Quorum sensing in Vibrio cholerae

Abstract

Vibrio cholerae is the causative agent of cholera, a disease that still threatens a large proportion of the world's population. The symptoms of cholera are mainly caused by the action of cholera toxin (CT), which is encoded by ctxA and ctxB on the CTX phage. However, the toxin-coregulated pilus (TCP), which is encoded by tcp on the Vibrio pathogenicity island (VPI), is crucial for V. cholerae to establish colonization in vivo. The expression of CT, TCP and several other gene products associated with virulence in V. cholerae is controlled by the transcription factor ToxR and an effector of ToxR function known as ToxS. Although quorum sensing was first discovered more than 25 years ago in Vibrio fischeri and Vibrio harveyi, and regulates the expression of virulence genes in many bacteria such as Pseudomonas aeruginosa, enteropathogenic Escherichia coli (EPEC) and Staphylococcus aureus, it was unclear whether quorum sensing also regulates the expression of virulence genes in V. cholerae. Recently, Zhu et al. [1xQuorum-sensing regulators control virulence gene expression in Vibrio cholerae. Zhu, J. et al. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 3129–3134Crossref | PubMed | Scopus (475)See all References][1] demonstrated that quorum sensing represses the expression of ToxR-regulated virulence genes in V. cholerae.Zhu et al. first determined whether luxO, a shared regulator that integrates two parallel two-component quorum-sensing circuits in V. harveyi, is involved in colonization in vivo, and then compared the gene expression profile of a luxO mutant against wild type using a V. cholerae whole-genome gene array. The authors found that the luxO mutant was profoundly defective in colonization in a mouse model. Additionally, the expression of the ToxR regulon was significantly depressed in the luxO mutant, whereas the expression of hapR, a close homolog of luxR in V. harveyi, was upregulated. These results suggested that quorum sensing affects V. cholerae pathogenesis in vivo, and that LuxO represses hapR. To determine the mechanism of CT repression in the luxO mutant, the authors overexpressed ToxR, TcpP and ToxT in the luxO mutant and found that overexpression of TcpP or ToxT, but not ToxR, restored CT production. This indicates that LuxO regulates the ToxR regulon by downregulating TcpP. Because LuxO represses hapR, the authors tested the possibility that Lux O negatively regulates hapR expression, and HapR in turn represses tcpP expression. Overexpression of HapR does indeed repress the expression of TCP and CT in wild-type V. cholerae, and a luxO/hapR double mutant produced wild-type levels of TCP and CT. This indicates that HapR acts downstream of LuxO to repress tcpP expression, and that this repression leads to repression of the ToxR virulence regulon. Zhu et al. also investigated the kinetics of HapR repression of CT, and found that HapR only repressed CT production at an early stage of growth, that is, at a low cell density. Finally, the authors demonstrated that in V. cholerae LuxO suppresses not only the expression of proteases, including HapA, at an early growth stage, but also motility and the ability to form biofilms. These results indicate that in V. cholerae quorum sensing controls multiple cellular processes and not simply virulence gene expression.The authors also provided a model for quorum-sensing-mediated regulation of V. cholerae virulence. At low cell density, LuxO is active and represses the expression of hapR; this repression activates the expression of TCP and CT, as HapR is a negative regulator of tcpP transcription. By contrast, at high cell density, LuxO is inactive, resulting in the expression of hapR, which, in turn, represses TcpP and the ToxR regulon and activates the expression of the Hap protease. In contrast to other bacterial species in which quorum-sensing activates virulence gene expression at high cell densities, this study provides the first evidence that quorum sensing represses the expression of ToxR-regulated virulence genes in V. cholerae. However, it remains to be determined how quorum signals are integrated to regulate virulence in vivo, and whether virulence genes are responsive to the autoinducers that accumulate at high cell densities.