Vibrio cholerae is the causative agent of cholera, a severe and devastating diarrheal disease. V. cholerae lives naturally in various aquatic habitats during interepidemic periods. Recent studies reveal that quorum-sensing systems, which exist in many bacteria and help them monitor their population densities and regulate various cellular functions, control V. cholerae pathogenesis, biofilm formation, and protease production. In this study we surveyed quorum-sensing systems in 16 geographically diverse V. cholerae strains from epidemic-causing O1 and O139 strains as well as non-O1/non-O139 and environmental isolates and discovered an unexpectedly high rate of dysfunctional components. We also found that a functional quorum-sensing system conferred a survival advantage on bacteria in biofilms when the bacteria were exposed to seawater, though quorum sensing was less important to survival in a planktonic state under the same conditions. These findings suggest that variations in quorum-sensing systems are due to environmental selective pressures and might be beneficial to V. cholerae's fitness under certain conditions found in its natural reservoirs.The gram-negative bacterium Vibrio cholerae is the causative agent of cholera, a severe diarrheal disease that affects millions of people and causes well over 100,000 deaths on an annual basis (8). V. cholerae communicates by monitoring its population density and alters its gene expression accordingly. This phenomenon, known as quorum sensing, is highly important for the V. cholerae infectious cycle in humans, controlling phenotypic factors critical to pathogenesis, such as virulence factors, biofilms, and protease production (19,26,32). It has been postulated that biofilms aid V. cholerae in the passage through the gastric barrier of the stomach, while quorum-sensing-controlled detachment from biofilms in the small intestine aids in the colonization of the intestinal epithelium and eventual infection of new hosts (32). In addition, quorum sensing and biofilms have also been implicated in the long-term survival of V. cholerae in its natural marine environment (19,26).Quorum sensing in V. cholerae is controlled by two central regulators, LuxO and HapR. LuxO responds to accumulated molecules in the environment called autoinducers via membrane receptors CqsS and LuxPQ. Two autoinducer molecules are important for quorum sensing in V. cholerae. These two molecules are cholera autoinducer 1 (CAI-1), a molecule of undetermined structure, and autoinducer 2 (AI-2), a furanosyl borate diester also produced by Vibrio harveyi and many other bacteria. At high cell densities and autoinducer concentrations, this system causes LuxO to turn off its repression of hapR (23,25). HapR can then repress the expression of aphA, whose gene products are key activators of virulence regulons (22). HapR also represses the vps (Vibrio polysaccharide synthesis) operon, thus negatively regulating biofilm formation (15,31,32). In addition, HapR directly up-regulates the expression of hapA, which produce...