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 and one horse which had revealed kidneys positive to an inhibitor test. Information on pre-slaughter medication with one antimicrobial drug was available 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 antimicrobials. Sample activity patterns compiled from inhibition zone diameters on test plates were compared with those obtained with standard antimicrobial solutions both visually and by locating the minimal sum of absolute pairwise differences over the tests. Chemical identification of residues was based on liquid chromatography. In kidney samples containing one microbiologically-identified antimicrobial the two methods gave fully consistent results with tetracyclines (15/15) and fluoroquinolenes (8/8). Preparation and storage of the kidney samples before chemical analyses appeared to influence the chemical 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 medication. The same residue as in the kidney samples was identifiable microbiologically in 41% of the muscle samples of the same carcasses. The results show that the microbiological method is well suited for identification of antibiotic residues. They indicate further that an enhanced resolution with a reduced combination of plates is attainable.