Detection of the mec-A gene and phenotypic detection of resistance in Staphylococcus aureus isolates with borderline or low-level methicillin resistance

Eighty-three isolates of Staphylococcus aureus for which MICs of methicillin of 4-16 mg/L had previously been recorded were tested for the presence of the mecA gene with a DNA probe and a PCR assay. There was complete agreement between the results obtained by these methods; 39 isolates were mecA-positive and 44 were mecA-negative. Using the presence of mecA as the defining standard, several phenotypic methods for determining resistance to methicillin were evaluated and a high-inoculum, agar-incorporation breakpoint test was found to offer the best combination of high sensitivity and high specificity. However twenty-seven of the 44 mecA-negative strains were methicillin-resistant according to agar dilution MICs (MIC > 4 mg/L on at least one of the four media used) but none had MICs exceeding 32 mg/L. One of the mecA-positive strains had a methicillin MIC of only 8 mg/L and did not appear to be heteroresistant. The clinical significance of these two groups of 'atypical' isolates may need further investigation. This study highlights the problems of detecting reliably S. aureus with low level methicillin resistance by phenotype methods and the usefulness of direct detection of the mecA gene.