Concentration-dependent synergy and antagonism between cefoperazone and imipenem against methicillin-resistant Staphylococcus aureus.

By the agar dilution checkerboard method, striking synergy between cefoperazone and imipenem was observed with 32 strains of methicillin-resistant Staphylococcus aureus (fractional inhibitory concentration indices, < or = 0.03 to 0.34; mean, 0.12 +/- 0.08). By the double-disk diffusion method, the synergy was confirmed, but about 60% of strains showed antagonism manifest by truncation of the zone of inhibition around cefoperazone. The mechanism may involve auxiliary factors distinct from those essential for the expression of methicillin resistance.