A rapid assay for detecting sulfonamides in tissues of slaughtered animals

Governments regulate antimicrobial residues in slaughtered animals with surveillance programs for detecting drugs in food-producing animals. Although initial screening bioassay systems are recognized for their sensitivities to antimicrobial drug groups, none are sensitive to sulfonamides at or near the maximum residue levels (MRLs) in the Codex Alimentarious. We have developed a sulfonamide-sensitive rapid assay using Bacillus stearothermophilus inoculated PM indicator agar containing bromcresol purple and trimethoprim, where the end point is a combination of color change in the agar and zone of microbial growth inhibition around the sampling disk. Five sulfonamides, plus 16 other antimicrobial drugs were tested in standard concentrations in water, bovine kidney, and ground beef. Sulfonamides were detected at concentrations near the MRLs, and they were presumptively identified using para-aminobenzoic acid. The rapid assay was extremely sensitive to beta-lactams that were presumptively identified using penase. The system also was sensitive to tetracyclines, aminoglycosides, and macrolides, of which tetracyclines and gentamicin were identified using enzyme-linked immunosorbent assay (ELISA). In trials on slaughterhouse tissues submitted for testing in Ontario's meat surveillance program, the rapid assay identified twofold the number of positive kidneys and threefold the number of positive diaphragm samples compared to a standard microbiological inhibition test (MIT) currently approved. Fifty-three of 471 carcasses were sulfonamide positive with the rapid assay, while no sulfonamides were detected with the MIT. ELISA and thin-layer chromatography were used on selected samples to confirm the rapid assay sulfonamide presumptive results.