β-lactam antibiotics and gastrointestinal colonization with vancomycin-resistant enterococci

We studied the effect of different subcutaneously administered beta-lactam antibiotics on the establishment of gastrointestinal colonization by vancomycin-resistant Enterococcus faecium C68 in a mouse model. Aztreonam, cefazolin, cefepime, and, to a lesser extent, ceftazidime, which neither have significant antienterococcal activity nor are secreted into human bile at high concentrations, did not promote significant vancomycin-resistant enterococci (VRE) colonization. Piperacillin-tazobactam, which has antienterococcal activity and is secreted in human bile at high concentrations, inhibited colonization after limited exposure to the inoculum but was associated with progressively increased VRE colony counts in stool samples after repeated exposure to the VRE inoculum. Ceftriaxone and cefotetan, which lack antienterococcal activity but are secreted into human bile at high concentrations, were associated with rapid and high-level colonization. These data suggest that the risk of VRE colonization varies during exposure to different beta-lactam antimicrobial agents and that the risk is related to biliary concentration and antienterococcal activity of the specific beta-lactam.