Facile electrochemical detection of Escherichia coli using redox cycling of the product generated by the intracellular β-d-galactosidase

•A facile electrochemical detection method for Escherichia coli.•Use of the intracellular β-d-galactosidase activity of E. coli.•Along with the signal amplification based on redox cycling.•Increased Gal expression level by treatment with isopropyl-β-d-thiogalactopyranoside. Various detection methods for pathogenic bacteria have been developed, most of which are based on cell culture, DNA amplification, and immunoassays. Although the methods allow highly sensitive and/or selective detection, they require long detection periods and complex detection procedures. This paper reports a facile electrochemical detection method for Escherichia coli (E. coli) without the use of DNA amplification or immunoassays. The detection method harnesses the intracellular β-d-galactosidase (Gal) activity of E. coli along with the signal amplification based on redox cycling. The Gal expression level is increased by treatment with a Gal expression-inducer (isopropyl-β-d-thiogalactopyranoside; IPTG); the enzymatic reaction of Gal is facilitated by the permeabilization treatment involving the use of chloroform and sodium dodecyl sulfate; the electrochemical signal is amplified by the electrochemical–chemical–chemical (ECC) and chemical–chemical redox cycling involving the Gal product, Ru(NH3)63+ and tris(2-carboxyethyl)phosphine. The detection limit obtained in the presence of the ECC redox cycling, with a total detection period of only 4.5h, is ca. 1 colony forming unit (CFU)/mL, indicating that ECC redox cycling allows for a sensitive detection.