Emergence of homogeneously methicillin-resistant Staphylococcus aureus.

Forty-seven clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA), collected between 1986 and 1990 from 29 institutions, were analyzed for susceptibility to various antibiotics. Twenty-six strains were homogeneously methicillin resistant (i.e., greater than or equal to 10% of the cells in these strains were able to grow on Mueller-Hinton agar containing 50 micrograms of methicillin per ml). The MICs of gentamicin, clindamycin, trimethoprimsulfamethoxazole, methicillin, and imipenem for homogeneous MRSA strains were higher than those for heterogeneously resistant strains. Both types of strains were, for the most part, susceptible to vancomycin and trimethoprim-sulfamethoxazole. Ciprofloxacin-resistant MRSA strains were not isolated prior to 1988 but made up 40% of the post-1987 strains. The level of methicillin resistance correlated well with the imipenem MIC, suggesting that susceptibility to imipenem may serve as a marker to identify and monitor the prevalence of homogeneous MRSA strains.