A tailored indigoidine-based whole-cell biosensor for detecting toxic cadmium in environmental water samples

Regulated expression of resistance genes is essential for the survival and adaptation of microbial communities under stress-inducing conditions, such as exposure to environmental contamination. By simulating environmental microorganisms sensing heavy metal pollutants, numerous whole-cell biosensors have been developed in the past two decades. However, most of biosensing signals are excessively dependent on instrumental determination, which remains a huge challenge. Here, we developed a visual whole-cell biosensor, which was assembled by fusing CadR-based Cd(II) sensory element with a biosynthetic gene cluster for indigoidine. The basal production of indigoidine was very low, which could contribute to the reduced detection background. The whole-cell biosensor responded strongly to Cd(II), but responded weakly to Zn(II), Pb(II), and Hg(II). The limit of detection was 0.049 mu M upon Cd(II) exposure in the early exponential phase, and was further decreased to be 0.024 mu M upon Cd(II) exposure in the lag phase. Bioavailable Cd(II) could be quantified using a nonlinear regression within a range of 6.25 to 200 mu M. By integrating indigoidine and mCherry into one biosensing construct, a versatile dual-signal output biosensor conductive to diversified detection was successfully developed and validated. The Cd(II) biosensing capability of indigoidine-based biosensor was verified in detecting environmental water samples. A high-throughput method based on a 96-well microplate was validated in quantitative detection. Naked-eye recognizable blue in the culture system was convenient for approximate quantification. Its ease-of-use, low cost, and minimal-equipment requirement make it a potential field-deployable biosensor for environmental cadmium pollution surveying. (C) 2022 The Author(s). Published by Elsevier B.V.

Automated summary

A visual whole-cell biosensor was developed for detecting heavy metal pollutants, with a strong response to Cd(II) and the ability to detect low concentrations at different growth stages. The biosensor showed advantages of high-throughput detection and approximate quantification.