Study Shows Persistent Nature of Antimicrobials

The active ingredients of anti-bacterial soaps and cleaning agents have come under scrutiny by the U.S. Environmental Protection Agency and the U.S. Food and Drug Administration, due to both environmental and human health concerns.

Two closely related antimicrobials, triclosan and triclocarban, are at issue. Triclosan (TCS) has a structural resemblance to dioxin, and triclocarban (TCC) is one of the top 10 pharmaceuticals and personal care products most frequently found in the environment and in U.S. drinking water resources.

Researcher Rolf Halden and co-workers at the Biodesign Institute at Arizona State University have traced the active ingredients of soaps – used as long ago as the 1960s – to their current location, the shallow sediments of New York City's Jamaica Bay and the Chesapeake Bay, the nation's largest estuary.

"Our group has shown that antimicrobial ingredients used a half a century ago, by our parents and grandparents, are still present today at parts-per-million concentrations in estuarine sediments underlying the brackish waters into which New York City and Baltimore discharge their treated domestic wastewater," said Halden, a new member of the institute's Center for Environmental Biotechnology. "This extreme environmental persistence by itself is a concern, and it is only amplified by recent studies that show both triclosan and triclocarban to function as endocrine disruptors in mammalian cell cultures and in animal models."

Aiding in his team's research was another type of contamination: the radioactive fallout from nuclear testing conducted in the second half of the last century. Using the known deposition history and half-lives of two radioactive isotopes, cesium-137 and beryllium-7, Halden and his collaborators Steven Chillrud, Jerry Ritchie, and Richard Bopp assigned the approximate time at which sediments observed to contain antimicrobial residues had been deposited in the two East Coast locations.

By analyzing vertical cores of sediment deposited over time in the two sampling locations on the East Coast, they showed that TCC, and to a lesser extent, TCS, can persist in estuary sediments. TCC was shown to be present at parts per million levels, which could represent unhealthy levels for aquatic life, especially the bottom feeders that are important to commercial fishing industries like shellfish and crabs.

In the Chesapeake Bay samples, the group noticed a significant drop in TCC levels that corresponded to a technology upgrade in the nearby wastewater treatment plant back in 1978. However, earlier work by the team had shown that enhanced removal of TCC and TCS in wastewater treatment plants leads to accumulation of the problematic antimicrobial substances in municipal sludge that often is applied on agricultural land for disposal. Lead author Todd Miller concludes that "little is actually degraded during wastewater treatment and more information is needed regarding the long-term consequences these chemicals may have on environmentally beneficial microorganisms."

Along the way of studying the deposition history of antimicrobials in sediments, the team also discovered a new pathway for the breakdown of antimicrobial additives of consumer products. Deep in the muddy sediments of the Chesapeake Bay, they found evidence for the activity of anaerobic microorganisms that assist in the decontamination of their habitat by pulling chlorine atoms one by one off the carbon backbone of triclocarban, presumably while obtaining energy for their metabolism in the process. "This is good news," said Halden, "but unfortunately the process does not occur in all locations and furthermore it is quite slow. If we continue to use persistent antimicrobial compounds at the current rate, we are outpacing nature's ability to decompose these problematic compounds."

While combining bioenergy production and pollutant destruction has its own appeal, Halden sees a simpler solution to combating the pollution his team discovered: limit the use of antimicrobial personal care products to situations where they improve public health and save lives.

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