GENETICALLY modified bacteria capable of cleaning up soil contaminated with toxic solvent residues have come a step closer with the publication of the genome sequence of a sewage-dwelling bug.
The bacterium Dehalococcoides ethenogenes is already being used inpilot projects on contaminated sites to remove perchloroethene (PCE) and trichloroethene (TCE) from the soil. These solvents were once thought to be inert and harmless, and were dumped in large quantities in the 1950s and 1960s. 鈥淭hen it emerged that they were both neurotoxic and carcinogenic,鈥� says Stephen Zinder of Cornell University in Ithaca, New York. 鈥淓very dry-cleaning store in the US probably has a PCE spill associated with it,鈥� he says. TCE contaminates airfields, where it was used to degrease aircraft parts.
Sites undergoing treatment with D. ethenogenes include the Kelly Air Force base in San Antonio, Texas, and a derelict industrial site in Caldwell, New Jersey. Although the bug appears to be working well, it is hard to culture in the quantities needed because the nutrient mix it thrives on in sewage is difficult to reproduce. Researchers would like to find out how it breaks down the pollutants, and possibly transfer the genes for its solvent-busting enzymes into more easily nurtured bacteria.
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鈥淥ne goal is to transfer the genes for solvent-busting enzymes into more easily nurtured bacteria鈥�
An important step towards this goal has been achieved by a team led by Rekha Seshadri at the Institute for Genomic Research in Rockville, Maryland, which has sequenced the bug鈥檚 genome (Science, vol 307, p 105). The team discovered genes for a battery of 19 different enzymes that the bug uses to break down the chlorinated solvent molecules. The main residues it leaves behind are ethene, a non-toxic gas better known for making fruit ripen, and small amounts of hydrogen chloride, which dissipate harmlessly in the environment.
Now the race is on to find out what each of the enzymes does. One option is to transfer the genes that code for the enzymes into a more easily cultured bug. The team is also looking for other natural bacteria that already contain some of these genes, and which are also easier to produce. Zinder, who is part of the team that sequenced the bug, says that two other strains of Dehalococcoides are already showing promise.
Until recently, the only way to decontaminate soil affected by chlorinated solvents was to apply chemical or filtering treatments. All existing treatments, he says, create additional waste that has to be burned or buried elsewhere.
鈥淭he beauty of bioremediation is that you degrade the pollutants in place,鈥� says Phil Dennis of SiREM, a company in Guelph, Ontario, which is developing its own bacterial anti-pollution treatments.
