Abstract
Antibiotic sensitivities of rough and smooth variants of Bacillus stearothermophilus var. calidolactis were investigated using penicillin, chloramphenicol, tetracycline and bacitracin. A disc assay procedure and incubation at 55°C for 6h was employed. The minimum concentrations of penicillin, bacitracin and chloramphenicol, detected by the rough and smooth pure variants, were 0.004, 0.005IU/mL; 35.0, 40.0IU/mL; and 1.0, 1.0μ/mL, respectively. These minimum detectable concentrations (MDC) indicated slight or no differences between the variants as to their antibiotic sensitivities. Responses to tetracycline, however, indicated that the smooth variant (MDC - 2.0μ/mL) was twenty times more sensitive to this antibiotic than the rough variant (MDC - 40.0μg/mL). Assay plates seeded with different proportions of rough and smooth variants, containing discs impregnated with 40.0μg tetracycline/mL, exhibited a secondary zone of inhibition. The presence of this inhibition zone appeared to be dependent upon the ratio of rough and smooth variants in the assay. The possible presence of an antagonistic agent produced by the smooth component, acting syner-gistically with tetracycline, is suggested.
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