Volunteers are being recruited(征募) to eat raw potatoes in the first human trials of a vaccine grown in genetically engineered vegetables. Researchers in Texas hope that people who eat the potatoes will be protected against common gut(肠，肠子) infections. They believe this technique could prove to be a cost-effective way of growing vaccines in developing countries where such diseases are still killers. Other researchers previously succeeded in using similar techniques to produce potential vaccines Now Hugh Mason and his colleagues at Texas A & M University(得克萨斯农业及机械大学) their plant vaccines on mice and plan to recruit 15 volunteers for a human trial. The team first tested the technique in tobacco plants. They took a strain of Escherichia coli(大肠杆菌) bacteria that causes food poisoning, and identified the part of the poison which binds to its victims gut cells. They then used a modified plant bacterium called Agrobacterium tumefasciens to transfer the segment of DNA which manufactures the binding protein into the tobacco plant. Under normal circumstances, these bacteria transfer packets of DNA into plant cells to force the plant to manufacture the nutrients they need.But in the modified bacteria, the DNA package includes the gene to pro- duce the binding protein. Once the foreign DNA segment was incorporated(结合，合并) into the tobacco's own DNA, the bacteria were killed off with antibiotics. Mason's team then grew these modified tobacco plants and found that they produced the E. coli binding protein. Proof of success came when the tobacco leaves were mashed up(捣碎) and squirted into the stomachs of mice. Mason says that within days the mice started producing specific antibodies to the E. coli poison, but suffered no ill effects from digesting the binding protein. Mason then produced genetically engineered potatoes and fed these to mice, with similar results. Mason's team have used plants to produce vaccines against a number of other infectious agents. For example, they have made a vaccine using a protein from the shell of the Norwalk virus, which causes diarrhoea(腹泻) in children. A third vaccine has also been produced in tobacco using a surface protein from the hepatitis B virus. But Mason says that so far they have only been able to produce small amounts of it in potatoes. Although a vaccine already exists against hepatitis B, a cheaper plant version could make mass immunisation(群众性免疫) possible. One problem with growing potatoes to produce vaccines is that cooking tends to destroy the protein component of the vaccine, so they must be eaten raw. Mason thinks that bananas may be a better option. 'One banana could potentially produce a whole host of different vaccines,' says Mason. The purpose of this text is______。