Detection of antibiotics in muscle tissue with microbiological inhibition tests: effects of the matrix.

The effects of the tissue matrix on detection limits of antibiotics with microbiological inhibition tests, intended for muscle tissue, were measured. Pieces of frozen meat were laid directly on top of paper disks impregnated with aqueous antibiotic solutions. Inhibition zones were compared with those obtained by the same standard solution without tissue. Only tetracyclines were detected as efficiently with as without muscle tissue. Inhibition zones of the beta-lactam antibiotics ampicillin and penicillin G, and the fluoroquinolone antibiotics enrofloxacin and ciprofloxacin were smaller when muscle tissue was added to low levels of standard solution. At higher levels the differences were not substantial. Inhibition zones of tylosin were smaller and irregular or had disappeared completely, while ceftiofur, sulfadimidine, erythromycin, lincomycin, and streptomycin were not detected in spiked muscle tissue at concentrations fivefold higher than the detection limits without tissue. These results indicate that ceftiofur, sulfonamides, streptomycin and some macrolide antibiotics cannot be detected in intact meat with the plates and bacterial strains prescribed in the European Four Plate Test, a test which was initially intended as a multi-residue method for muscle tissue. Two plates of this system are not suitable for screening purposes; a third one detects tetracyclines and beta-lactam antibiotics in spiked tissue; the fourth one is sensitive for beta-lactam antibiotics and for some but not all macrolides. Samples spiked with the fluoroquinolones enrofloxacin and ciprofloxacin can be detected with an additional plate, not included in the Four Plate Test.