A microbiological assay system for assessment of raw milk exceeding EU maximum residue levels

A microbiological multiplate system is described, which can be used to evaluate the milk residue state near the established EU MRLs for beta-lactam antibiotics, tetracyclines, macrolides, aminoglycosides, quinolones, polymyxins, and other antibiotics like rifamycin and novobiocin. The multiplate system consists of a Bacillus stearothermophilus var, calidolactis plate at pH 8.0, a Bacillus cereus plate at pH 6.0, a Micrococcus luteus plate at pH 8.0, Bacillus subtilis plates at pH 7.0 (sulpha plate) and pH 8.0, an Escherichia coli plate at pH 6.0 and a Staphylococcus epidermidis plate at pH 6.0. Among the 48 antimicrobial drugs tested the multiplate system did not reach the required sensitivities for sulphanilamide, cefquinone, spectinomycin and colistin with respect to established MRLs for milk. The mean variation coefficient of reproducibility for the 7 test plates ranged between 4.5 (M. luteus plate) and 9.8% (E. coli plate) at MRL level. Based on the inhibition zone patterns an indication can be given of the type of drug residue involved. For each test plate a so-called action level is proposed below which the raw milk fulfils the EU MRL criteria, except for cefquinome, spectinomycin, tylosin, colistin, trimethoprim and sulphanilamide. (C) 1999 Elsevier Science Ltd. All rights reserved.