HIGH-LEVEL PENICILLIN RESISTANCE AND PENICILLIN-GENTAMICIN SYNERGY INENTEROCOCCUS-FAECIUM

Thirty-seven Enterococcus faecium strains with different levels of pen icillin susceptibility were studied in time-kill experiments with a fi xed concentration (5 mug/ml) of gentamicin combined with different pen icillin concentrations (6 to 600 mug/ml). Synergy was defined as a rel ative decrease in counts of greater than 2 log10 CFU per milliliter af ter 24 h of incubation when the combination of the antibiotics was com pared with its most active component alone. The minimal synergistic pe nicillin concentrations found were 6 mug/ml for 16 of 16 strains for w hich penicillin MICs were less-than-or-equal-to 25 mug/ml, 20 to 100 m ug/ml for 14 of 17 strains for which penicillin MICs were 50 to 200 mu g/ml, and 200 to 500 mug/ml for 4 of 4 strains for which MICs penicill in were >200 mug/ml. Penicillin-gentamicin synergy was observed even i n high-level penicillin-resistant E. faecium strains at penicillin con centrations close to one-half the penicillin MIC. The possibility of t reating infections caused by high-level penicillin-resistant E. faeciu m strains with penicillin-gentamicin combinations in particular cases may depend on the penicillin levels attainable in vivo.