Impact of dialysis fluid on the susceptibility of Staphylococcus epidermidis biofilms to rifampin.

Routine laboratory testing of the susceptibility of infecting strains of S. epidermidis to antibiotics is based on the assumptions that the infection is in contact with the stable environment of blood or tissue fluid and that the bacteria are in a susceptible phase of growth. In infections of the continuous ambulatory peritoneal dialysis (CAPD) intraperitoneal catheter, the bacteria are commonly in the resistant biofilm phase of growth, are applied to the surface of the catheter, and are exposed to the nonphysiological impact of peritoneal dialysis solutions. The possible impact of these two variables is not examined in routine testing of antibiotic susceptibility. We describe an assay of antibacterial action on a strain of S. epidermidis in the biofilm phase, exposed to a neutral control environment and to dialysis solutions, both fresh and spent. Rifampin, which has an incomplete action in a neutral control environment, was used as a sensitive indicator of possible synergistic or antagonistic interaction of the biofilm bacteria with the dialysis environments. We demonstrated that peritoneal dialysis solutions (1.5% dextrose), both fresh and spent, were synergistic with rifampin action against S. epidermidis biofilms, converting the incomplete killing action to a total bactericidal outcome, clinically essential for cure. Fresh dialysis solution was more active than spent, but in both instances the synergy was complete, indicating that the crucial complementation of the bactericidal action of rifampin was independent of pH. These findings have implications in the management of catheter-associated infections due to rifampin-susceptible strains of S. epidermidis.