A microbiological six-plate method for the identification of certain antibiotic groups in incurred kidney and muscle samples

A microbiological method was developed for group level identification of antibiotics in incurred kidney and muscle samples from cattle and pigs. The method was composed of six test bacterium-plate growth medium combinations and the result was recorded as a profile of growth inhibition zones. The sample profiles were compared to two sets of references: one constructed with standard antibiotic solution profiles, and the other with these combined with profiles of microbiologically and chemically identified residues from incurred samples. The algorithm employed in profile comparison located the minimal sum of absolute pairwise differences over the tests, with the addition of a number of experimentally observed intra-test criteria. Chemical identification and quantitation of incurred residues was based on liquid chromatography. The method identified penicillin G as a penicillinase sensitive penicillin, enrofloxacin and ciprofloxacin belonging to fluoroquinolone group, and oxytetracycline belonging to tetracycline group. Each of these residues was microbiologically identified below the Maximum Residue Limit (MRL) for kidney tissue. Combining sample profiles with the standard reference data set did not enhance the resolution. Microbiological and chemical identification test results were in good agreement. The results of this study show that a microbiological identification method is a useful tool in preliminary characterisation of antibiotic residues in animal tissues.