Abstract
Cefoxitin is today the substance of choice for the phenotypic detection of methicillin-resistant Staphylococcus aureus (MRSA). We investigated the influence of incubation temperature in the standard range, i.e. 35-37 degrees C, and time, i.e. 18-20 h, versus a full 24 h. Cefoxitin disc testing was examined at incubation temperatures of 35 and 36 degrees C and times of 18-20 and 24 h, respectively, for 94 mecA-negative and 49 mecA-positive S. aureus on Iso-Sensitest agar using a semi-confluent inoculum. Cefoxitin inhibition zones on Iso-Sensitest agar were larger at temperatures above 35 degrees C; two isolates (4%, 95% confidence interval=0.5-14%) incubated at 36 degrees C were falsely categorized as susceptible to methicillin. Incubation time across 18-24 h did not impact results. Detection of methicillin resistance in S. aureus using the cefoxitin disc method with a semi-confluent inoculum on Iso-Sensitest agar is influenced by incubation temperature, and the temperature should not exceed 35 degrees C for the reliable detection of MRSA.
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