Analysis of the role of flagellar activity in virulence gene expression inVibrio cholerae

Expression of virulence factors and motility of Vibrio cholerae are intimat ely linked by an as yet uncharacterized mechanism. Several lines of evidenc e indicate that the activity of the flagellum of V. cholerae might have an impact on virulence gene regulation, as alterations of the motility phenoty pe, either by mutation or by inhibitory drugs, result in varied levels of v irulence factor production, The Na+-driven polar flagella of some Vibrio sp ecies are proposed to act as mechanosensors, sensing media viscosity. It ha s been suggested that the V. cholerae flagellum might act as a 'voltmeter', responding to changes in membrane potential, or might sense some environme ntal conditions that lead to the repression of virulence factors in V. chol erae. To test these hypotheses, P-galactosidase levels of several types of non-motile mutant derivatives of a V. cholerae toxT::lacZ reporter strain w ere analysed following changes in media viscosity, membrane potential and o ther environmental conditions. Like the parental strain, the non-motile str ain showed increased toxT:: lacZ expression in high-viscosity media, sugges ting that the sensing of media viscosity does not occur via the flagella. O ther molecules that might be able to detect changes in media viscosity coul d include mechanosensitive (MS) ion channels found in the bacterial membran e. However, a V. cholerae derivative strain mutated in two putative MS chan nels still showed increased toxT::lacZ expression in high-viscosity media, indicating that these putative ion channels of V. cholerae are not involved in the viscosity effect and suggesting an as yet uncharacterized mechanism for sensing of media viscosity. The flagellum does not appear to act as a voltmeter, as beta -galactosidase activities of the non-flagellate derivati ve strain were found to be similar to those of the parental strain after ar tificially changing the sodium membrane bioenergetics. Similarly, several e nvironmental conditions known to reduce toxin expression were equally effec tive in reducing toxT:: lacZ expression in the motile or nonmotile strains. In conclusion, the flagellum of V. cholerae does not act as a mechanosenso r, voltmeter or signal transducer for environmental conditions. Thus, alter native mechanisms for the detection of these conditions must exist that lik ely do not involve the ToxR molecule, as the sensing of all of the tested p arameters occurred when the TcpP/H proteins alone activated the toxT::lacZ reporter gene.