A chitosan gel traps bacteria on an electrode and lets them talk in electricity

Living bacteria can sense chemicals and produce electrical signals in response. That makes them, in principle, ideal biosensors - more versatile, more durable, and cheaper than sensors built from purified proteins or enzymes. In practice, building devices around living organisms is messy. The chemical mediators bacteria use to transfer electrons get washed away in liquid environments. Some mediators are toxic. And keeping bacteria in place without killing them is its own engineering challenge.

A team at Rice University, led by Rafael Verduzco, has solved several of these problems at once with a single material: a hydrogel made from chitosan, the polymer found in the hard outer shells of crabs and shrimp. The study was published in Advanced Materials.

One gel, three jobs

The chitosan-based hydrogel performs triple duty. First, it physically encapsulates the bacteria near the electrode surface, preventing them from escaping into the surrounding liquid while allowing analytes - the target substances being measured - to flow in. Second, the chitosan is chemically modified with anchor points where redox mediators can attach. These mediators accept electrons from bacteria and pass them along to the electrode, creating a readable electrical signal. Third, chitosan is biocompatible, non-toxic, and derived from a renewable source, making it safe for use in food and environmental monitoring.

The innovation came from doctoral student Xinyuan Zuo, who recognized that a redox-active polymer could simultaneously solve the structural problem (keeping bacteria in place) and the signal transduction problem (moving electrons from bacteria to electrode). By attaching redox mediators directly to the chitosan backbone, the team created a material that acts as both a cage and a wire.

Detecting a preservative in milk

To demonstrate the system, the researchers built a sensor for sakacin P, an antimicrobial peptide commonly used as a food preservative. They used an engineered strain of Lactobacillus plantarum - a probiotic bacterium found in fermented dairy products - that produces a small electrical current when it encounters sakacin P.

The hydrogel with its embedded bacteria was attached to an electrode and placed into milk. Within a few hours, the electrode registered an electrical signal. The bacteria had detected the preservative and communicated that detection as current.

Both the polymer and the bacterium are food-safe, which matters for applications in food quality monitoring. And because the hydrogel platform is not species-specific, it can work with other electroactive bacteria for different sensing applications.

The broader potential

Verduzco sees the platform as a general-purpose interface between living microbes and electronic systems. Potential applications extend beyond sensing: electroactive bacteria could be harnessed for chemical production, environmental remediation (breaking down pollutants), or isolation and destruction of harmful compounds in wastewater.

The hydrogel's ability to maintain electrical communication in liquid environments is particularly important. Previous microbial bioelectronic systems worked well in controlled laboratory settings but lost performance when deployed in the wet, dynamic conditions of real-world applications - precisely the environments where bacterial sensors would be most useful.

Limitations and open questions

The study demonstrates the concept in a benchtop setting. Long-term stability - how long the bacteria remain viable and electrically active within the hydrogel, and how many detection cycles the system can sustain - has not been extensively characterized. Bacterial populations can drift genetically over time, potentially altering their sensing properties.

Signal specificity is another consideration. The engineered L. plantarum strain responds to sakacin P, but real-world samples contain complex mixtures of compounds that could interfere with detection. Selectivity in noisy environments is a common challenge for biosensors that this study does not fully address.

Manufacturing scalability and cost are unclear, though chitosan itself is inexpensive and abundant as a byproduct of the seafood industry. The redox modification adds chemical processing steps, but none that appear prohibitively complex.

The fundamental contribution is the material itself: a biocompatible, electron-conducting hydrogel that keeps bacteria alive, in place, and electrically connected. That platform is the enabling piece. What gets built on it - whether food safety sensors, environmental monitors, or microbial chemical factories - depends on which bacteria are loaded and what they are engineered to detect or produce.