Comparison of Different Agar Diffusion Methods for the Detection of Antimicrobial Residues in Slaughter Animals

The different agar diffusion methods were compared using antibiotic and sulphonamide-impregnated filter-paper discs and the kidneys of healthy and emergency-slaughtered pigs and cows after slaughter. No method used seemed to be sensitive to all antimicrobial drugs preimpregnated onto discs. Tetracycline yielded a greater zone of inhibition at pH 6 than at pH 8 and aminoglycosides, erythromycin, polymyxin B and lin cornycin at pH 8 than at pH 6. It therefore seems necessary to use different pHs (6 and 8). The addition of trimethoprim to the medium is necessary for the detection of sulphonamides. Bacillus subtilis BGA used as the test organism was more sensitive to sulphonamides on the “Test agar for the inhibitor test” containing trimethoprim than on the “Iso-Sensitest agar” also containing trimethoprim. The addition of trimethoprim to “Test agar for the inhibitor test” is recommended at pH 8 but not at pH 6 because false-positive cases (with inhibition zones > 2 mm) were observed at pH 6 with trimethoprim on the kidneys of healthy pigs.