Detection and quantification of tetracyclines by whole cell biosensors

Three different mini‐Tn 5 plasmids, containing a tetracycline‐inducible promoter, P tet and a regulatory gene, tetR , in operon fusions with a reporter gene system ( lacZYA , luxCDABE or gfp ), were constructed. These biosensor constructs responded to low levels of tetracyclines by producing β‐galactosidase, light or green fluorescent protein. They did so in a quantitative manner, thus enabling the quantification of tetracyclines in the immediate surroundings of the biosensor organism. All three constructs were transferred successfully to different Gram‐negative bacteria by conjugation. An Escherichia coli strain containing the P tet ‐ lac construct was used to determine oxytetracycline in milk as a demonstration of the application of these biosensors.