The need for speed is obvious when detecting contaminants in the environment. Biosensors can detect a variety of molecules that trigger the synthesis of pigment proteins that in turn generate an optical signal. However, a sensor dependent on protein expression takes up to half an hour to generate an optical signal that is inherently difficult to detect. Quicker biosensors are essential for remediation.
Investigators at Rice University have engineered bacteria and hooked them up to electrodes to create bioelectronic sensors that can detect contaminants in a matter of minutes. Caroline Ajo-Franklin, PhD, and Jonathan Silberg, PhD, professors of biosciences at Rice University are the senior authors, and Joshua Atkinson, PhD, and Lin Su, PhD, are the lead authors of the study published in the journal Nature on November 2.
The findings show bacterial cells can be programmed to produce a detectable electrical current in response to the presence of specific chemical contaminants. Moreover, these biosensors could scavenge energy from the environment to power themselves as they monitor rivers, farms, industry and wastewater treatment plants to ensure the safety and security of water bodies.
The investigators used the common Escherichia coli to produce a chemical-induced electrical current by programming a synthetic electron transport chain.
“I think it’s the most complex protein pathway for real-time signaling that has been built to date,” said Silberg. “The chain has eight components that control electron flow, but there are other components that build the wires that go into the molecules. There are a dozen-and-a-half components with almost 30 metal or organic cofactors. This thing’s massive compared to something like our mitochondrial respiratory chains.”
“It’s literally a miniature electrical switch,” said Ajo-Franklin. “You put the probes into the water and measure the current. It’s that simple. Our devices are different because the microbes are encapsulated. We’re not releasing them into the environment.”
One of the challenges the scientists encountered was attaching the bacteria to the electrodes. “They don’t naturally stick to an electrode,” said Ajo-Franklin. “We’re using strains that don’t form biofilms, so when we added water, they’d fall off.”
To get around this problem, Xu Zhang, PhD, a postdoctoral researcher in Ajo-Franklin’s lab and a co-author of the study, encapsulated the sensors in a sheath of inert agarose that allowed contaminants in but held the sensors in place, reducing the noise.
In their prototype, the researchers designed the bacterial sensor to detect thiosulfate in two minutes. Thiosulfate is used in water treatment and can cause algal blooms. They tested their initial device in water collected from Galveston beach, Houston’s Brays and Buffalo bayous. The live sensor was able to sense thiosulfate at levels less than 0.25 millimoles per liter, which is far lower than levels that are toxic to fish.
In another design, the researchers programmed E. coli to detect an endocrine disruptor, 4-hydroxytamoxifen (4-HT).The signals were greatly enhanced when Su included custom-synthesized, conductive nanoparticles in the encapsulated biosensor. Using the biosensor, the researchers were able to detect the endocrine disruptor in urban waterway samples within three minutes—ten times faster than the earlier state-of-the-art devices.
Among other potential applications, Silberg sees engineered microbes monitoring the gut microbiome, sensing viruses, and improving wastewater plants in the future.
“Real-time monitoring becomes pretty important with those transient pulses,” said Silberg. “And because we grow these sensors, they’re potentially pretty cheap to make.”
Silberg’s team is collaborating with Rafael Verduzco, PhD, a Rice professor of chemical and biomolecular engineering, Kirstin Matthews, PhD, a senior Baker Institute fellow, and Lauren Stadler, PhD, assistant professor of civil and environmental engineering at Rice, to develop real-time wastewater monitoring.