Antibiotics and gastrointestinal colonization by vancomycin-resistant enterococci

Although several classes of antimicrobial agents have been associated with colonization or infection with glycopeptide-resistant enterococci (GRE) in individual clinical studies, the agents most commonly implicated are extended-spectrum cephalosporins and compounds with potent activity against anaerobic bacteria, including ticarcillin-clavulanic acid. In some clinical studies, formulary alterations designed to minimize the use of extended-spectrum cephalosporins or ticarcillin-clavulanic acid have resulted in significant decreases in colonization and infection by GRE. Experimental data using a mouse model of GRE gastrointestinal colonization indicate that persistence of high-level GRE colonization of the mouse gastrointestinal tract is promoted by exposure to agents with potent activity against anaerobic bacteria, suggesting that reduction of competing flora is the major factor leading to persistence of high-level colonization. One study performed in humans is consistent with this model and suggests that high levels of colonization may promote spread of resistant organisms in the nosocomial setting. Establishing colonization with GRE in uncolonized mice correlates with exposure to agents that are (a) secreted into the bile in significant concentrations and (b) have negligible activity against the colonizing enterococcal strain. Differences between piperacillin-tazobactam and ceftriaxone in the establishment model can be attributed directly to differences in their anti-enterococcal activity. Modification of antimicrobial prescribing practices may play an important role in facilitating successful infection control efforts to limit GRE in the nosocomial setting.