Enterococcus faecalis autolysin EpaU binds the enterococcal polysaccharide antigen via its teichoic acid-like repeats and modulates c-di-AMP signaling

The cell wall of the gram-positive bacterium Enterococcus faecalis is decorated with the enterococcal polysaccharide antigen (EPA), consisting of a core rhamnan backbone linked covalently with a strain-variable teichoic acid-like (TA) polymer. Current models propose that the TA decoration is a repeating polymer composed of two alternating subunits, designated TAI and TAII, which are attached to the rhamnan core via a mild-acid labile phosphodiester bond from the initiating TAI subunit. In this study, we characterize the EpaU autolysin encoded within the EPA biosynthetic gene cluster. We demonstrate that the cell wall-binding domain of EpaU associates with the intact TA domains of EPA synthesized with the aid of the glycosyltransferases EpaR and EpaX. We further show that EpaU is a potent autolysin that binds generally over the E. faecalis cell surface, suggesting that it functions as a remodeling peptidoglycan hydrolase. The absence of EpaU leads to increased ampicillin resistance and elevated intracellular levels of the second messenger c-di-AMP. These data suggest that E. faecalis possesses a mechanism that senses the integrity of the peptido glycan meshwork and employs c-di-AMP to regulate cell turgor, potentially altering the antibiotic resistance.