Synergistic enhancement of in vitro antimicrobial activity of imipenem and cefazolin, cephalothin, cefotiam, cefamandole or cefoperazone in combination against methicillin-sensitive and -resistant Staphylococcus aureus.

Synergistic enhancement of the in vitro antimicrobial activity of imipenem combined with cephalosporins against methicillin-resistant Staphylococcus aureus (MRSA) has been reported. In order to investigate which cephalosporin is more effective in enhancing the activity of imipenem against MRSA, the in vitro antimicrobial activities of imipenem, cefazolin, cephalothin, cefotiam, cefamandole and cefoperazone, alone and in combination, against methicillin-sensitive Staphylococcus aureus (MSSA) and MRSA were assessed. Using the checkerboard Mueller-Hinton agar dilution method, strong synergy was found in 97% to 100% of MRSA strains for imipenem and all tested cephalosporins except cefoperazone; fractional inhibitory concentration (FIC) indices were < or = 0.5. Among the cephalosporins studied, cefamandole most markedly increased the activity of imipenem against MRSA, followed, in order of decreasing effect, by cefotiam, cephalothin, cefazolin, and cefoperazone. The synergistic effect of imipenem combined with cefamandole or cefotiam was confirmed using the broth dilution method with 2% of NaCl.