Previously, we showed that Enterococcus faecium clade B strains outcompeted health care-associated clade A1 strains in murine gastrointestinal colonization. Here, parenterally administered piperacillin-tazobactam and ceftriaxone significantly promoted colonization by clade A1 over clade B strains except that ceftriaxone, at the dose used, did not favor the least β-lactam-resistant A1 strain. The advantage that β-lactam administration gives to more highly ampicillin-resistant E. faecium over ampicillin-susceptible strains mirrors what occurs in hospitalized patients administered these antibiotics.
Keywords: E. faecium; ampicillin resistant; ceftriaxone; mouse GI colonization; piperacillin-tazobactam.