Antibiotics Driving Resistant Bacteria In Urban Sewers

Brett Smith for – Your Universe Online

A new study from Chicago-based researchers has found that a confluence of sewage overflows and widespread antibiotic use is causing the proliferation of antibiotic-resistant bacteria in waterways around the Windy City.

Recently published in the journal Environmental Science and Technology, the study looked at the antibiotic triclosan, which is in about half of liquid soaps and functions by slowing or stopping the growth of bacteria, fungi, and mildew. Triclosan frequently gets into streams and rivers through domestic waste-water, leaky sewers and sewage overflows.

“The bacterial resistance caused by triclosan has real environmental consequences,” said study author Emma Rosi-Marshall, an aquatic ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York. “Not only does it disrupt aquatic life by changing native bacterial communities, but it’s linked to the rise of resistant bacteria that could diminish the usefulness of important antibiotics.”

In the study, researchers collected samples at three sites in the Greater Chicago area: the urban North Shore Channel, the suburban West Branch Dupage River, and the rural Nippersink Creek. The team found a correlation between urbanization and a rise in both triclosan concentrations in sediments and the ratio of bottom-dwelling bacteria resistant to the antibiotic. The rural creek had the lowest levels of triclosan-resistant bacteria, while the urban site sampled downstream of 25 combined sewer overflows had the highest levels.

Combined sewers ferry sewage, industrial wastes, and storm water to a treatment plant using a single pipe. When excessive runoff from high rainfall or snowmelt gets into these sewers, the overflows that result send untreated sewage flowing directly into rivers and streams.

The Chicago researchers found that their local sewer overflows are a major source of triclosan pollution in the North Shore Channel. The team’s findings also support previous work that suggests sewage treatment plants can efficiently remove triclosan from wastewater.

“We detected much lower levels of triclosan at a site downstream of a sewage treatment facility as compared to a site downstream of combined sewer overflows,” said John Kelly of Loyola University Chicago. “And we demonstrated a strong link between the presence of triclosan in the environment and the development of triclosan resistant bacteria.”

Almost 800 cities in the United States use combined sewer overflows, an infrastructure feature that the Environmental Protection Agency cites as a major water pollution concern.

The study researchers also conducted artificial stream experiments at Loyola to confirm field tests that indicated triclosan exposure triggers an increase in triclosan-resistant bacteria. In addition to the proliferation of resistant bacteria, researchers also saw a drop in the diversity of river-bottom bacteria and a change in the makeup of bacterial communities. The researchers noted a six-fold increase in cyanobacteria and a spectacular die-off of algae.

“Cyanobacteria are less nutritious than algae and can produce toxins,” Rosi-Marshall explained. “In triclosan-polluted streams and rivers, changes in microbial communities could negatively affect ecological function and animal communities.”

The study is the latest result of study researchers’ on-going effort to learn more about the ecological and human health consequences of synthetic antimicrobials.