2-PERCENT SODIUM-CHLORIDE IS REQUIRED FOR SUSCEPTIBILITY TESTING OF STAPHYLOCOCCI WITH OXACILLIN WHEN USING AGAR-BASED DILUTION METHODS

The need to add NaCl to agar media to ensure accuracy of results when testing staphylococci with oxacillin was investigated. The results of four antimicrobial susceptibility testing methods (agar and broth dilu tion, E test, and disk diffusion) in which the growth medium contained 0, 2, 4, or 5% NaCl were compared with the results of a hybridization assay using a mec gene probe. We tested 223 strains of staphylococci, 128 of which were mec gene positive. A total of 7 of the 128 positive strains were coagulase-negative staphylococci with 24-h oxacillin MIC s of less-than-or-equal-to 2 mug/ml. Ninety-five isolates were mec gen e negative, including seven strains of Staphylococcus aureus with oxac illin MICs of greater-than-or-equal-to 4 mug/ml. The oxacillin MICs fo r mec gene-positive, oxacillin-resistant strains of staphylococci incr eased two- to fourfold with the addition of NaCl to the test medium, w hile the MICs for mec gene-negative strains did not change in the pres ence of added salt. Very major error rates for the agar dilution and E test methods in the absence of salt ranged from 18.2 to 20.2%. Major error rates for mec gene-negative S. aureus isolates were >17% for all test methods when 4 or 5% NaCl was added to the test medium. The addi tion of 2% NaCl to Mueller-Hinton agar for testing of oxacillin result ed in very major error rates of <1% for the agar dilution and E test m ethods although the major error rates for the two methods with added N aCl were 8.5 and 6.9%, respectively. The disk diffusion test did not p erform well in this study, showing essential error rates of greater-th an-or-equal-to 18.3%. We recommend the addition of 2% NaCl to Mueller- Hinton agar when testing staphylococci with oxacillin by either the ag ar dilution or E test method. NaCl should not be added for the disk di ffusion test.