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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.

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.

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.

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., i Karolis Vaitkevicius diss. utgör detta delarbete 2 med titel: Purification and characterization of the metalloprotease PrtV from Vibrio cholerae.

Manuscript in preparation.

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.

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.

Background: Gram-negative bacteria release large amounts of outer membrane vesicles (OMVs) during normal growth. OMVs from pathogenic bacteria are known to carry different biologically active toxins and enzymes into the surrounding environment. We hypothesized that OMVs may therefore be able to mediate the transport of bacterial products into host cells. We present here an analysis of the V. cholerae OMV-associated virulence factor PrtV. Methodology/Principal Findings: We observed that PrtV, a M6 family, zinc-binding metalloprotease, is secreted from V. cholerae wild type strain C6706 into the culture supernatant in association with OMVs. The association of PrtV with OMVs was determined by immunoblotting and electron microscopy using immunogold labeling. In addition, we observed that the PKD-domain(s) of PrtV has a role in the protein’s association with OMVs. We also demonstrated that OMV-associated PrtV was biologically active because HCT8 cells treated with OMVs from the wild type V. cholerae strain C6706 exhibited altered morphology, whereas cells treated with OMVs from the prtV isogenic mutant showed no morphological changes. Moreover, our data suggest that OMV-associated PrtV might be transported into target eukaryotic cells by a vesicle fusion mechanism in association with lipid raft microdomains in the plasma membrane. Conclusion/Significance: Our findings suggest that OMVs can deliver biologically active PrtV into target host cells.