DETECTION OF METHICILLIN RESISTANCE IN COAGULASE-NEGATIVE STAPHYLOCOCCI INITIALLY REPORTED AS METHICILLIN-SUSCEPTIBLE USING AUTOMATED METHODS

Reliable detection of methicillin resistance in congulase-negative sta phylococci (CNS) is required for appropriate therapy of serious infect ions from these pathogens. To determine the most accurate method of me asuring methicillin resistance in CNS initially reported as methicilli n susceptible by automated methods, we compared mecA detection by poly merase chain reaction (PCR) with phenotypic methods. One hundred eight y-eight blood culture isolates of CNS that were initially reported as susceptible to methicillin using commercial methods (Vitek or MicroSca n) were tested by agar dilution, disk diffusion, oxacillin salt agar s creen plate, and a multiplex PCR assay rising primer sets for mecA and 16S rRNA. Sixteen isolates (8.5%) previously reported as methicillin susceptible by automated methods contained the mecA gene. MICs of thes e isolates ranged from 0.5 mu g/mL to greater than or equal to 128 mu g/mL. Ten of these isolates had MICs equal to or below the NCCLS break point of 2 mu g/mL. Six of the 10 isolates (4 with MICs of 0.5 mu g/mL and 2 with MICs of 2 mu g/mL) did not grow on any of the oxacillin sc reen plates after 48 h of incubation at 30 degrees C or 35 degrees C. All sis isolates were induced to grow in the presence of oxacillin at 128 mu g/mL by serial passaging on plates containing increasing concen trations of antibiotic. Retesting with MicroScan and Vitek detected me thicillin resistance in 7 and 10 isolates, respectively. Disk diffusio n testing with incubation for 48 h proved to be the next best method a fter PCR for detection of methicillin resistance (15 of 16 isolates). Commercial automated methods and some methods recommended by National Committee for Clinical Laboratory Standards may not detect methicillin resistance in CNS that carry the mecA gene and have MICs just below b reakpoint. (C) 1998 Elsevier Science Inc.