ABSTRACT
Vibrio cholerae, the agent of cholera, is a normal inhabitant of aquatic environments, in which it survives under a wide range of conditions of pH and salinity. In this work, we identified the nhaA gene in a wild-type epidemic strain of V. cholerae O1. nhaA encodes a protein of 382 amino acids that is very similar to the proteins NhaA of Vibrio parahaemolyticus, Vibrio alginolyticus ( approximately 87% identity), and Escherichia coli (56% identity). V. cholerae NhaA complements an E. coli nhaA mutant, enabling it to grow in 700 mM NaCl, pH 7.5, indicating functional homology to E. coli NhaA. However, unlike E. coli, the growth of a nhaA-inactivated mutant of V. cholerae was not restricted at various pH and NaCl concentrations, although it was inhibited in the presence of 120 mM LiCl at pH 8.5. Nevertheless, using a nhaA'-lacZ transcriptional fusion, we observed induction of nhaA transcription by Na(+), Li(+), and K(+). These results strongly suggest that NhaA is an Na(+)/H(+) antiporter contributing to the Na(+)/H(+) homeostasis of V. cholerae. nhaA-related sequences were detected in all strains of V. cholerae from the various serogroups. This gene is presumably involved in the survival and persistence of free-living bacteria in their natural habitat.