The image at left shows genetically engineered bacteria that consume corn sugar and produce a polyester that can be used to make biodegradable plastics, including the types used in shopping bags. (The polyester--called polyhydroxyalkanoate, or PHA--is visible inside the bloated cells.) After years of research and development, the bacteria are almost ready for use on the commercial scale. In a joint venture with ­Metabolix of Cambridge, MA, which makes the microbes, Archer Daniels Midland is building a plant adjacent to its corn mill in Clinton, IA, that will use them to generate 110 million pounds of PHA annually. The new plant will produce more than 300 times as much PHA as an existing ­Metabolix pilot plant. "We'll reduce greenhouse­-gas emissions by about two-thirds and petroleum usage by about 80 percent compared to traditional petroleum­-based plastics," says ­Metabolix vice president Brian Igoe. And bags made from Metabolix's polymer will degrade even if they drift into wetlands or the ocean. The compound will cost three times as much as petroleum-based polymers. Peter Fairley