Only two out of more than 200 V. cholerae serogroups, classified on the basis of LPS structure, are associated with epidemic or pandemic cholera. These toxigenic serogroups carry phage-derived pathogenicity islands coding for the main virulence factors for establishment of cholera disease – cholera toxin (CTX) and toxin-coregulated pilus (TCP). The latter also serves as a bacterial surface receptor for the CTXΦ – the filamentous phage which carries the cholera toxin genes into otherwise harmless to human, environmental bacterium V. cholerae. In its natural aquatic habitat V. cholerae is subject to predator grazing, bacteriophage killing, temperature and pH changes, seasonality of plankton blooms and other environmental factors. Therefore understanding V. cholerae pathogenic and virulence potential requires the knowledge of its interaction not only with human host but also members of aquatic environment and environmental factors.
V. cholerae is capable of killing the nematode Caenorhabditis elegans. Using a reverse genetics approach, we demonstrated that the quorum sensing regulated protease PrtV is essential for this killing. Other proteases did not seem to contribute to virulence in this model. The data from this study suggest that the PrtV could be important to V. cholerae in its natural niche for its resistance to the grazing predators.
The PrtV protease belongs to an M6 family of metallopeptidases which is represented by an Immune Inhibitor A protease from the insect killing bacterium Bacillus thuringiensis. To characterize the protease in more detail, the PrtV was cloned, overexpressed in V. cholerae and purified from the culture supernatant. The enzyme was calcium stabilized and inhibited by metal ion chelators. In tests with in vitro cultured cells of the human intestinal cell line HCT8, the PrtV protein showed a cytotoxic effect leading to cell detachment and death. Using human blood plasma as a source of potential substrates, and by tests with purified candidate substrate proteins, we have identified fibrinogen (all α, β and γ chains), fibronectin and plasminogen to be degraded by the protease. Additionally, PrtV was found to alter the stability of V. cholerae cytolysin implicating its role in modulation of the reactogenicity of V. cholerae secreted factors.
Pigmentation has been considered to be important in microbial pathogenesis because it has been associated with virulence in many microorganisms. Using transposon mutagenesis we identified the mutated locus of a pigment producing V. cholerae strain to encode a gene of a tyrosine catabolic pathway. The mutation in a putative homogentisate 1,2-dioxigenase gene lead to accumulation of homogentisic acid, its spontaneous oxidation and formation of a dark pigment. The pigment producing strain was altered in its ability to survive UV exposure and H2O2 stress, and was more efficient in colonizing the suckling mouse intestine compared to the wild type strain. Under the in vitro growth conditions the major virulence factor TcpA and CT expression was found to be somewhat enhanced too.