The in vitro activities of two cephamycins, cefotetan and cefoxitin, against 107 anaerobic bacteria were evaluated. The aerobically incubated thioglycolate disk elution technique of Kurzynski et al. (Antimicrob. Agents Chemother. 10:727-732, 1976) was also evaluated to establish the appropriate number of 30-micrograms disks to be added to each tube, assuming that strains with MICs of less than or equal to 16 micrograms/ml are susceptible. Optimal predictive values were obtained when two tubes were prepared for each antimicrobial agent, one with two 30-micrograms disks and the other with three 30-micrograms disks. After 48 h of incubation, resistant strains grew in both tubes and susceptible strains provided no growth or growth less than or equal to 50% of that in the control broth. Growth in one tube but not in the other was considered an equivocal test result indicating the need for additional tests; 5 to 7% of our strains gave equivocal results. Reproducibility studies confirmed that broth microdilution tests were more reproducible than disk elution susceptibility tests.
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