DIVERSITY OF METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS ISOLATED IN A CANADIAN HOSPITAL

Three neonates and three other patients located elsewhere in the hospi tal became infected with Staphylococcus aureus. Initial automated micr odilution susceptibility testing with oxacillin and disk diffusion tes ting with amoxicillin-clavulanic acid indicated the isolates had borde rline oxacillin resistance (MICs 4 mu g/ml), presumably due to hyperpr oduction of beta-lactamase. Chromosomal DNA restriction fingerprinting and phage typing revealed the neonatal isolates to be identical; wher eas, the other patients were infected with three different strains. Fu rther analysis of the four strains by Southern hybridization with a me cA specific oligoprobe and a quantitative beta-lactamase assay demonst rated that two strains carried the mecA gene (coding for low affinity penicillin-binding protein 2a), and two strains were hyperproducers of beta-lactamase, including one which was mecA gene positive. One strai n neither carried the mecA gene nor hyperproduced beta-lactamase. The two mecA gene positive strains displayed oxacillin MICs of 16 mu g/ml on dilution susceptibility testing in 4 % NaCl supplemented Mueller-Hi nton agar. Hence, they were considered intrinsically methicillin-resis tant Staphylococcus aureus. Both oxacillin and amoxicillin-clavulanic acid MICs were increased on NaCl supplementation. Results of amoxicill in-clavulanic acid disk diffusion susceptibility testing did not corre late with quantitative beta-lactamase production. It is recommended th at clinical laboratories do not use amoxicillin-clavulanic acid disk d iffusion assays to differentiate suspected borderline resistance due t o beta-lactamase hyperproduction from mecA gene expression of PBP-2a s ince additional mechanisms may account for resistance.