Criteria for disk susceptibility tests and quality control guidelines for the cefoperazone-sulbactam combination.

For in vitro susceptibility tests with cefoperazone and sulbactam (a beta-lactamase inhibitor), 75/30-micrograms disks may be used with the interpretive zone size breakpoints that are currently used for 75-micrograms cefoperazone disks. For dilution tests, a 2:1 ratio of cefoperazone to sulbactam is recommended. For quality control purposes, MIC limits that are used to monitor cefoperazone tests were also applied to tests with the combination of drugs. For gram-negative control strains, zone size limits were calculated to be 1 mm smaller than those used for cefoperazone disks. To monitor the sulbactam portion of the combination, Acinetobacter calcoaceticus subsp. anitratus ATCC 43498 was selected; zones with 75/30-micrograms disks were 26 to 32 mm in diameter, and broth microdilution MICs ranged from 1.0/0.5 to 8.0/4.0 micrograms/ml. With cefoperazone alone, MICs for Acinetobacter calcoaceticus subsp. anitratus were 16 to 64 micrograms/ml and zones ranged from 14 to 18 mm in diameter. For anaerobic dilution tests, only Bacteroides thetaiotaomicron ATCC 29741 is recommended for cefoperazone-sulbactam; MICs ranged from 8.0/4.0 to 32/16 micrograms/ml.