MICs of selected antibiotics for Bacillus anthracis, Bacillus cereus, Bacillus thuringiensis, and Bacillus mycoides from a range of clinical and environmental sources as determined by the Etest.

This paper presents Etest determinations of MICs of selected antimicrobial agents for 76 isolates of Bacillus anthracis chosen for their diverse histories and 67, 12, and 4 cultures, respectively, of its close relatives B. cereus, B. thuringiensis, and B. mycoides derived from a range of clinical and environmental sources. NCCLS breakpoints are now available for B. anthracis and ciprofloxacin, penicillin, and tetracycline; based on these breakpoints, the B. anthracis isolates were all fully susceptible to ciprofloxacin and tetracycline, and all except four cultures, three of which had a known history of penicillin resistance and were thought to originate from the same original parent, were susceptible to penicillin. Based on NCCLS interpretive standards for gram-positive and/or aerobic bacteria, all cultures were susceptible to amoxicillin-clavulanic acid and gentamicin and 99% (one with intermediate sensitivity) of cultures were susceptible to vancomycin. No group trends were apparent among the different categories of B. cereus (isolates from food poisoning incidents and nongastrointestinal infections and food and environmental specimens not associated with illness). Differences between B. anthracis and the other species were as expected for amoxicillin and penicillin, with all B. anthracis cultures, apart from the four referred to above, being susceptible versus high proportions of resistant isolates for the other three species. Four of the B. cereus and one of the B. thuringiensis cultures were resistant to tetracycline and a further six B. cereus and one B. thuringiensis cultures fell into the intermediate category. There was a slightly higher resistance to azithromycin among the B. anthracis strains than for the other species. The proportion of B. anthracis strains fully susceptible to erythromycin was also substantially lower than for the other species, although just a single B. cereus strain was fully resistant. The Etest compared favorably with agar dilution in a subsidiary test set up to test the readings, and it compared with other published studies utilizing a variety of test methods.