Antimicrobial Residues in Milk. Comparison of Different Agar Diffusion Methods

Different agar diffusion methods were compared in order to find a sensitive method for the detection of various antimicrobial residues in milk. A total of 588 producer milk samples were analyzed using subsets of the most sensitive methods. With the IDF method, 2 positive cases (0.34 %) appeared among the producer milk samples, with the Thermocult method 13 positive cases (2.21 %) and with the Test agar pH 8 method with trimethoprim and glucose 4 positive cases (0.68 %). A combination of the IDF method and the Test agar pH 8 method resulted in 6 positive cases (1.02 %) and a combination of the Thermocult method and the Test agar pH 8 method in 17 positive cases (2.89 %). With penicillinase 41 % of the positive cases were identified as β-lactam antibiotics and with p-aminobenzoic acid 18 % of the positive cases were identified as sulphonamides. 41 % of the positive cases remained unexplained. The best combination for the detection of antimicrobial agents in milk seems to be that of the Thermocult method and the Test agar pH 8 method with trimethoprim and glucose.