Comparison of three genetically modified Escherichia coli biosensor strains for amperometric tetracycline measurement

▸ In this manuscript we report a genetically modified whole-cell based amperometric biosensor. ▸ The nuoA-based bioassays offered high sensitivity and specificity to the specific targets (tetracycline, copper and silver). ▸ This is the first time that a respiratory gene has been used as a reporter gene allowing the whole-cell sensor to directly quantify a target substrate by amperometric measurement of cell respiration responses. Three separate genetic strategies, based upon the induced expression of three different genes (lacZ, selA and nuoA) were tested to provide the SciTox assay with sensitive and specific detection of the antibiotic tetracycline (Tet). All three strategies relied on gene induction from the Tn10 tetA promoter. Both lacZ and nuoA biosensors responded specifically and sensitively to sub-inhibitory concentrations of Tet. However, the selA-based assay was not sensitive enough to detect Tet in the SciTox assay. The detection limits for Tet of the lacZ and nuoA biosensor strains were 0.11μgml−1 and 0.0026μgml−1, respectively, and their linear ranges were 0.1–1μgml−1 and 0–0.01μgml−1, respectively. While lacZ has previously been used as a reporter gene in an amperometric bioassay, nuoA is a novel and more sensitive reporter gene. This is the first report in which a respiratory gene was used as a reporter gene in an amperometric biosensor. The results indicate that this approach can produce a highly sensitive detection system. In order to test whether the new system could be used to detect other chemicals, the nuoA gene was re-engineered to be driven by the copper-inducible copA promoter. Using this strain, the SciTox assay was found to be able to specifically detect copper and silver ions.