Inhibition of beta-galactosidase biosynthesis in Escherichia coli by tetracycline residues in milk.

Low levels of tetracyclines found as residues in milk inhibited the biosynthesis of beta-galactosidase in Escherichia coli. To produce the same effect, other antibacterials had to occur in concentrations that were more than 10-fold higher. This relative selectivity was exploited for the development of a screening test for tetracyclines in milk based on a chemiluminometric assay of beta-galactosidase. The method was validated with spiked samples of raw milk and applied to field samples contaminated with tetracyclines.