Abstract
In this study, we constructed a recombinant Escherichia coli strain with different promoters inserted between the chromate-sensing regulator chrB and the reporter gene luxAB to sense low hexavalent chromium (Cr(VI)) concentrations (<0.05 mg/L); subsequently, its biosensor characteristics (sensitivity, selectivity, and specificity) for measuring Cr(VI) in various water bodies were evaluated. The luminescence intensity of each biosensor depended on pH, temperature, detection time, coexisting carbon source, coexisting ion, Cr(VI) oxyanion form, Cr(VI) concentration, cell type, and type of medium. Recombinant lux-expressing E. coli with the T7 promoter (T7-lux-E. coli, limit of detection (LOD) = 0.0005 mg/L) had the highest luminescence intensity or was the most sensitive for Cr(VI) detection, followed by E. coli with the T3 promoter (T3-lux-E. coli, LOD = 0.001 mg/L) and that with the SP6 promoter (SP6-lux-E. coli, LOD = 0.005 mg/L). All biosensors could be used to determine whether the Cr(VI) standard was met in terms of water quality, even when using thawing frozen cells as biosensors after 90-day cryogenic storage. The SP6-lux-E. coli biosensor had the shortest detection time (0.5 h) and the highest adaptability to environmental interference. The T7-lux-E. coli biosensor—with the optimal LOD, a wide measurement range (0.0005–0.5 mg/L), and low deviation (−5.0–7.9%) in detecting Cr(VI) from industrial effluents, domestic effluents, and surface water—is an efficient Cr(VI) biosensor. This unprecedented study is to evaluate recombinant lux E. coli with dissimilar promoters for their possible practice in Cr(VI) measurement in water bodies, and the biosensor performance is clearly superior to that of past systems in terms of detection time, LOD, and detection deviation for real water samples.
Highlights
Hexavalent chromium (Cr(VI)) is widely used in metal refining, leather tanning, wood preservation, and chemical and refractory processing; it is applied to produce stainless steel, textile dyes, mimeographs, and plastics and as an anticorrosion agent [1,2]
The luminescence intensities of the three recombinant E. coli strains were proportional to the bacterial growth
The results indicated that the luminescence intensity of three recombinant E. coli biosensors rapidly increased, plateaued at 0.5 h, and decreased during incubation
Summary
Hexavalent chromium (Cr(VI)) is widely used in metal refining, leather tanning, wood preservation, and chemical and refractory processing; it is applied to produce stainless steel, textile dyes, mimeographs, and plastics and as an anticorrosion agent [1,2]. Cr(VI) has been listed as a hazardous substance by the Agency for Toxic Substances and Disease Registry since 2011. Cr(VI) can damage DNA and cause varied toxicity, mutagenicity, and carcinogenicity, because it can quickly pass through the cell membrane and enter the cell given its high solubility, bioavailability, and mobility [3,4,5]. In Taiwan, environmental protection agencies have set 0.5 mg/L as the maximum allowable level (MAL) for Cr(VI) in industrial and domestic effluents and 0.05 mg/L as the MAL in surface water and fishery water. The need for sensitive Cr(VI) detection is high, but the design of such sensors is challenging
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