Unusual susceptibility of methicillin-resistant Staphylococcus aureus to erythromycin, clindamycin, gentamicin, and tetracycline at 30°C but not at 35°C

Methicillin-resistant Staphylococcus aureus (MRSA) is an important pathogen in hospital environments, and optimal detection of MRSA requires nonroutine methods in clinical microbiology laboratories. One such method is an incubation temperature of 30° C in contrast to the more commonly used temperature of 35°C. To determine the percentage of MRSA isolates that would be missed if only one temperature were used, we evaluated methicillin resistance and susceptibility of 2,397 S aureus isolates by agar dilution at 30 and 35°C. Of the clinical isolates, 93% showed matching MICs of methicillin at both temperatures. Another 6.8% (162) showed different MICs at 30 and 35°C, with 60 of the 162 isolates (2.5% of all isolates studied) being resistant to methicillin at 30°C but susceptible at 35° C (temperature-discrepant MRSA). MICs of other antimicrobial agents, measured at 35° C, revealed an unusual pattern of susceptibility of these temperature-discrepa nt MRSA isolates. In contrast to previously reported resistance of MRSA at 35° C to erythromycin, clindamycin, gentamicin, and tetracycline, the temperature- discrepant MRSA isolates were susceptible to these agents. This resistance pattern may be of value in identifying questionable MRSA isolates when only one incubation temperature is used.