There will be a five-year window of vulnerability from 2002 in which people will be especially at risk from antibiotic-resistant "superbugs", a scientist has warned.

"We're not going to see new antibiotics until at least 2007 in any significant numbers," Dr George Post, of SmithKline Beecham, told the Festival of Science in Sheffield.

"So as more and more bugs become resistant to more and more antibiotics, we will have a definite window of vulnerability before the new antibiotics begin to be introduced," the chief scientific officer of the British drugs giant added.

"Superbugs" cause infections untreatable with any existing drugs and have emerged, in part, through the indiscriminate prescribing of antibiotics by doctors. This has given the bacteria more opportunities to evolve and become resistant to the drugs.

Leading drug companies have boosted their budgets to develop new antibiotics but the complexities of bacteria and the time it takes to develop and test new drugs mean it will be years before new antibiotics are on the market.

Viral warfare

The new class of antibiotics will have to be different from current drugs and be able to kill bacteria in a completely different way.

One possible approach, said Dr Martin Westwell of Oxford University, is to use bacteriophages. These are viruses that kill specific bacteria but do not harm humans.

"This year the first person in the West to be cured of a bacterial infection by using a virus has actually occurred," he claimed.

The woman had an antibiotic-resistant infection that was cured with phage therapy. The virus was injected into her body and it killed the bacteria.

Scientists in the former Soviet republic of Georgia have been developing bacteriophage therapy since the 1920s.

Dr Westwell said it could work against "superbugs" because, unlike antibiotics, every time bacteria develop a defence against the phage, it will itself evolve a new way of killing the bacteria.