FIG. 1.

Frequencies of mutation to clindamycin resistance of S. aureus strains, including 18 with erm(C) (solid circles), 10 with erm(A) (solid triangles), and 7 controls (solid diamonds). Open circle and triangle represent RN4220erm(C) and RN4220erm(A), respectively. Approximately 10⁸ to 5 × 10⁹ CFU of cells was spread onto agar plates containing 20 μg/ml of clindamycin. After 48 h of incubation at 35°C, colonies were counted, and the mutation frequencies were determined relative to the total count of viable organisms plated. Each data point represents the mean mutation frequency calculated from three experiments for one strain.