Scientists have identified the first bacteria that can break the toughest chlorine bonds in polychlorinated biphenyls (PCBs), some of the most stubborn pollutants in the world. The discovery, reported in the January issue of Environmental Microbiology, may help bioremediation of these toxic and persistent compounds, which were used in industry for more than 70 years until their ban in the United States in 1979.
PCBs have long eluded practical solutions for rendering them harmless. Although certain oxygen-requiring microbes can break down compounds containing fewer than four chlorine atoms, most commercial mixtures of PCBs possess up to 10 chlorine atoms per molecule. Lab studies of contaminated marine and estuarine sediments--where PCBs have accumulated and lingered for decades--indicated that unknown organisms in the oxygen-deprived sediments could break down the large PCBs, but no one had isolated or identified the responsible organisms.
To detect the PCB-munchers, a team of microbiologists led by Harold May of the Medical University of South Carolina, Charleston, and Kevin Sowers of the University of Maryland Biotechnology Institute in Baltimore looked for signs of dechlorination in a sediments dredged from Baltimore Harbor. When chlorine-busting bugs appeared to be active, the researchers extracted bacterial DNA from the sample. By process of elimination they found that a bug called 017 was always on the scene when PCBs were being broken down.
The researchers suggest that this bacterium will pave the way for improved techniques to clean up contaminated sites. "If we know the organisms are there but there's not much activity, we may be able to add nutrients that are limiting growth or possibly treat dredged sediments in bioreactors by adding mixtures of degrading organisms," Sowers says.
The discovery is "extremely exciting," says Linda Chrisey, a molecular biologist with the Office of Naval Research in Arlington, Virginia, adding "it provides us with tools for assessing the role of these organisms in 'real' environments and the potential for novel strategies for addressing the problem of PCB contamination in sediments."