DETECTION OF THE MEC-A GENE AND PHENOTYPIC DETECTION OF RESISTANCE INSTAPHYLOCOCCUS-AUREUS ISOLATES WITH BORDERLINE OR LOW-LEVEL METHICILLIN RESISTANCE

Eighty-three isolates of Staphylococcus aureus for which MICs of methi cillin of 4-16 mg/L had previously been recorded were tested for the p resence of the mecA gene with a DNA probe and a PCR assay. There was c omplete agreement between the results obtained by these methods; 39 is olates were mecA-positive and 44 were mecA-negative. Using the presenc e of mecA as the defining standard, several phenotypic methods for det ermining resistance to methicillin were evaluated and a high-inoculum, agar-incorporation breakpoint test was found to offer the best combin ation of high sensitivity and high specificity. However twenty-seven o f the 44 meed-negative strains were methicillin-resistant according to agar dilution MICs (MIC > 4 mg/L on at least one of the four media us ed) but none had MICs exceeding 32 mg/L. One of the mecA-positive stra ins had a methicillin MIC of only 8 mg/L and did not appear to be hete roresistant. The clinical significance of these two groups of 'atypica l' isolates may need further investigation. This study highlights the problems of detecting reliably S. aureus with low level methicillin re sistance by phenotype methods and the usefulness of direct detection o f the meed gene.