In-vitro susceptibility of isolates of methicillin-resistant Staphylococcus aureus 1988-1993

MICs for 423 strains of methicillin-resistant Staphylococcus aureus (MRSA) isolated in Hong Kong during 1988-1993 were performed for 15 antimicrobial agents: erythromycin, chloramphenicol, tetracycline, minocycline, gentamicin, netilmicin, trimethoprim, rifampicin, fusidic acid, ciprofloxacin, vancomycin, teicoplanin, sparfloxacin, clinafloxacin and RP 59500 (quinupristin/dalfopristin). Susceptibility to antibiotics generally remained stable throughout the study period, with the exception of the quinolones. Resistance to ciprofloxacin (breakpoint 4 mg/L) increased from a low of 9% in 1988 to a high of 82% in 1993. For sparfloxacin the corresponding figures were 9% and 78%, respectively. Six (1%) clinafloxacin-resistant strains were found. MIC(50)s and MIC(90)s of clinafloxacin increased from less than or equal to 0.06 mg/L and 0.25 mg/L in 1988 to 1.0 mg/L and 2.0 mg/L, respectively, in 1993. All 423 strains were phage typed (typability 70%) and a diversity of phage types which changed during the observation period, with 13 dominating types, was observed. Ciprofloxacin resistance occurred in 12 of the dominating types, in 46 non-typable strains, and also in 23 strains of different, sporadically occurring types, indicating that the emergence of quinolone resistance was not due to dissemination of a single or few MRSA clones. The usefulness of quinolones in the treatment of MRSA infections is likely to be seriously constrained by the emergence of resistance. MICs for RP-59500 were less than or equal to 2 mg/L for all isolates, suggesting that this agent merits further evaluation as an anti-MRSA agent. All MRSA remained susceptible to vancomycin and teicoplanin throughout the study period.