These next-generation engineered bacteria can detect water contaminants in real time

Perhaps if the leak had been quickly detected when there were only minute changes, the situation would not have escalated to the level it did. The same can be said for similar catastrophes worldwide, whereby around two million tons of waste are pumped into water sources like rivers, lakes, and oceans daily.

This is where a newly developed "living bioelectronic sensor" could help. Researchers from Rice University have engineered bacteria to sense and report on the presence of various contaminants within minutes by releasing a detectable electrical current.

In an exclusive interview, Interesting Engineering (IE) spoke with the lead researchers, Dr. Caroline Ajo-Franklin and Prof. Joff Silberg, to gain a deeper understanding.

Engineered bacteria send a jolt of electricity

"We used both genetically engineered bacteria and specially designed materials to create the bioelectronic sensors," the researchers told IE.

They explained that in their study, bacteria were created to sense particular pollutants or 'environmental information.' These would then respond by generating an electrical current, or 'electrical information,' and all in real-time.

"Customized materials were designed to gather that current and transport it to a device that collected and quantified it," they explained. Still, reaching this point had its challenges and involved teaming up with engineers outside of their field.

A critical adjustment: Materials engineers encapsulated the sensors into agarose

"[Our engineered E.coli] don't naturally stick to an electrode," Ajo-Franklin explained. "We're using strains that don't form biofilms, so when we added water, they'd fall off."

The electrodes produced more noise than the signal at that time.

Together with co-author Dr. Xu Zhang, a postdoctoral researcher in Ajo-Franklin's lab, they created a lollipop-shaped agarose capsule. Agarose is a high-molecular-weight polymer extracted from the cell walls of some red algae. Significantly, this arrangement held the sensors in place while allowing pollutants to enter, lowering noise.

'We're not releasing them into the environment"

The design ensured that the materials didn't harm or kill the bacteria. Likewise, the materials couldn't be contaminated by the bacteria. Two things Ajo-Franklin and Silberg expressed to IE as essential.

"You put the probes into the water and measure the current," Ajo-Franklin said. "It's that simple. Our devices are different because the microbes are encapsulated. We're not releasing them into the environment."

E.coli was 'coded' to signal the presence of thiosulfate and endocrine

"I think it's the most complex protein pathway for real-time signaling that has been built to date," said Silberg in a recent press release.

He described the process as a wire that directs electrons to flow from a cellular chemical to an electrode- but that the wire is broken 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. 

Firstly, the Rice team successfully programmed E. coli to express a synthetic route that only produces electricity when it comes into contact with thiosulfate. 

In contrast to levels that are hazardous to fish, this biological sensor could detect this toxin at concentrations lower than 0.25 millimoles per liter.

Ten times faster than current state-of-the-art chemical sensors

Another experiment involved recoding the E.coli to detect an endocrine pollutant. This involved an additional step whereby lead author Lin Su custom-made conductive nanoparticles.

These were then combined with the cells in the agarose lollipop. The team concluded that the E.coli sensor functioned well, significantly improving the signals.

Whoa. E. coli + nanomaterials = Direct sensing of endocrine disruptors in water within 3 minutes.

Love to see real-world deployment, too!

From @AjoFranklinLab in Nature. https://t.co/QPnfCXzYjF pic.twitter.com/kSPhJ6hG02

— Niko McCarty 🧫 (@NikoMcCarty) November 2, 2022

The researchers claimed that compared to the earlier state-of-the-art equipment, these enclosed sensors detected this contamination up to ten times faster.

Better yet, they told IE, "The microbes are approximately 0.000001 meters in length. [Unlike mass spectrometers that are meter scale], they are much smaller than instruments used for chemical sensing today."

'Amazing' proteins: Responding to a single prompt in a sea of information

"Biology is really good at sensing molecules," expressed Silberg. "That's an amazing thing. Think about how complex the cell is and how proteins evolve that can respond to a single prompt in a sea of information."

He described how the team's goal is to take advantage of that "exquisite ability." In this way, they look forward to paving the way for more elaborate biomolecules, which they will then employ to create effective synthetic biology technologies.

From tackling algal blooms in rivers to applications in national defense

"We think this device and ones like it could have important applications in safeguarding the health of our rivers and oceans and thus our health," revealed Ajo-Franklin and Silberg to IE.

They explained that because their device can respond so quickly (in minute time scales) to contaminants, it could provide an early warning system for the unintended release of chemicals or toxins from various sources.

This includes contaminants released by organisms in the environment, such as algal blooms that can pollute waters. "It could even prevent further accidental release," they said.

From a more health-related perspective, Silberg sees the engineered bacteria monitoring the gut microbiome and sensing contaminants like viruses in wastewater.

"We also think this research could provide additional information for applications in national defense and for improving processes in advanced biomanufacturing," they said.

"A step towards solving some of the global challenges that confront us"

"We see the device we created as a step towards solving some of the global challenges that confront us. Pollution is one of the greatest current threats to human and ecological health," they told IE.

The researchers explained how pollutants could gradually accumulate in the human body, eventually harming health. Additionally, they can have both immediate and long-lasting impacts on ecosystems.

"Being able to quickly detect a variety of different pollutants will help avert or mitigate impacts on the environment and on human health."

The next steps will include coding the bacteria to sense other pollutants

The study represents a proof-of-concept effort where the team showed that they could convert chemical information in the environment into electrical information using a small (micrometer scale) microbe.

"We only sensed two chemicals but demonstrated both can be done in real-time," said Ajo-Franklin and Silberg. Still, as you can imagine, E. coli is only suitable for testing some water contaminants.

"There is a need for fast, easy-to-design variants of this system that can sense other chemicals across environmental settings with a wide range of characteristics."

Additionally, the team will look into the potential of their biosensors to detect multiple chemicals simultaneously.

Taking the proof-of-concept effort to "a whole new level"

The researchers revealed that this latter aspect would likely require the programming of microbes that grow optimally in different settings. Materials that will support seamless electrical integration while not being fouled by these different environmental settings will also need to be developed.

According to a recent press release, the team is currently collaborating with Rafael Verduzco- a Rice professor of chemical and biomolecular engineering, materials science, and nanoengineering.

Verduzco leads a $2 million dollar National Science Foundation grant that will bring together other bioscientists as well as civil and environmental engineers to develop real-time wastewater monitoring.

"The type of materials we can make with Raphael takes this to a whole new level," Ajo-Franklin revealed in a statement.

"This project is a great example of how collaborations between scientists with different expertise enable big technological leaps forward," the researchers concluded.