If a chemical spill in a river goes unnoticed for 20 minutes, it might be too late to remediate. Living bioelectronic sensors developed by a team of researchers at the Rice University can help. A team led by Rice synthetic biologists Caroline Ajo-Franklin and Jonathan (Joff) Silberg and lead authors Josh Atkinson and Lin Su, both Rice alumni, have engineered bacteria to quickly sense and report on the presence of a variety of contaminants. Their study in Nature shows the cells can be programmed to identify chemical invaders and report within minutes by releasing a detectable electrical current.
Such “smart” devices could power themselves by scavenging energy in the environment as they monitor conditions in settings like rivers, farms, industry, and wastewater treatment plants and to ensure water security, according to the researchers.
The environmental information communicated by these self-replicating bacteria can be customized by replacing a single protein in the eight-component, synthetic electron transport chain that gives rise to the sensor signal.
“I think it’s the most complex protein pathway for real-time signaling that has been built to date,” said Silberg, Director of Rice’s Systems, Synthetic and Physical Biology Ph.D. Program. “To put it simply, imagine a wire that directs electrons to flow from a cellular chemical to an electrode, but we’ve broken the wire in the middle. When the target molecule hits, it reconnects and electrifies the full pathway.”
“It’s literally a miniature electrical switch,” Ajo-Franklin said.
“You put the probes into the water and measure the current,” she said. “It’s that simple. Our devices are different because the microbes are encapsulated. We’re not releasing them into the environment.”
The researchers’ proof-of-concept bacteria was Escherichia coli, and their first target was thiosulfate, a dichlorination agent used in water treatment that can cause algae blooms. And there were convenient sources of water to test: Galveston Beach and Houston’s Brays and Buffalo bayous.
With the physical constraints in place, the labs first encoded E. coli to express a synthetic pathway that only generates current when it encounters thiosulfate. This living sensor was able to sense this chemical at levels less than 0.25 millimoles per liter, far lower than levels toxic to fish.
In another experiment, E. coli was recoded to sense an endocrine disruptor. This also worked well, and the signals were greatly enhanced when conductive nanoparticles custom-synthesized by Su were encapsulated with the cells in the agarose lollipop. The researchers reported these encapsulated sensors detect this contaminant up to 10 times faster than the previous state-of-the-art devices.
The team sees engineered microbes performing many tasks in the future, from monitoring the gut microbiome to sensing contaminants like viruses, improving upon the successful strategy of testing wastewater plants for SARS-CoV-19 during the pandemic.
To that end, the team is collaborating with Rafael Verduzco, a Rice Professor of chemical and biomolecular engineering and of materials science and nanoengineering who leads a recent $2 million National Science Foundation grant with Ajo-Franklin, Silberg, bioscientist Kirstin Matthews and civil and environmental engineer Lauren Stadler to develop real-time wastewater monitoring.
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