrtV was cytotoxic against the human intestinal cell line HCT8. By using human blood plasma, fibrinogen, fibronectin and plasminogen were identified as candidate substrates for the PrtV protease.

Outer membrane vesicles (OMVs) are released to the surroundings by most Gram-negative bacteria through “bulging and pinching” of the outer membrane.  OMVs have been shown to contain many virulence factors important in pathogenesis. Therefore, we investigated the association of PrtV with OMVs. PrtV was not associated with OMVs from the wild type O1 strain. In contrast, in an LPS mutant lacking two sugar chains in the core oligosaccharide PrtV was found to be associated with the OMVs. The OMV-associated PrtV was shown to be proteolytically and cytotoxically active.

V. cholerae strains are grouped into >200 serogroups. Only the O1 and O139 serogroups have been associated with pandemic cholera, a severe diarrhoeal disease.  All other serogroups are collectively referred to as non-O1 non-O139 V. cholerae. Non-O1 non-O139 V. cholerae can cause gastroenteritis and extraintestinal infections, but unlike O1 and O139 strains of V. cholerae, little is known about the virulence gene content and their potential to become human pathogens. We analysed clinical and environmental non-O1 non-O139 isolates for their putative virulence traits. None of them carry the genes encoding CT or the TCP, but other putative virulence factors were present in these isolates. The incidence of serum resistance was found to vary considerably and was independent of encapsulation. Three strains were strongly serum-resistant, and these same strains could also kill C. elegans.

BACKGROUND: Cytolethal distending toxin (CDT) is one of the well-characterized virulence factors of Campylobacter jejuni, but it is unknown how CDT becomes surface-exposed or is released from the bacterium to the surrounding environment.

RESULTS: Our data suggest that CDT is secreted to the bacterial culture supernatant via outer membrane vesicles (OMVs) released from the bacteria. All three subunits (the CdtA, CdtB, and CdtC proteins) were detected by immunogold labeling and electron microscopy of OMVs. Subcellular fractionation of the bacteria indicated that, apart from the majority of CDT detected in the cytoplasmic compartment, appreciable amounts (20-50%) of the cellular pool of CDT proteins were present in the periplasmic compartment. In the bacterial culture supernatant, we found that a majority of the extracellular CDT was tightly associated with the OMVs. Isolated OMVs could exert the cell distending effects typical of CDT on a human intestinal cell line, indicating that CDT is present there in a biologically active form.

CONCLUSION: Our results strongly suggest that the release of outer membrane vesicles is functioning as a route of C. jejuni to deliver all the subunits of CDT toxin (CdtA, CdtB, and CdtC) to the surrounding environment, including infected host tissue.

BACKGROUND: The type VI secretion system (T6SS) has emerged as a protein secretion system important to several gram-negative bacterial species. One of the common components of the system is Hcp, initially described as a hemolysin co-regulated protein in a serotype O17 strain of Vibrio cholerae. Homologs to V. cholerae hcp genes have been found in all characterized type VI secretion systems and they are present also in the serotype O1 strains of V. cholerae that are the cause of cholera diseases but seemed to have non-functional T6SS.

METHODOLOGY/PRINCIPAL FINDINGS: The serotype O1 V. cholerae strain A1552 was shown to express detectable levels of Hcp as determined by immunoblot analyses using polyclonal anti-Hcp antiserum. We found that the expression of Hcp was growth phase dependent. The levels of Hcp in quorum sensing deficient mutants of V. cholerae were compared with the levels in wild type V. cholerae O1 strain A1552. The expression of Hcp was positively and negatively regulated by the quorum sensing regulators HapR and LuxO, respectively. In addition, we observed that expression of Hcp was dependent on the cAMP-CRP global transcriptional regulatory complex and required the RpoN sigma factor.

CONCLUSION/SIGNIFICANCE: Our results show that serotype O1 strains of V. cholerae do express Hcp which is regarded as one of the important T6SS components and is one of the secreted substrates in non-O1 non-O139 V. cholerae isolates. We found that expression of Hcp was strictly regulated by the quorum sensing system in the V. cholerae O1 strain. In addition, the expression of Hcp required the alternative sigma factor RpoN and the cAMP-CRP global regulatory complex. Interestingly, the environmental isolates of V. cholerae O1 strains that showed higher levels of the HapR quorum sensing regulator in comparison with our laboratory standard serotype O1 strain A1552 where also expressing higher levels of Hcp.

We suggest that VCC is capable of causing an inflammatory response characterized by increased permeability and production of IL-8 and TNF-alpha in tight monolayers. Pure VCC at a concentration of 160 ng/ml caused an inflammatory response that reached the magnitude of that caused by Vibrio-secreted factors, while higher concentrations caused epithelial cell death. The inflammatory response was totally abolished by treatment with PrtV. The findings suggest that low doses of VCC initiate a local immune defense reaction while high doses lead to intestinal epithelial lesions. Furthermore, VCC is indeed a substrate for PrtV and PrtV seems to execute an environment-dependent modulation of the activity of VCC that may be the cause of V. cholerae reactogenicity.

