By Andy Coghlan WEED KILLERS that harm fish when they leak from fields into streams could soon be destroyed by spraying herbicide-eating bacteria onto the soil. Allan Walker and his colleagues at Horticulture Research International, a government-owned research agency in Wellesbourne, Warwickshire, have discovered a strain of bacteria that degrades phenylureas, the most widely used herbicide family in Europe. Phenylureas persist in soil for up to a year, and these residues are frequently washed out into streams. One well-known member of the phenylurea family is diuron, which is used to keep surfaces such as railway lines and paths free of weeds. It is weakly carcinogenic and known to be harmful to fish. Isoproturon, the most widely used phenylurea herbicide, used by wheat and barley farmers to control blackgrass, may also pose a hazard to fish. Walker and his colleagues discovered a diuron-destroying strain of Arthrobacter in fields that had previously been treated with the herbicide. They later established that the new strain, codenamed D47, has a taste for other phenylureas too. “The Arthrobacter species we’ve found can degrade them all,” says Walker. The researchers have traced the herbicide-destroying genes to a tiny loop of DNA, called a “plasmid”, that is unique to ArthrobacterD47. Their results will soon be published in the journal Soil Biology and Biochemistry. But Walker says that several important questions will have to be answered before the bacteria can be used to cleanse fields of residual herbicides. “Would they persist, for example, and prevent herbicides applied the following season from doing their job?