Standardized Antimicrobial Disc Susceptibility Testing of Anaerobic Bacteria. I. Susceptibility of Bacteroides fragilis to Tetracycline

A modified Bauer-Kirby-Sherris-Turck method for disc susceptibility testing of anaerobic bacteria is presented. When tetracycline was used against 100 strains of Bacteroides fragilis as a model, reasonably reproducible results were obtained after overnight incubation in both the GasPak atmosphere and an atmosphere achieved by adding 10% CO2 to a mixture of 10% H2 and 90% N2. The minimal inhibitory concentration for the strains determined by the agar dilution technique correlated well with the results of disc tests performed in the GasPak atmosphere with 30-μg tetracycline discs. Among 63 strains isolated from 1970 to the present, only 24 (38.1%) were found to be susceptible to tetracycline.