Microbial communities in urban streams are developing resistance to drugs as a result of pharmaceutical pollution.

When a team of researchers measured pollution levels in a group of streams as part of the Baltimore Ecosystem Study, they found urban streams hosted a greater variety and larger concentration of drugs than suburban streams.

Scientists used passive sensors, which measure levels of painkillers, stimulants, antihistamines, and antibiotics over a two-week period. Researchers also measured the responses of microbial samples to caffeine, cimetidine, ciprofloxacin and diphenhydramine.

"Stream microbial communities are sensitive to pharmaceuticals, which can suppress both respiration and primary production," John J. Kelly, a researcher at Loyola University Chicago, said in a news release. "We used respiration as a proxy to assess microbes' ability to maintain biological function in the presence of pharmaceuticals."

Caffeine and cimetidine depressed respiration rates of microbes from both suburban and urban streams, but ciprofloxacin, an antibiotic, only depressed respiration rates in suburban samples. Microbes from urban streams had developed a tolerance.

The microbial diversity of both urban and suburban samples was affected differently by each contaminant.

The findings -- detailed this week in the journal Ecosphere -- suggest urban microbial communities are better able to adapt to the presence of pharmaceutical pollution.

"We suspect that since urban streams have received frequent pharmaceutical inputs over long timescales, pockets of drug-resistant microbes have developed in these streams," said Emma Rosi, an aquatic ecologist at the Cary Institute of Ecosystem Studies. "They are ready to colonize substrates, even when drugs are present. When faced with pharmaceutical exposure, these resistant microbes can maintain ecological function, even when other species have been eliminated."

Microbial communities, which often form biofilms on rocks and stream beds, perform a variety of ecological functions. They break down contaminants, as well as organic matter, recycling nutrients. They anchor freshwater food chains.

Microbial communities are capable of adapting to environmental change, but the adaptations aren't always positive.

"Different types of microbes can withstand different types and concentrations of synthetic chemicals," Rosi said. "When we expose streams to pharmaceutical pollution, we are unwittingly altering their microbial communities. Yet little is known about what this means for ecological function and water quality."

The latest research showed pharmaceutical pollution is encouraging higher concentrations of Aeromonas genus, a group of microbes associated with human disease and gastrointestinal problems.

"Effectively managing our freshwaters requires an understanding of how contaminants, including pharmaceuticals, impact microbial communities," Kelly said. "Our findings show that biofilms can be surprisingly resilient. The broader ecological impacts of changes in microbial species composition, as well as the effects of suppressed microbial functioning in more rural streams, remain important questions to explore."

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