A research team led by Chuanwu Xi of the University of Michigan School of Public Health sampled water containing the bacteria Acinetobacter at five sites in and near Ann Arbor's wastewater treatment plant.
They found the bacteria resistant to multiple antibiotics (aka superbugs) up to 100 yards downstream from the discharge point into the Huron River.
However, it is important to understand much work is still needed to assess what risk, if any, the presence of superbugs in aquatic environments poses to humans.
"We still need to understand the link between aquatic and human multiple drug resistant bacteria," said Xi, assistant professor of public health.
The team found
while the total number of bacteria left in the final discharge effluent declined dramatically after treatment, the remaining bacteria was significantly more likely to resist multiple antibiotics than bacteria in water samples upstream.
In fact, some strains resisted as many as seven of eight antibiotics tested. The bacteria in samples taken 100 yards downstream also were more likely to resist multiple drugs than bacteria upstream.
"Twenty or 30 years ago, antibiotics would have killed most of these strains, no problem," he said.
The Acinetobacter was chosen as the subject of the team's research because it is a growing cause of hospital-acquired infections and because of its ability to acquire antibiotic resistance.
The problem isn't that treatment plants don't do a good job of cleaning the water—it's that they simply aren't equipped to remove all antibiotics and other pharmaceuticals entering the treatment plants.
The treatment process encourages bacteria to grow and break down the organic matter. This makes it a fertile breeding ground.
Good and bad bacteria grow and replicate along side each other. In the confined space, bacteria share resistant genetic materials, and remaining antibiotics and other stressors may select multi-drug resistant bacteria.
The next step
is to see how far downstream the superbugs survive and try to understand the link between aquatic and human superbugs. This study did not look past 100 yards.
Xi's colleagues include visiting scholar Yongli Zhang; Carl Marrs, associate professor of public health; and Carl Simon, professor of mathematics.