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

Abstract

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 μM upon Cd(II) exposure in the early exponential phase, and was further decreased to be 0.024 μM 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 μM. 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.