Abstract
Enterococci are Gram-positive gastrointestinal tract colonizers of humans and animals. Vancomycin-resistant enterococci (VRE) are important nosocomial pathogens and can cause life-threatening infections. To control hospital-associated infections, skin antisepsis and bathing utilizing chlorhexidine is recommended for VRE patients in acute care hospitals. Previously, we reported that exposure to inhibitory chlorhexidine levels induced the expression of vancomycin resistance genes in VanA-type Enterococcus faecium. However, vancomycin susceptibility actually increased for VanA-type E. faecium in the presence of chlorhexidine. Hence, a synergistic effect of the two antimicrobials was observed. In this study, we tested various models to elucidate the mechanism(s) of synergism between chlorhexidine and vancomycin. We deleted each of the pbp genes from a model VanA-type VRE E. faecium strain. We found that deletion of ddcP, a membrane-bound carboxypeptidase, resulted in partial loss of synergism. Interestingly, addition of excess D-lactate, but not D-alanine, enhanced synergism. Furthermore, we isolated a synergy escaper mutant in E. faecium and utilized whole genome sequencing to determine that a mutation in a gene encoding an ATPase of phosphate-specific transporters (pstB) also resulted in loss of synergism. Our study is significant because understanding the mechanisms for chlorhexidine-induced vancomycin resensitization in VRE could lead to new combinatorial therapeutics to treat VRE infections.