Determination of oxytetracycline residues in milk with the use of HPLC method and two microbial inhibition assays

Abstract

The aim of this study was to compare the sensitivity of the disc diffusion method, the Four-plate test (FPT), and the HPLC method for the determination of oxytetracycline residues in milk. Milk samples were collected from six cows with mastitis which were intramammary treated with Oxymykoin for 5 d. Milk samples were collected and tested for the presence of OTC residues at intervals of 0, 24, 48, 72, 96, 120, 144, 168 and 192 h after the first drug administration. The highest average concentration of 195.68 mg OTC/kg was detected by HPLC method 5 d after the first drug administration (the last day of therapy). Two positive samples (zone diameters ≤ 1 mm) were detected using the microbiological assay with Bacillus stearothermophilus var. calidolactis C 953 in milk samples before the first drug administration (0 h). When the same samples were tested with the dialysis membrane, negative results were obtained. As seen from the results of this study, positive milk samples confirmed by HPLC method in which the concentrations of OTC residues exceeded the MRLs were not always detected by the agar diffusion test with Bac. stearothermophilus.