In-vitro activity of imipenem combined with beta-lactam antibiotics for methicillin-resistant Staphylococcus aureus

The in-vitro activity of imipenem combined with beta-lactam antibiotics was studied for 25 strains of methicillin and imipenem-resistant Staphylococcus aureus (MRSA) in comparison with that of fosfomycin combined 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 less than or equal to 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 concentration of sodium chloride in the medium or beta-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 beta-lactam antibiotics were more effective than that of fosfomycin with cefmetazole.