Evaluation of the European four-plate test as a tool for screening antibiotic residues in meat samples from retail outlets.

A modified 4-plate test was used to screen 4795 meat samples from retail outlets in the European Community (EC). This microbial inhibition test uses 3 media seeded with Bacillus subtilis at different pH values (6, 7.2, or 8) and a fourth medium seeded with Micrococcus luteus. Positive samples were confirmed by a receptor test for macrolides, a thin-layer chromatographic method for sulfonamides, or an enzyme-linked immunosorbent assay test for tetracyclines. Inhibition on M. luteus plates, often by beef and veal samples, could not be confirmed. Circumstantial evidence indicated these test results had to be considered presumptively false positives. Of the samples, 95 inhibited at least one plate seeded with B. subtilis. Usually, samples were positive on more than one plate: 70 samples were positive on all 3 plates, and only 6 samples did not inhibit the plate at pH 6. The majority of positive results on plates seeded with B. subtilis, 77 of 89 samples tested, contained tetracycline antibiotics. One sample also contained sulfadimidine. Two other samples contained high levels of enrofloxacine and ciprofloxacine. The 4-plate test is not sensitive enough to detect sulfonamides and quinolones at the EC maximum residue limits, but higher levels may cause inhibition. The 4-plate test can be used to detect meat samples containing tetracycline residues, but the method is too complicated when used only for that purpose.