The Use of Different Microbial Assays in Combination with the Charm II Test in the Detection of Antibiotic Residues in Herd Milk

Three microbial assays, i.e. Delvo test SP, BR-test AS and Valio T101, were used to screen approximately 40 000 milk samples for inhibitory substances at ten different milk grading laboratories. Samples found positive by any of the assays were then analysed by the Charm H test for the presence of beta-lactams, tetracycline. s, aminoglycosides, macrolides and sulfa drugs. Positive samples were also analysed by the screening assays used routinely at Swedish dairies, i.e. Delvo test P and Arla microtest. The results revealed considerable differences between the methods regarding their sensitivity to various antibiotics, beta-Lactams were the prevailing type of antibiotics found, irrespective of the microbial assay method used. A surprisingly large number of samples were positive for tetracyclines and/or macrolides. These substances were best detected by Arla microtest and Valio T101, while beta-lactams were somewhat better detected by the Delvo tests. A future control system with an alternative use of more than one microbial screening assay in combination with the Charm H test is suggested.