Microbial diffusion assay for antibiotics in feeds using a simplified design

Results are compared for the microbiological analysis of antibiotics in feeds using the AOAC plate diffusion assay and the simplified 2-plate assay. Five antibiotics, bacitracin, chlortetracycline, oxytetracycline, penicillin, and streptomycin, were used to supplement feed extracts at levels of 100 and 25 micrograms antibiotic/g feed (bacitracin at 100 micrograms/g only). For bacitracin at the one level and for penicillin at both levels, the 2-plate design yielded significantly more accurate results than those of the AOAC assay. The same was true for the 25 micrograms/g level of oxytetracycline and the 100 micrograms/g level of streptomycin. For streptomycin at the 25 micrograms/g supplementation, the AOAC assay results showed better accuracy. There was no significant difference in results between the 2 designs for oxytetracycline at 100 micrograms/g and chlortetracycline at either level. The accuracy and precision of the results for the 2-plate design are equivalent to or better than those obtained using the AOAC design; in addition, the 2-plate assay is less labor-intensive, is more cost-effective, and is able to determine reasonable conditions of similarity.