Activity of retapamulin against Streptococcus pyogenes and Staphylococcus aureus evaluated by agar dilution, microdilution, E-test, and disk diffusion methodologies.

The in vitro activity of retapamulin against 106 Staphylococcus aureus isolates and 109 Streptococcus pyogenes isolates was evaluated by the agar dilution, broth microdilution, E-test, and disk diffusion methodologies. Where possible, the tests were performed by using the CLSI methodology. The results of agar dilution, broth microdilution, and E-test (all with incubation in ambient air) for S. aureus yielded similar MICs, in the range of 0.03 to 0.25 microg/ml. These values corresponded to zone diameters between 25 and 33 mm by the use of a 2-microg retapamulin disk. Overall, 99% of the agar dilution results and 95% of E-test results for S. aureus were within +/-1 dilution of the microdilution results. For S. pyogenes, the MICs obtained by the agar and broth microdilution methods (both after incubation in ambient air) were in the range of 0.008 to 0.03 microg/ml, and E-test MICs (with incubation in ambient air) were 0.016 to 0.06 microg/ml. For S. pyogenes, 100% of the agar dilution MIC results were within +/-1 dilution of the broth microdilution results. E-test MICs (after incubation in ambient air) were within +/-1 and +/-2 dilutions of the broth microdilution results for 76% and 99% of the isolates, respectively. E-test MICs for S. pyogenes strains in CO(2) were up to 4 dilutions higher than those in ambient air. Therefore, it is recommended that when retapamulin MICs are determined by E-test, incubation be done in ambient air and not in CO(2), due to the adverse effect of CO(2) on the activity of this compound. Diffusion zones (with incubation in CO(2)) for S. pyogenes were 18 to 24 mm. Retapamulin MICs for all strains by all methods (with incubation in ambient air) were < or =0.25 microg/ml. These results demonstrate that S. pyogenes (including macrolide-resistant strains) and S. aureus (including methicillin-resistant and vancomycin-nonsusceptible strains) are inhibited by very low concentrations of retapamulin and that all four testing methods are satisfactory for use for susceptibility testing.