13 May 2009

Superbugs thriving in wastewater treatment plants

by Kate Melville

In the first study of its kind, University of Michigan researchers have established that wastewater treatment plants are providing a perfect environment for the emergence of antibiotic-resistant superbugs that eventually end up in neighboring streams and lakes.

Chuanwu Xi and co-researchers, 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 so-called superbugs - bacteria resistant to multiple antibiotics - up to 100 yards downstream from the discharge point into the Huron River. But Xi is cautious about 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.

Xi found that 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. 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.

Multiple antibiotic-resistant bacteria has emerged as one of the top public health issues worldwide in the last few decades as the overuse of antibiotics and other factors have caused bacteria to become resistant to common drugs. Xi's group chose to study Acinetobacter because it is a growing cause of hospital-acquired infections and because of its ability to acquire antibiotic resistance.

Xi explained that 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 is fertile ground for the creation of superbugs because it encourages bacteria to grow and break-down organic matter. However, the good bacteria grow and replicate along with the bad. In the confined space, bacteria share resistant genetic materials, and remaining antibiotics and other stressors may select multi-drug resistant bacteria.

The next step, said Xi, is to see how far downstream the superbugs survive and try to understand the link between aquatic and human superbugs.

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Source: University of Michigan