Simple whole-cell test detects toxic metals using a bacterial protein switch

Human activities have disrupted natural geochemical cycles and caused dangerous buildup of metals in the environment, creating a need for simple, sensitive, and eco-friendly monitoring tools. Existing biosensors often suffer from poor efficiency, instability, or require complex equipment and skilled users. To meet this need, a research team led by Sasmita Nayak explored a novel approach that borrows a natural molecular switch: intein-mediated protein splicing. Inteins are protein segments that can cut themselves out of a larger protein and rejoin the remaining parts. The team adapted this spontaneous post-translational process so that metal-dependent inhibition of splicing would lead to a measurable change in living cells. In short, when certain toxic metals block splicing of a native Mycobacterium tuberculosis SufB protein, the host cells lose viability. By coupling that loss of viability to a simple color change readout, the researchers designed a whole-cell biosensor that promises to be low-cost, easy to use, and suitable for routine screening of environmental and industrial effluents.

The core of the method tested how metal ions affect the splicing activity of an Mtb SufB precursor protein. The team found that toxic metal ions Cd 2₊ and Hg 2₊ attenuated splicing activity across a concentration range from 25 µM to 2 mM, while Pb 2₊ and Cr 3₊ did not produce the same effect. Building on that result, they created a colorimetric biosensor platform using an attenuated Mtb strain (H37Ra) as the indicator cells and the Alamar Blue assay as the readout. Metal-induced SufB splicing inhibition caused loss of viability in H37Ra cells, producing a detectable Alamar Blue color change; adding metal-specific chelators reversed the effect, confirming specificity. The team also tested multiplexing by including known splicing inhibitors Cu 2₊ , Zn 2₊ , and Pt 4₊ alongside Cd 2₊ and Hg 2₊ across various concentrations. A simple 96-well plate format allowed multiplexed qualitative detection, and colorimetric absorbance measurements provided quantification.

This approach yields a practical, user-friendly assay that uses whole-cell native organisms carrying a metal-sensing precursor protein to screen for toxic metals. Because it relies on an intrinsic biochemical response—intein splicing inhibition of Mycobacterium tuberculosis SufB protein—and couples that to cell viability and a standard Alamar Blue colorimetric readout, the platform avoids complicated instruments and skilled operators. Its 96-well plate compatibility supports high-throughput testing, and the ability to multiplex multiple metals in one run could speed environmental and industrial monitoring. The reversibility of the signal by metal-specific chelators demonstrates specificity and offers a built-in control. Overall, the designed biosensor promises low cost, sensitivity, and robustness for routine metal screening in standard biological laboratories handling environmental or effluent samples.