[In vitro effects of combinations of antibiotics against highly-fosfomycin-resistant, methicillin-resistant Staphylococcus aureus. With special reference to efficacies of combinations of imipenem/cilastatin and cephems].

: The author evaluated in vitro effects of combinations of antibiotics against 27 clinical isolates of highly-fosfomycin (FOM)-resistant, methicillin-resistant Staphylococcus aureus (MRSA). In combinations of FOM and cephems (CEPs), MICs of CEPs were decreased by FOM significantly, and synergistic effects were observed when the MIC of FOM was below 50 micrograms/ml. Combinations of imipenem/cilastatin (IPM/CS) and CEPs, however, was superior to those of FOM and CEPs in terms of synergistic effects. In these combinations, good synergism was found when the MIC of IPM/CS was below 25 micrograms/ml and those of CEPs were relatively low. In combinations of aminoglycoside and CEPs, no apparent synergism was observed. Most significant synergistic effect was achieved in the combination between IPM/CS and cefamandole. We believe that both fractional inhibitory concentration indices and achievement rates of effective serum concentrations should be taken into account to determine an effective combination of antibiotics.