OIL does not just provide the fuel that powers the internal-combustion engines in cars. Its by-products are also the basis of many of the materials, such as plastics, from which cars are made. One of these petroleum by-products is isoprene. This is used to make the synthetic rubber in car tyres. About a billion tyres are made every year—each one requiring about 26 litres (or seven American gallons) of oil. Now a way has been found to make greener tyres by using genetically modified bugs to produce isoprene biologically.

The work is being carried out by Genencor, an industrial biotech company based in California, in collaboration with Goodyear, one of the world's biggest tyremakers. In the laboratory Genencor has produced enough of what it calls BioIsoprene for Goodyear to build and successfully test prototype tyres made with the new material. Genencor is now completing a dedicated pilot factory which will be used to develop a mass-production process, which could be operating commercially around 2015.

Isoprene is a monomer, which is a substance whose molecules can be linked together chemically to form a polymer. Latex, a material obtained from rubber trees, is a natural polymer of isoprene. Isoprene can also be artificially polymerised to make synthetic rubber. Tyres are made of a combination of natural and synthetic rubber to provide the grip and endurance required by carmakers. Typically around a quarter of each tyre is made from isoprene derived from petrochemicals, and about 60% of worldwide isoprene production is used by tyremakers. The rest goes into making other products, including glues, disposable nappies and surgical gloves.

Genencor used a genetically modified form of E-coli, a favourite species of bacteria in microbial genetics, to produce BioIsoprene. By splicing in genes from other bugs, the company was able to engineer synthetic metabolic pathways—ones that do not exist in nature—that enable the bacteria to produce isoprene from the sugars found in plant materials such as sugar cane, corn cobs and switchgrass, a tall-growing variety native to North America. The modified organism ferments sugars in the biomass to produce isoprene, in such a way that it bubbles out as a gas. This makes it easier to collect and purify compared with separating it out from a liquid. Isoprene needs to be extremely pure to make synthetic rubber.

Having the flexibility to use different crops and agricultural by-products as the feedstock for the process is important, says Karl Sanford of Genencor. The company wanted to avoid the contentious issue of using crops that might otherwise provide food. Moreover, supply chains for biomass are still being developed, so the availability of feedstock will change.

Research is continuing into using other micro-organisms, such as yeast, which might be engineered to create similar metabolic pathways. If biologically derived isoprene becomes more widely available, Mr Sanford thinks, industry could find additional uses for it. And if production costs end up being as low as the company hopes, isoprene might itself become a feedstock from which to produce synthetic fuels, such as petrol or diesel. So as well as helping build cars in a more environmentally friendly way, it might eventually help power them in a greener way, too.