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 tested by agar dilution, microbroth dilution, E-test, and disk diffusion using 5, 10 and 50 μg disks, as outlined by the NCCLS, in order to correlate the different MIC methods and to establish tentative species specific interpretive zone diameter breakpoints for fusidic acid. MIC results of the three methods were comparable. For Staphylococcus aureus, using MIC breakpoints of ≤ 0.5 mg/L for susceptible and ≥ 2 mg/L for resistant tentative interpretive zone diameters of ≥ 20 mm and ≥ 21 mm for susceptible and ≤ 17 mm and ≤ 18 mm for resistant are suggested for the 5 μg and the 10 μg disk, respectively. The 50 μg disk did not separate susceptible from resistant isolates. For streptococci a uniform MIC distribution of 2–8 mg/L was found.