Gentamicin- and methicillin-resistant Staphylococcus aureus: in vitro susceptibility to antimicrobial drugs.

Thirteen nosocomially significant, gentamicin- and methicillin-resistant (GRMR) Staphylococcus aureus isolates, all of phage group III/M (lysotype 42E/47/53/54/75/77/83A/84/85/94/96), were uniformly resistant against augmentin, erythromycin, fosfomycin, gentamicin, methicillin, oxacillin, penicillin G, tetracycline, and tobramycin, but differed in susceptibility to cefamandole, ciprofloxacin, clindamycin, imipenem, josamycin, the synthetic chinolone Ro 23-6240, and ofloxacin. All isolates were susceptible to chloramphenicol, coumermycin, fusidic acid, novobiocin, rifampin, teicoplanin, trimethoprim-sulfamethoxazole (cotrimoxazole), and vancomycin. One isolate was of intermediate susceptibility to netilmicin. On a weight-for-weight basis, the 7 most active drugs were rifampin, coumermycin, cotrimoxazole, novobiocin, teicoplanin, fusidic acid, and vancomycin (in decreasing order) in terms of minimal inhibitory concentrations. With regard to minimal bactericidal concentrations, coumermycin, rifampin, vancomycin, teicoplanin, cotrimoxazole, ofloxacin, and ciprofloxacin (in decreasing order) were the 7 most potent antimicrobial drugs. Freshly defibrinated human blood [65% (v/v)] combined with chloramphenicol and rifampin, respectively, resulted in a weak additive effect (time kill curves). Indifferent effects were observed following combination of blood with ciprofloxacin, cotrimoxazole, coumermycin, fusidic acid, imipenem, netilmicin, novobiocin, ofloxacin, compound Ro 23-6240, teicoplanin, and vancomycin. Rifampin combined with novobiocin, teicoplanin, and vancomycin, respectively, in the presence of 65% (v/v) human blood, resulted in an additive effect. Combinations of rifampin with 9 other antimicrobial drugs in blood yielded essentially indifferent effects.