Quantitative assay for antibiotics used commonly in treatment of bovine infections

Delvotest-P and Bacillus stearothermophilus Difco disc assay procedures were utilized, and assay sensitivity for detecting various antibiotics was tested. Bacillus stearothermophilus spores used in the disc assay were purchased from two laboratories and compared. The dyes methylene blue, 2,3,5-triphenyltetrazolium chloride, orange-O, and bromcresol purple were incorporated separately into agar used for the disc assay to determine if sharper, clearer zones could be produced. None of the dyes had an observable effect on zone size or clarity. Using the Difco disc assay, quantitation of penicillin, cephapirin, and oxytetracycline was accomplished with reliable precision by relating the size of the zone of inhibition to the log of the antibiotic concentration. The Delvotest-P and Difco disc assays were equal in sensitivity, as were spore suspensions used from the two laboratories.