Susceptibility Testing of Anaerobic Bacteria with 100-μg Carbenicillin Disks

A total of 245 strains of anaerobic bacteria were examined for their susceptibility to carbenicillin by the disk test method and by minimum inhibitory concentration (MIC) determinations. Standard-curve studies with a strain of Bacteroides fragilis subsp. fragilis that was minimally susceptible to carbenicillin and Escherichia coli (ATCC 25922) demonstrated that a disk containing 100 mug of carbenicillin was suitable for testing susceptibility of anaerobes to carbenicillin. Thus, the diameter of zones around the 100-mug carbenicillin disks and MIC values were determined under the following test conditions: Mueller-Hinton agar supplemented with sheep blood, vitamin K(1), and hemin; an incubation temperature of 35 C; and an atmosphere of 80% N(2), 10% H(2), and 10% CO(2). The strains were separated into two populations by correlating zone diameters and geometric mean MICs. The disk test more clearly separated the resistant and susceptible populations and was more reproducible than the MIC test. Thus, a statistical analysis based on the distribution of zone diameters of susceptible and resistant strains was used to derive an interpretive scheme for anaerobic bacteria tested with 100-mug carbenicillin disks. The following interpretive scheme is recommended for testing anaerobes with 100-mug disks of carbenicillin: resistant, 8 mm or less; indeterminate, 9 to 12 mm; and susceptible, 13 mm or greater.