Contamination of Animal Feed with Undeclared Tetracyclines—Confirmatory Analysis by Liquid Chromatography–Mass Spectrometry after Microbiological Plate Test

The presence of tetracycline (TC) antibiotics was determined in animal feed that had been previously screened with a microbiological plate test. Feed samples were screened by a microbiological plate test on a pH 6.0 culture medium seeded with Bacillus cereus ATCC 11778 able to pre-reveal the presence of tetracyclines. Subsequently, confirmation and quantification were performed using a validated HPLC method with mass spectrometric detection. In 2013–2018, 353 feed samples were analysed to detect antibacterial substances, of which 186 (52.7%) were suspected to contain tetracyclines. Forty-two out of 186 (22.6%) samples analysed by the chromatographic method contained undeclared tetracyclines, which were determined at concentrations from 0.3 to 49 mg kg(−1). The most frequently identified contaminating tetracyclines were doxycycline and chlortetracycline.