Using bacteria to clean the environment

In recent years, concerns have heightened about increasing amounts of drugs in the environment, particularly in water. While the impact of this environmental pollution is not well understood, some evidence indicates that these drugs may be entering the food chain. Researchers  believe that most of the drugs that end up in fresh water first accumulate at wastewater treatment facilities. Therefore, there is a need to eliminate the drugs at these facilities.

Ashley Robinson, a senior biochemistry major at Hamline University who plans to start graduate school in the fall, started doing research in her sophomore year. She is presenting a poster at the 2021 ASBMB Annual Meeting on this topic, the third research project she has worked on with Betsy Martínez–Vaz.

Kathryn Malody

Ashley Robinson works in the biological safety hood at the Martínez–Vaz lab.

The researchers’ goal was to find bacteria that break down metformin, a drug commonly used to treat diabetes in the U.S. and around the world. Little research has been done on the impact of pollution with metformin and its byproduct, guanylurea, which are not fully metabolized by humans and thus are excreted into wastewater systems. “We consider them to be emerging pollutants,” Robinson said.

Ashley Robinson & James Aukema

This graphic represents the topics of Robinson’s research. With increasing prescription of the Type 2 diabetes drug metformin (top), both metformin and its predominant metabolite, guanylurea (bottom), are water pollutants of emerging concern worldwide. The researchers recently isolated a strain of Pseudomonas that can completely degrade guanylurea from a wastewater treatment facility (middle). They identified and characterized a guanylurea-degrading enzyme, guanylurea hydrolase.

Studies have demonstrated the potential for metformin to disrupt some hormones, she explained. The drug  is considered an endocrine disruption agent in some small fishes, and guanylurea has been shown to interfere with the nitrogen cycle in soil. Little is known about its bioaccumulation potential.

“Can these molecules pass up the food chain?” Robinson said. “That is one concern that we have.”

The research team collected samples at a local wastewater treatment facility from several stages of the treatment process. The bacteria in the samples were then grown in the lab under limiting conditions, meaning the bacteria were not given all the nutrients they needed. In this case, their only source of nitrogen was metformin, so most of the bacteria that survived were species that could use metformin as a nitrogen source. The team then used metagenomics to identify the enzymes involved in the breakdown of guanylurea and its transformation product guanidine. They identified three enzymes: guanylurea hydrolase, carboxyguanidine deiminase and allophanate hydrolase.

Robinson and her colleagues are now working to identify the enzyme that breaks down metformin in the initial step that forms guanylurea. They hope the enzymes they find could be used to break down metformin and guanylurea at wastewater treatment facilities, keeping these pollutants out of freshwater systems.