Researchers have discovered the persistence of antibiotic-resistant GMO genes in sewage sludge

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Two researchers work in a laboratory
Graduate student, Sandra Un Jan Contreras, and assistant professor Courtney Gardner in Gardner's lab.

Antibiotic-resistant genes that have been included in genetically modified foods are able to withstand conventional wastewater treatments.

The results, contained in a research paper recently published in the journal Biotechnology and bioengineering, could indicate a previously unknown way that bacteria can become resistant to life-saving antibiotics. The work is based on the graduate work done at Duke University by Courtney Gardner, now an assistant professor in the Department of Civil and Environmental Engineering of the WSU. Together with Professor Tim Ginn of the Department of Civil and Environmental Engineering, he recently received a 3-year grant from the USDA to continue the research.

Antibiotic resistance, which bacteria adopt in response to drug interactions, is increasing worldwide and threatens the ability to treat many common infections and diseases.

"We have established that extracellular DNA released from digestion appears to be ubiquitous in wastewater treatment in the United States – it is much more persistent in the environment than we initially thought," said Gardner. "Historically, I think this is likely to have contributed to the spread of antibiotic resistance in the environment.

"The extent of this contribution is still unknown – it's something we're trying to determine," he added.

Unlike Europe, which has largely banned the cultivation of genetically modified crops for human consumption, GMOs are a common part of the food supply in the United States. Although the practice is becoming less common, in the past companies have added antibiotic-resistant genes in their modifications as useful markers to differentiate genetically modified plant cells. About 130 lines of genetically modified crops contain these genes. The researchers found that when people eat these foods, the embedded genetic material moves through the digestive system and that these genes can be released into the environment, even in wastewater treatment plants.

Meanwhile, half of the biosolids produced in the United States after wastewater treatment are used as agricultural fertilizer every year, providing a potential pathway for the movement of antibiotic-resistant genes and bacteria in the environment.

In the new study, funded by the National Science Foundation, researchers added antibiotic-resistant genes to the reactors to mimic the most common wastewater treatment for 30 and 60-day treatments that generate class A biosolids. They added antibiotics common, such as penicillin, which are also often found in wastewater.

The researchers found that fragments of antibiotic resistance genes, especially longer strands, persisted during the treatment process. They believe that DNA strands cling to particles in the soil or to sediments in digesters. There they lie protected from the processes that generally kill microbes and, instead, become a reservoir of genetic material.

Furthermore, the researchers found that the genes seemed to be absorbed by bacteria in the sludge. Researchers theorize that bacteria in wastewater, such as staphylococcus, are stressed when they encounter antibiotics. By collecting antibiotic-resistant DNA strands, they gain a selection advantage.

In their new study, Gardner and Ginn will follow and shape long-range transport of antibiotic-resistant genes in agricultural fields. They will also study the silencing of RNA molecules, which are used in modern genetically modified crops to silence the undesirable traits of crops. The researchers want to know if the residual genetic material of the silencing of RNA molecules could bind with other bacteria in the environment to inadvertently silence the desirable traits.

"Our genetic and bacterial concern is that there are probably some non-target matching sequences between the silencing RNA and environmental microbes that could cause the loss of a gene and possibly a functional gene," he said. Gardner.

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