Pathoadaptive Conditional Regulation of the Type VI Secretion System in Vibrio cholerae O1 Strains
ABSTRACTThe most recently discovered secretion pathway in Gram-negative bacteria, the type VI secretion system (T6SS), is present in many species and is considered important for the survival of non-O1 non-O139Vibrio choleraein aquatic environments. Until now, it was not known whether there is a functionally active T6SS in wild-typeV. choleraeO1 strains, the cause of cholera disease in humans. Here, we demonstrate the presence of a functionally active T6SS in wild-typeV. choleraeO1 strains, as evidenced by the secretion of the T6SS substrate Hcp, which required several gene products encoded within the putativevasgene cluster. Our analyses showed that the T6SS of wild-typeV. choleraeO1 strain A1552 was functionally activated when the bacteria were grown under high-osmolarity conditions. The T6SS was also active when the bacteria were grown under low temperature (23°C), suggesting that the system may be important for the survival of the bacterium in the environment. A test of the interbacterial virulence ofV. choleraestrain A1552 against anEscherichia coliK-12 strain showed that it was strongly enhanced under high osmolarity and that it depended on thehcpgenes. Interestingly, we found that the newly recognized osmoregulatory protein OscR plays a role in the regulation of T6SS gene expression and secretion of Hcp fromV. choleraeO1 strains.