OPTIMIZING TESTING OF METHICILLIN-RESISTANT STAPHYLOCOCCUS SPECIES

Selection of the appropriate NaCl concentration for test medium for ox acillin susceptibility testing of Staphylococcus aureus and coagulase- negative staphylococci has been problematic when using different antim icrobial susceptibility testing methods. Broth microdilution, using ca tion-adjusted Mueller-Hinton broth + 2% NaCl, is the currently recomme nded reference method. There is currently no recommendation for the ad dition of NaCl to agar for dilution susceptibility tests when Staphylo coccus species are tested with oxacillin. We examined the effects of a dding 0, 2%, 4%, and 5% NaCl to Mueller-Hinton agar and broth for agar dilution, Etest, and broth microdilution tests. The results of these tests were compared with the reference broth microdilution results and with the results of a hybridization assay using a mec gene probe. We tested 223 strains of staphylococci, 128 of which were mec gene positi ve and had oxacillin minimum inhibitory concentrations (MICs) greater than or equal to 4 mu g/ml, Seven strains of S. aureus were mec probe negative but were oxacillin resistant. Seven coagulase-negative strain s (three S. epidermidis, one S. haemolyticus, and three S. simulans) w ere mec probe positive and were oxacillin susceptible. The MICs for ox acillin-resistant strains increased two- to fourfold with the addition of 2% NaCl, but the MICs for oxacillin-susceptible strains were uncha nged. Major and very major interpretative rates ranged from 18.2% to 2 0.2% for agar dilution and Etest without NaCl added to the medium, and these rates decreased to < 1% with the addition of 2% NaCl to the med ium. The addition of 4% or 5% NaCl caused major error rates of > 17% f or all test methods. Our results indicate that 2% NaCl should be added to Mueller-Hinton agar Etest and agar dilution to obtain optimal agre ement with reference broth microdilution test and mec gene probe resul ts.