Weight control for bacterial plastic.

In their quest for better ways to synthesize biodegradable plastics, scientists are looking to polymers produced naturally by certain bacteria. So far, most research has focused on increasing the polymer yield, but a new study takes a step toward influencing the plastic's final properties.

Researchers at the Massachusetts Institute of Technology have found that they can control the size of the polymer's molecules by genetically manipulating production of a key enzyme. The larger polymers made by these modified bacteria can weigh up to four times more than those synthesized naturally.

"I think we'd be able to go higher and lower by further manipulating the genetics," says MIT chemist Kristi D. Snell.

A polymer's molecular size strongly influences the properties of the finished plastic. The MIT group's report, which appears in the January Nature Biotechnology, suggests that biodegradable plastics made by bacteria could be practical for a wide range of applications.

"What exactly controls the molecular weight of biopolymers of this kind is really not well understood," says Stephen Padgette of Monsanto Co. in St. Louis. "This research is probably a first step in trying to understand that."

The MIT group studied a polymer called PHB, which the bacterium Alcaligenes eutrophus produces to store energy. The researchers genetically engineered Escherichia coli to produce PHB. Controlling enzyme activity is easier in a bacterium that doesn't normally manufacture PHB, Snell says.

The researchers inserted additional genes in E. coli to make them produce different amounts of PHA synthase, an enzyme that links the individual polymer units into chains. In cells with lots of PHA synthase, the enzyme joined the polymer units into a larger number of shorter chains, whereas in cells with less synthase, the enzyme made longer chains.

Altogether, bacteria make more than 100 polymers of the PHA type, many of which hold promise as components of biodegradable plastics. "We believe our method of controlling for molecular weight will apply to the whole class," Snell says.

Researchers have also turned plants into plastics factories by giving them bacterial genes (SN: 12/24&31/94, p. 420). Because bacteria must be provided with organic molecules before they will synthesize polymers, "the ultimate goal is to have the plants take carbon dioxide, sunlight, and water and produce the material," Padgette says.