INVITRO ACTIVITY OF IMIPENEM COMBINED WITH BETA-LACTAM ANTIBIOTICS FOR METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

The in-vitro activity of imipenem combined with β-lactam antibiotics was studied for 25 strains of methicillin and imipenem-resistant Staphylococcus aureus (MRSA) in comparison with that of fosfomycincombined with cefmetazole. Using the chequerboard agar dilution method, strong synergy was seen for all strains for imipenem with cefoperazone, cefotiam, cefpiramide or piperacillin. All fractional inhibitory concentration indices (FIC indices) of these combinations were ≤ 0.12. For the combination of imipenem with cefotiam the synergy was found to be bactericidal, but was not affected by the temperature of incubation, the concentra tion of sodium chloride in the medium or β-lactamase production. However, for the combination of fosfomycin with cefmetazole only 44% of the strains with a mean FIC index of 0.55, showed synergy. The remaining 56% of strains showed either partial synergy (44%) or additive activity (12%). The combinations of imipenem with the β-lactam antibiotics were more effective than that of fosfomycin with cefmetazo. © 1991 The British Society for Antimicrobial Chemotherapy.