INNOVATION : Plastic crop quest seeks rich pickings

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It sounds like the ultimate green dream: a plastics factory stretching over acre after acre, but without a chemical vat or an oil-filled pipeline in sight. Instead, there is nothing but a vast field of potatoes.

This bizarre vision, which brings a whole new meaning to the image of the chemical plant, is being actively pursued by a group of researchers at the Swiss Federal Institute of Technology in Zurich.

In the process envisaged by the Swiss group, led by Bernhard Witholt, the potatoes use carbon-rich compounds, which they would otherwise convert into starch, as the raw material for making a plastic that is not only tough but biodegradable. The potatoes perform this trick by virtue of genetic engineering.

Realisation of the vision remains a long way off, but the principles behind it are already well established. Dr Witholt's group has been producing biodegradable plastics, composed of polymers called PHAs, on a small scale by feeding bacteria on octane, akey component of petrol.

Microbiologists have become accustomed to the idea that if one searches hard enough, one can find microbes that feed on just about any substance imaginable.

Recently a species of bacteria was identified that ingests uranium, the main ingredient of nuclear reactor fuel.

Compared with this, Dr Witholt's oil-consuming bugs are mild specimens. They are members of a species called Pseudomonas oleovorans, which are widespread in soils and, as the name suggests, have a voracious appetite for oil.

They thrive around oil spills and for this reason have been comandeered for cleaning them up - a strategy that involves providing nutrients to assist the bacteria to multiply and devour the spillage.

This approach, called bioremediation, was used to deal with the huge oil spill from the Exxon Valdez in Prince William Sound, Alaska, in 1989.

The microbes convert oil into PHAs, which they use as an energy source, much as our bodies produce fats. The polymers reside inside the bacteria in the form of small granules, which can be extracted by breaking the cells apart. The granules can then be processed into flexible plastic.

Dr Witholt believes it should be possible to transfer the plastic-making genes from these bacteria into plants.

This approach to producing biodegradeable plastics has already been explored by researchers at the biotechnology company Zeneca, which has transferred genes from similar bacteria into oilseed rape plants.

As well as making oil, the plants make the plastic polyhydroxybutyrate (PHB), a member of the PHA family of polymers. There is also a suggestion of transferring the genes to trees, allowing companies to grow plastic in plantations just as producers do with rubber.

The type of PHA produced by Dr Witholt's bacteria differs from PHB in that the molecules that go into making it contain longer chains of carbon atoms. This makes the polymer more flexible than PHB.

Flexibility is clearly an important property for producing things like plastic bags. The biodegradability of these plastics may also allow them to be used in microcapsules for administering drugs. The slow degradation of the capsule in the body could allow the drug to be delivered internally at a steady rate over a long period.

At present, Dr Witholt says, industrial companies are taking a careful approach to bacterial PHA - his group has talked to the chemicals companies, but there are no plans for large-scale production yet, in contrast to Zeneca's PHB project.

He says that bacterial PHA will be competitively priced.