Bacteria have long promised to be a powerful ally for cleaning up sites contaminated by pesticides and chemical weapons. But the bacterial enzymes that can break down the toxic chemicals are often sealed inside the cells themselves, where they can't attack the pollutants. Work reported in the October issue of Nature Biotechnology points to a possible solution: genetically engineering the bacteria to display the beneficial enzymes on the outside of their cell membranes, where they can get right to work.
Wilfred Chen, a chemical engineer at the University of California, Riverside, was looking for a better way to exploit the bacterial enzyme organophosphorus hydrolase (OPH), which readily breaks down a class of pesticides and chemical weapons known as organophosphates. Unless the OPH is purified from the bacteria, the clean-up process is slow. Extracting the enzyme is also time-consuming, however: Each enzyme batch takes about 2 weeks to make. Chen figured that he could speed up the process if he modified the common bacterium Escherichia coli to put OPH on its cell surface, so the enzyme wouldn't need to be purified.
So he and his co-workers created a new gene coding for a hybrid protein that combined the active region of OPH with other sections designed to anchor the protein in the bacterial cell membrane, with the active region on the membrane's outer surface. The group then introduced the new gene into E. coli and tested the modified bacteria. They found that bacteria fringed with OPH detoxified the pesticides parathion and paraoxon nine times more efficiently than bacteria containing OPH within their cells. Once Chen induced the altered bacteria to start making OPH, they continue doing so for up to 1 month, he says.
To show that the system can clean contaminated water, Chen plans to suspend the altered bacteria in a column, then pour solutions containing organophosphate pesticides or nerve gases over it. The water running out the other end should be clean. "This system cuts down on effort and cost," says Chen, though he admits, "it is not the ultimate solution." The system cannot clean toxins attached to soil particles, he says, because the bacteria are designed to work in a column.
Simon Silver, a microbiologist at the University of Illinois, Chicago, says Chen's work is a first step toward a useful technology. "It really promises to be useful, but they haven't made it useful yet," Silver says, who's waiting to see how the altered bacteria work in field trials.