Quorum-regulated biofilms enhance the development of conditionally viable, environmental Vibrio cholerae

The factors that enhance the waterborne spread of bacterial epidemics and sustain the pathogens in nature are unclear. The epidemic diarrheal disease cholera caused by Vibrio cholerae spreads through water contaminated with the pathogen. However, the bacteria exist in water mostly as clumps of cells, which resist cultivation by standard techniques but revive into fully virulent form in the intestinal milieu. These conditionally viable environmental cells (CVEC), alternatively called viable but nonculturable cells, presumably play a crucial role in cholera epidemiology. However, the precise mechanism causing the transition of V. cholerae to the CVEC form and this form's significance in the biology of the pathogen are unknown. Here we show that this process involves biofilm formation that is dependent on quorum sensing, a regulatory response that is controlled by cell density. V. cholerae strains carrying mutations in genes required for quorum sensing and biofilm formation displayed altered CVEC formation in environmental water following intestinal infections. Analysis of naturally occurring V. cholerae CVEC showed that organisms that adopt this quiescent physiological state typically exist as clumps of cells that comprise a single clone closely related to isolates causing the most recent local cholera epidemic. These results support a model of cholera transmission in which in vivo-formed biofilms convert to CVEC upon the introduction of cholera stools into environmental water. Our data further suggest that a temporary loss of quorum sensing due to dilution of extracellular autoinducers confers a selective advantage to communities of V. cholerae by blocking quorum-mediated regulatory responses that would break down biofilms and thus interfere with CVEC formation.