Correlation of MIC methods and tentative interpretive criteria for disk diffusion susceptibility testing using NCCLS methodology for fusidic acid

Three hundred and eight staphylococci and 43 Streptococcus pyogenes were te sted by agar dilution, microbroth dilution, E-test, and disk diffusion usin g 5, 10 and 50 jig disks, as outlined by the NCCLS, in order to correlate t he different MIC methods and to establish tentative species specific interp retive zone diameter breakpoints for fusidic acid. MIC results of the three methods were comparable. For Staphylococcus aureus, using MIC breakpoints of less than or equal to 0.5 mg/L for susceptible and greater than or equal to 2 mg/L for resistant tentative interpretive zone diameters of greater t han or equal to 20 mm and greater than or equal to 21 mm for susceptible an d less than or equal to 17 mm and less than or equal to 18 mm for resistant are suggested for the 5 mug and the 10 mug disk, respectively. The 50 mug disk did not separate susceptible from resistant isolates. For streptococci a uniform MIC distribution of 2-8 mg/L was found. (C) 2001 Elsevier Scienc e Inc. All rights reserved.