Qualitative detection of tetracycline residues in milk with a luminescence-based microbial method: The effect of milk composition and assay performance in relation to an immunoassay and a microbial inhibition assay

Performance of Tet-Lux, a newly developed microbiological test for the dete ction of tetracycline residues in raw milk, based on tetracycline-controlle d luminescence activation of the test bacteria, was evaluated in bovine mil ks with variable amounts of somatic cells, bacteria, fat, protein, and natu ral inhibitory compounds. The sensitivity of Tet-Lux was also compared to a commercially available tetracycline immunoassay (Snap, Idexx Laboratories Inc.) and to a microbial inhibition test (Delvotest SP, Gist-Brogades). The re were slight differences in the luminescence signals between different mi lk samples, but no single factor could be pointed out to be responsible for them. There appeared to be a modest inverse relationship between luminesce nce and increasing fat and protein content. The amount of somatic cells, ba cteria, and the natural inhibitors lysozyme and lactoferrin did not affect the luminescence response. The test fulfilled the sensitivity requirement s pecified by the European Union (maximum residue limit 100 ng/ml for tetracy clines). The Tet-Lux test was clearly more sensitive to all tetracyclines t ested (oxytetracycline, tetracycline, chlortetracycline, doxycycline, demec locycline, methacycline, minocycline) than Delvotest SP, and for five tetra cyclines out of seven more sensitive than Snap. The test provides a fast, s imple, and robust microbial method for the qualitative detection of tetracy cline residues in milk.