Calcium promotes exopolysaccharide phase variation and biofilm formation of the resulting phase variants in the human pathogen <i>Vibrio vulnificus</i>

Abstract

Vibrio vulnificus is a Gram-negative bacterium found in estuaries and coastal waters and is associated with human disease caused by ingestion of raw shellfish. Pathogenesis is directly related to the presence of capsular polysaccharide (CPS). Encapsulated virulent strains exhibit an opaque colony phenotype, while unencapsulated attenuated strains appear translucent. A third colony type, rugose, is caused by expression of rugose extracellular polysaccharide (rEPS) and forms robust biofilms. Vibrio vulnificus undergoes phase variation associated with altered levels of CPS and rEPS, and we show here that calcium (Ca²(+) ) significantly increases the rate of CPS and rEPS phase variation in this species. Interestingly, multiple phenotypic responses to increased [Ca²(+) ] were observed among strains, which suggests the existence of underlying cognate genetic or epigenetic differences. Certain translucent isolates contained deletions at the group I CPS operon, inferring increased [Ca²(+) ] upregulates existing phase variation mechanisms. Expanding on a previous observation (Kierek and Watnick, Proc. Natl. Acad. Sci. USA 100: 14357-14362, 2003), increased [Ca²(+) ] also enhanced biofilm formation for all phase variants. Our results show that Ca²(+) promotes both polysaccharide phase variation and biofilm formation of the resulting phase variants, thereby likely serving a dual role in persistence of V. vulnificus in the environment.