Detection of antimicrobial agents by a specific microbiological method (Eclipse100®) for ewe milk

Drug residues in milk may have public health and technological implications. Thus, the aim of this work was to assess the specific microbiological method for ewe milk (Eclipse 100®) by means of the study of 'false positive' results (specificity), the effect of the preservative acidiol upon specificity and the calculation of the detection limits in comparison with the MRLs set out by the EU. Milk samples used were taken from 80 Manchega ewes every two weeks as of 45 days post-partum until the end of lactation. The animals received no pharmacological treatment throughout lactation. Each milk sample was divided into two aliquots (with or without acidiol). From a total of 508 samples analysed, no 'false positive' results were obtained, although doubtful results were higher in the samples with acidiol (8.9%) than in preservative-free milk samples (0.6%). The detection limits of Eclipse 100® were calculated for 27 antimicrobial agents. For each drug, eight concentrations were tested on 24 milk samples (192 samples for chemotherapeutics). The detection limits determined by means of logistic regression models were 7 μg/kg amoxycillin, 5 μg/kg penicillin 'G', 68 μg/kg cloxacillin, 28 μg/kg oxacillin, 86 μg/kg cefadroxil, 115 μg/kg cephalexin, 110 μg/kg cefoperazone, 85 μg/kg cefuroxime, 3140 μg/kg gentamicin, 18700 μg/kg kanamycin, 9100 μg/kg neomycin, 10100 μg/kg streptomycin, 750 μg/kg erythromycin, 18100 μg/kg spiramycin, 230 μg/kg tylosin, 5100 μg/kg ciprofloxacin, 4000 μg/kg enrofloxacin, 76200 μg/kg flumequine, 9500 μg/kg norfofloxacin, 170 μg/kg sulfadimethoxine, 750 μg/kg sulfamethazine, 370 μg/kg sulfanilamide, 250 μg/kg sulfathiazole, 1500 μg/kg chlortetracycline, 260 μg/kg doxycycline, 560 μg/kg oxytetracycline and 480 μg/kg tetracycline. It may be concluded that the Eclipse 100® test did not present 'false positive' results and that the preservative acidiol caused a decrease in specificity of the method. The Eclipse 100® test is also suitable for detection of residues of beta-lactam antibiotics, sulfadimethoxine and sulfathiazole, as their detection limits are similar to EU-MRLs, whereas the limits for residues of aminoglycosides, macrolides, tetracyclines and quinolones in ewe milk were higher than EU-MRLs.