Microbiological and chemical identification of antimicrobial drugs in kidney and muscle samples of bovine cattle and pigs

Microbiological and chemical identification of antimicrobial drug residues was attempted in 95 kidney and 76 muscle samples from 58 cattle, 36 pigs an d one horse which had revealed kidneys positive to an inhibitor rest, infor mation on pre-slaughter medication with one antimicrobial drug was availabl e for 63% of the carcasses. Microbiological identification was performed by agar diffusion using 17 or 18 combinations of eight test bacteria, varying medium pH and three substances blocking the action of certain antimicrobia ls. Sample activity patterns compiled from inhibition zone diameters on tes t plates were compared with those obtained with standard antimicrobial solu tions both visually and by locating the minimal sum of absolute pairwise di fferences over the rests. Chemical identification of residues was based on liquid chromatography. In kidney samples containing one microbiologically-i dentified antimicrobial the two methods gave fully consistent results with tetracyclines (15/15) and fluoroquinolenes (8/8). Preparation and storage o f the kidney samples before chemical analyses appeared to influence the che mical identification of penicillin G. The results were consistent in 37 of the 41 samples stored without homogenization at -70 degrees C. The residue was identified by chemical means only in six and neither microbiologically nor chemically in four kidney samples with information on pre-slaughter med ication. The same residue as in the kidney samples was identifiable microbi ologically in 41% of the muscle samples of the same carcasses. The results show that the microbiological method is well suited for identification of a ntibiotic residues. They indicate further that an enhanced resolution with a reduced combination of plates is attainable.