Winter 2009

Water Resources and Sea Grant Research


Studying the Bugs that Fight Back

The more antibiotics the bacteria encounter, the faster their rapidly evolving populations develop resistance, making the drugs potentially ineffective…

By Carolyn Rumery Betz

Antibiotics fight dangerous bacteria in humans, farm animals, and farm-raised fish, but their use—and overuse—has led to bacteria that fight back—by the billions. The more antibiotics the bacteria encounter, the faster their rapidly evolving populations develop resistance, making the drugs potentially ineffective when they are most needed.

The bodily waste of antibiotic users contain both drug residue and resistant bacteria, and even after they pass through waste treatment processes, they are still found in the environment in high enough concentrations to be considered pollutants.

Katherine “Trina” McMahon, associate professor of civil and environmental engineering at UW–Madison, is trying to determine whether these pollutants could find their way into our water supply. She and her students are using cutting-edge technology to examine the genetic make-up of bacteria that are known to be resistant to the antibiotic tetracycline. If the resistance genes are mobile, they could move from one bacterium to another, allowing their rapid reproduction rates to spread their drug-resistant traits even faster.

In contrast to traditional methods of culturing bacteria in the lab, McMahon used molecular techniques to extract DNA from tetracycline-resistant bacteria found in two completely different settings—a new subdivision and four aquaculture facilities, all located in Wisconsin.

With funding from the Water Resources Institute, McMahon tested water samples from a subdivision’s individual on-site septic tanks, groundwater monitoring wells, and the homes’ water taps to see if tetracycline-resistance genes were present and, if so, at what concentrations. The objective was to ­determine if the nearby septic tanks and drain fields could contaminate the drinking water with the resistant bacteria.

The analysis showed no human health risks from antibiotic-resistant bacteria in the drinking water systems in this particular subdivision. More antibiotic-resistance genes were found in the septic tanks and monitoring wells than in the drinking water supplies. The sources of the genes could be the ubiquitous presence of bacteria in the human gut, the use of antibiotics by homeowners, past agricultural activity, or natural background levels.

With Sea Grant funding, McMahon applied the same genetic testing in four trout hatcheries; two of the facilities had at one time treated their fish with the antibiotic oxytetracycline, and the other two had not. The unexpected finding was that water in all four facilities tested positive for the tetracycline-resistance genes. McMahon determined that the likely source was the fish food, even if it was labeled as nonmedicated feed. Fish food is made up of processed fish mostly of unknown origin, so McMahon theorizes that at least some of the fish that made up the feed must have been exposed to antibiotics during some phase of their lives.

Hatchery managers in Wisconsin use antibiotics sparingly and need to know that the medicine will be effective when most needed. McMahon feels confident that her findings will be put to good use in the hatcheries.

“If the source was the atmosphere, there wouldn’t be much we could do. But when the source of the gene is as obvious as the feed,” said McMahon, “that is something that we can control.”



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