The Vibrio metalloprotease PrtV was purified from the culture supernatant of a Vibrio cholerae derivative that is deficient in several other secreted peptidases, including the otherwise abundant hemagglutinin/protease HapA. The PrtV is synthesized as a 102 kDa protein, but undergoes several N- and C-terminal processing steps during V. cholerae envelope translocation and prolonged incubation. Purified V. cholerae PrtV protease forms of 81 or 73 kDa were stabilized by calcium ions. Removal of calcium resulted in further rapid autoproteolysis. The two major products of autoproteolysis of the PrtV protease were approximately 37 and 18 kDa and could not be separated under non-denaturing conditions, indicating they are interacting domains. In an assay using cultured cells of the human intestinal cell line HCT8, the PrtV protein showed a cytotoxic effect leading to cell death. Using human blood plasma as a source of potential substrates of mammalian origin for the PrtV protease, we found that the extracellular matrix components fibronectin and fibrinogen were degraded by the enzyme. Additional tests with individual protein substrates revealed that plasminogen was also a possible target for the PrtV protease.

Vibrio cholerae is the causal bacterium of the diarrheal disease cholera, and its growth and survival are thought to be curtailed by bacteriovorous predators, e.g., ciliates and flagellates. We explored Caenorhabditis elegans as a test organism after finding that V. cholerae can cause lethal infection of this nematode. By reverse genetics we identified an extracellular protease, the previously uncharacterized PrtV protein, as being necessary for killing. The killing effect is associated with the colonization of bacteria within the Caenorhabditis elegans intestine. We also show that PrtV is essential for V. cholerae in the bacterial survival from grazing by the flagellate Cafeteria roenbergensis and the ciliate Tetrahymena pyriformis. The PrtV protein appears to have an indirect role in the interaction of V. cholerae with mammalian host cells as judged from tests with tight monolayers of human intestinal epithelial cells. Our results demonstrate a key role for PrtV in V. cholerae interaction with grazing predators, and we establish Caenorhabditis elegans as a convenient organism for identification of V. cholerae factors involved in host interactions and environmental persistence.

The ClyA protein is a pore-forming cytotoxin expressed by Escherichia coli and some other enterobacteria. It confers cytotoxic activity toward mammalian cells, but it has remained unknown how ClyA is surface exposed and exported from bacterial cells. Outer-membrane vesicles (OMVs) released from the bacteria were shown to contain ClyA protein. ClyA formed oligomeric pore assemblies in the OMVs, and the cytotoxic activity toward mammalian cells was considerably higher than that of ClyA protein purified from the bacterial periplasm. The redox status of ClyA correlated with its ability to form the oligomeric pore assemblies. In bacterial cells with a defective periplasmic disulphide oxidoreductase system, the ClyA protein was phenotypically expressed in a constitutive manner. The results define a vesicle-mediated transport mechanism in bacteria, and our findings show that the localization of proteins to OMVs directly may contribute to the activation and delivery of pathogenic effector proteins.

The broad group non-O₁ non-O₁₃₉ Vibrio cholerae presumably causes clinical diseases due to properties distinct from V. cholerae O₁ and O₁₃₉ serogroups in which cholera toxin is the hallmark of the infection. In this study, we screened for the presence of virulence-associated factors in V. cholerae non-O₁ non-O₁₃₉ strains isolated in Sweden. Six isolates from patients with different clinical manifestations and two environmental isolates were studied. None of the V. cholerae non-O₁ non-O₁₃₉ isolates carried the ctx or tcpA genes, but gene sequences for other putative virulence factors such as OmpU, cytolysin (VCC) and RTX were present. Significant differences in serum resistance were observed among the isolates independent of their encapsulation. The isolates were tested on Caenorhabditis elegans as an alternative host for analysis of factors associated with protection from natural predator grazing and environmental survival of V. cholerae. Three isolates caused lethality to C. elegans and also showed the strongest ability to resist killing by serum. We also observed that actin cross-linking activity and the level of protease secretion were different among the strains.

Gram negative bacteria produce outer membrane vesicles (OMVs) during normal bacterial growth. Recent studies have shown that OMVs can transport biologically active toxins and enzymes to the environment and into the host. We have initiated analysis of OMV associated proteins in V. cholerae. In this study, V. cholerae wild type strain P27459 and the O-antigen ligase mutant waaL, which lacks the O-antigen of the LPS were analyzed for the OMV associated release of the secreted protease PrtV. OMVs from the waaL mutant showed a higher amount of associated secreted PrtV protein than the OMVs from wild type V. cholerae indicating that LPS might be involved in vesicle association of the PrtV protein. We showed that the OMV associated PrtV protein is biologically active.