Unusual susceptibility of methicillin-resistant Staphylococcus aureus to erythromycin, clindamycin, gentamicin, and tetracycline at 30 degrees C but not at 35 degrees 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 degrees C in contrast to the more commonly used temperature of 35 degrees 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 degrees 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 degrees C, with 60 of the 162 isolates (2.5% of all isolates studied) being resistant to methicillin at 30 degrees C but susceptible at 35 degrees C (temperature-discrepant MRSA). MICs of other antimicrobial agents, measured at 35 degrees C, revealed an unusual pattern of susceptibility of these temperature-discrepant MRSA isolates. In contrast to previously reported resistance of MRSA at 35 degrees 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.