Disc diffusion method to screen for high-level resistance to clindamycin and erythromycin in the Bacteroides fragilis group

High-level clindamycin resistance in Bacteroides species was investigated by measuring zone sizes surrounding 2 μg clindamycin and 60 μg erythromycin discs, using a nonstandardized disc diffusion method, and by determining minimal inhibitory concentrations (MIC). The absence of a zone of inhibition surrounding either disc was predictive for all isolates having high-level resistance to both antibiotics (MIC > 256 μg/ml), characteristic of macrolide-lincosamide-streptogramin (MLS) cross-resistance. Although zone size could not be used as an absolute predictor of MIC, a clindamycin zone diameter of