by Michael Keller

The world is awash in antibiotics. We take them to fight off the bacteria that mean to colonize us. We feed them to animals to prevent the outbreak of disease in densely packed factory-farming operations. Even many of our cleaning and body care products, controversially, now contain them.

But many antibiotics don’t get fully metabolized within humans or animals and, through excretion, find their way into waste and surface waters. It’s a major environmental concern whose full ecological implications still aren’t clear.

And the problem creates a vicious cycle. Evolution gives our microbial adversaries the strategic advantage—the ability to adapt to our weapons and render them harmless. So we engage in a microscopic arms race, battering increasing numbers of antibiotic-resistant bugs with more and more drug compounds to keep them at bay.

So you could call it a small case of poetic justice when researchers figure out how to use the cellular machinery that renders some bacteria drug-resistant to reclaim antibiotics from contaminated water.

University of Cincinnati environmental engineers have pulled a key protein, called AcrB, from antibiotic-resistant bacteria. AcrB protects the bacteria that have it by pumping drugs that would normally kill them out of the cell without causing harm.

“Antibiotic-resistant bacteria survive because of this protein pump,” environmental engineering professor David Wendell tells Txchnologist. “AcrB is a selective garbage disposal that binds and pumps out only these deleterious compounds for the bacteria. It’s an amazing piece of evolution, actually.”

Snatching a weapon for our use

The engineers realized that if they could turn the pump around and embed it into the walls of empty sacs of membrane called vesicles, they could actively filter antibiotics from the surrounding environment.

Wendell and doctoral student Vikram Kapoor isolated the AcrB protein from antibiotic-resistant E. coli, a bacterium found in the intestines of mammals that can cause food poisoning when accidentally ingested. They then latched to AcrB another bacterial protein called delta-rhodopsin that takes sunlight and converts it into energy to power the pump—making the filter solar-powered. Finally, they embedded the two-protein complex into the walls of vesicles they had gleaned from E. coli.

When they placed the microscopic systems into an antibiotic bath and shined a light on them, amazingly, they sprang into action, pumping the drug into the vesicle sacs and dropping its concentration in the surrounding solution. They detailed their findings in the journal Nano Letters. “Essentially, it’s the same pumping that happens in resistant E. coli. The key difference it that things are getting pumped in instead of getting pumped out,” Wendell says.

(Using the mechanism bacteria use to shrug off powerful antibiotics, scientists have developed solar-powered nanofilters that remove antibiotics from lakes and rivers twice as efficiently as the best existing technology. Courtesy Michael Woods/ACS.)

Working better than standard methods

In the lab, their filters, each smaller than a human hair, removed nearly twice as much antibiotic as activated carbon, the conventional material used to scrub water of contaminants. The vesicle system filtered out more than 70 percent of antibiotic compared to a maximum of 44 percent removed by carbon per milligram of either type of filter. “It’s roughly two times as good as carbon,” Wendell says. “That translates to needing half the weight of our vesicles compared to carbon to filter out the same amount of compounds.”

The UC team’s pump isn’t limited to just antibiotics, though. Wendell says other bacterial protein pumps similar to AcrB have different binding sites, which can transport hormones or some heavy metals like silver and copper from water into the vesicles for containment and recycling. “There are a couple—maybe five or six—different models,” he says. “You just swap out the protein pump and you could recover these things.”

Once the contaminants are trapped inside the vesicles, they can be released on demand by applying a bit of mild detergent that makes the sacs leak them out. The addition of another ingredient rehabilitates the vesicles so they can be reused for another round of filtration.

Wendell envisions their innovation filling big bags that can be dipped in rivers and lakes—like teabags that are left underwater until the vesicles are filled with pure antibiotic compounds. Then they could be pulled up and the antibiotics, amazingly, could be recovered and reused. “You can recycle the antibiotics from these vesicles,” he says. “Farmers, healthcare facilities, they all need antibiotics and these drugs cost money. Our filter closes the loop.”

Top Image: Heavy rusty water flap via Shutterstock.