CONCENTRATION-DEPENDENT SYNERGY AND ANTAGONISM BETWEEN CEFOPERAZONE AND IMIPENEM AGAINST METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

By the agar dilution checkerboard method, striking synergy between cef operazone and imipenem was observed with 32 strains of methicillin-res istant Staphylococcus aureus (fractional inhibitory concentration indi ces, less than or equal to 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 in hibition around cefoperazone. The mechanism may involve auxiliary fact ors distinct from those essential for the expression of methicillin re sistance.