Bugs clean up nerve agents

Bespoke bacteria tackle poisonous organophosphates.

Bacteria could digest chemical-weapons stockpiles, say Californian chemists. Their genetically engineered bacteria might also scrub pesticides from farm equipment.

A bin full of bugs could make a cheap, green bioreactor to break down residues left in agricultural aircraft, tractors or animal dips, says Ashok Mulchandani of the University of California in Riverside.

Mulchandani and his colleagues have given Escherichia coli bacteria the power to break down organophosphates. Developed – but now banned – as chemical-warfare agents, the use of mild forms of these compounds as insecticides has led to concern that they may harm farm workers or contaminate food.

Current methods for disposing of organophosphates include rinsing and incineration. „Even washing equipment is one of the sources of environmental contamination,“ says David Coggon of the University of Southampton, who chairs the UK government’s Advisory Committee on Pesticides.

Better bacteria

Mulchandani’s team had previously given E. coli an enzyme that naturally breaks down organophosphates¹, taken from wild soil microbes. Now they have fine-tuned their bacteria.

The group added a protein that binds the bacteria to cellulose, stopping them from being washed away by chemicals. Stick-on bacteria increased the degradation rate tenfold².

They also customized the wild enzyme, organophosphorus hydrolase. By creating many slight genetic variations, the team found one form that chews up the pesticide methyl parathion 25 times faster than the original³.

„It’s a cute way to solve the problem,“ says chemical engineer George Georgiou of the University of Texas in Austin. E. coli is cheap and efficient compared with purifying the enzyme or using the soil microbes.

Deadly agent

In 1995, members of the Aum Shinrikyo cult released the nerve gas sarin – an organophosphate that cripples the central nervous system – on the Tokyo subway, killing 12 people.

Some fear that organophosphates could feature in future terrorist attacks. US military researchers are therefore pursuing ways to decontaminate affected sites.

Releasing genetically modified (GM) bugs into the environment is not the solution, admits Mulchandani, because of public concern over GM organisms. Instead, researchers are exploring the use of purified forms of the enzyme, despite the additional expense.

Some of the more toxic agricultural organophosphates have already been phased out in Britain and the United States, such as methyl parathion on food that is picked by hand. This trend is likely to continue, thinks Coggon: „I think their use is likely to decline as better alternatives become available“.

References

1. Richins, R.D. et al. Biodegradation of organophosphate pesticides using surface-expressed organophosphorus hydrolase. Nature Biotechnology, 15, 984 – 987, (1997).
2. Wang, A.A., M-H, Mulchandani, A. & Chen, W. Specific adhesion to cellulose and hydrolysis of organophosphate nerve agents by a genetically engineered Esceherichia coli strain with a surface-expressed cellulose-binding domain and organophosphorus hydrolase. Applied and Environmental Microbiology, 68, 1684 – 1689, (2002).
3. Cho, C., M-H, Mulchandani, A. & Chen, W. Bacterial cell surface display of organophosphorus hydrolase for selective screening of improved hydrolysis of organophophate nerve agents. Applied and Environmental Microbiology, 68, 2026 – 2030, (2002).