DETECTING METHICILLIN-RESISTANT STAPHYLOCOCCUS-EPIDERMIDIS - DISC DIFFUSION, BROTH BREAKPOINT OR POLYMERASE CHAIN-REACTION

Growth conditions are important for the expression of resistance to me thicillin among staphylococci. Consequently a phenotypic susceptibilit y test has to be chosen carefully to avoid false susceptible results. In this study we wanted to devise rapid and simple phenotypic tests wh ose results completely correlate with the presence of the methicillin resistance gene, mecA. A simplified polymerase chain reaction (PCR) me thod not needing separate DNA extraction from the tested bacteria was used to amplify a 449 bp region of the mecA gene. One hundred and ten strains of S. epidermidis were tested. The results were in complete ag reement with those from a broth tube breakpoint test, known to identif y more strains as resistant than does the method recommended by NCCLS. In disc diffusion tests it was possible to clearly distinguish resist ant from susceptible strains by using discs containing oxacillin, ceph alexin and cephradine. A 5 mug cephradine disc was further analysed by testing another 441 consecutive clinical isolates of staphylococci.'A ll resistant coagulase-negative staphylococci grew out to the edge of this disc, whereas susceptible strains showed an inhibition zone at le ast 10 mm in diameter. The 5 mug cephradine disc is recommended for ro utine work. The PCR method and broth tube breakpoint test are both rel iable reference methods.