Protection of Enterococcus faecalis in mixed cultures with carbapenemase-producing Escherichia coli and Bacteroide fragilis: effect of the bacterial load.

INTRODUCTION: This study explores effects of pH and inoculum size on imipenem versus tigecycline activity against E. coli, B. fragilis and E. faecalis, both in individual and mixed cultures. METHODS: MIC/MBCs (mg/L) of tigecycline and imipenem were 0.12/≥ 16 and 4/4 for E. coli, 0.12/0.5 and ≥ 16/≥ 16 for B. fragilis, and 0.12/≥ 16 and 2/≥ 16 for E. faecalis, respectively. Killing curves in supplemented Brucella broth were performed at pH 7 or 5.8, with two final inocula (≈ 105 or ≈ 107 cfu/ml) of each isolate (individual cultures) and with 1:1:1 mixed inocula. Tubes were 48 h incubated at 37 oC in anaerobiosis. Final concentrations (estimated concentrations in colon) were 1.50 mg/L for tigecycline and 26.40 mg/L for imipenem, with antibiotic-free curves as controls. Experiments were performed in triplicate. RESULTS: Imipenem showed inoculum effect against E.coli and B. fragilis, with reductions in initial inocula in experiments with standard inocula contrasting with increases in experiments with high inocula (both individual and mixed cultures). Against E. faecalis no inoculum effect for imipenem was observed in individual cultures, with marked reductions in initial inocula regardless inoculum size. However in mixed experiments the indirect protection of E. faecalis by the two gramnegatives resulted in bacterial regrowth. This protection was inoculum-dependant since it occurred with high but not with standard inocula. Tigecycline reduced initial inocula of the three isolates regardless culture type (individual/mixed) or experimental conditions (pH/inocula size), with lower reductions for the tolerant E. faecalis. CONCLUSION: Carbapenemase activity was inoculum-dependant for self-protection and indirect protection of E. faecalis.