Proposed standardized method for testing and interpreting susceptibility of Bacteroides fragilis to tetracycline

One hundred twenty-four strains of Bacteroides fragilis were examined for susceptibility to tetracycline disks and by minimal inhibitory concentration (MIC) determinations. MIC values and zone sizes around 30-mug tetracycline disks were determined by using selected test conditions which included Mueller-Hinton agar supplemented with sheep blood, vitamin K, and hemin and an incubation temperature of 35 C in an atmosphere of 80% N(2), 10% H(2), and 10% CO(2). Strains were separated into two distinct populations by geometric mean MIC and dis tests. Of the 124 strains, 78 were resistant and 46 were susceptible. The resistant strains had geometric mean MIC's of 8 mug/ml or greater, whereas the geometric mean MIC's of sensitive strains were 5 mug/ml or less. The disk test proved to be more reproducible than the MIC test and completely separated the resistant and susceptible populations. An interpretive scheme for B. fragilis to tetracycline was statistically derived on the basis of the distribution of zone sizes of susceptible and resistant strains: resistant, 18 mm or less; indeterminant, 19 to 20 mm; and susceptible, 21 mm or greater. These zone sizes compared closely with the Kirby-Bauer criteria for aerobic bacteria.