Increased Intracellular Cyclic di-AMP Levels Sensitize Streptococcus gallolyticus subsp. gallolyticus to Osmotic Stress and Reduce Biofilm Formation and Adherence on Intestinal Cells.

Cyclic di-AMP is a recently identified second messenger exploited by a number of Gram-positive bacteria to regulate important biological processes. Here, we studied the phenotypic alterations induced by the increased intracellular c-di-AMP levels in , an opportunistic pathogen responsible for septicemia and endocarditis in the elderly. We report that an c-di-AMP phosphodiesterase knockout mutant, which displays a 1.5-fold higher intracellular c-di-AMP levels than the parental strain UCN34, is more sensitive to osmotic stress and is morphologically smaller than the parental strain. Unexpectedly, we found that a higher level of c-di-AMP reduced biofilm formation of on abiotic surfaces and reduced adherence and cell aggregation on human intestinal cells. A genome-wide transcriptomic analysis indicated that c-di-AMP regulates many biological processes in , including the expression of various ABC transporters and disease-associated genes encoding bacteriocin and Pil3 pilus. Complementation of the in-frame deletion mutant with a plasmid carrying in from its native promoter restored bacterial morphology, tolerance to osmotic stress, biofilm formation, adherence to intestinal cells, bacteriocin production, and Pil3 pilus expression. Our results indicate that c-di-AMP is a pleiotropic signaling molecule in that may be important for pathogenesis. is an opportunistic pathogen responsible for septicemia and endocarditis in the elderly and is also strongly associated with colorectal cancer. can form biofilms, express specific pili to colonize the host tissues, and produce a specific bacteriocin allowing killing of commensal bacteria in the murine colon. Nevertheless, how the expression of these colonization factors is regulated remains largely unknown. Here, we show that c-di-AMP plays pleiotropic roles in , controlling the tolerance to osmotic stress, cell size, biofilm formation on abiotic surfaces, adherence and cell aggregation on human intestinal cells, expression of Pil3 pilus, and production of bacteriocin. This study indicates that c-di-AMP may constitute a key regulatory molecule for host colonization and pathogenesis.