Superbugs 'slow to disappear'

The E.coli bacteria was tested for resistance

Strict guidance to curb the use of antibiotics has had little effect on the strength of existing "superbug" strains, scientists have found.

Many different types of bacteria which have the capacity to harm humans are becoming increasingly resistant to the antibiotics traditionally used to combat them.

One culprit is the overuse of the antibiotics themselves which, by wiping out only the weaker strains, accelerates the natural selection of stronger, more resistant strains.

This research suggests that we may not be able to get of resistance which has already become well-established

Dr David Livermore, Public Health Laboratory Service

Throughout the 1990s doctors were warned to reduce their antibiotic prescribing to address the problem.

A study published in The Lancet medical journal suggests the warnings did influence clinical practice.

In 1991 there were 320,000 prescriptions for sulphonamide antibiotics - a common treatment for E.coli bacteria infection.

In 1999 this had fallen to 7,000 prescriptions.

Worse, not better

But the researchers sampled approximately 360 different strains of E.coli, most of them harmless versions which live in the gut, and found that, if anything, the problem of resistance had become worse, despite the fall in antibiotic prescribing.

The number of strains displaying resistance reached 165 in 1999, compared to 143 in 1991.

Dr David Livermore, one of the study authors, wrote: "Whilst we can - and should - try to slow the development of resistance to antibiotics by less and better prescribing, this research suggests that we may not be able to get of resistance which has already become well-established.

"One of the reasons is that the genes which make bacteria resistant to sulphonamides become linked to those giving resistance to other antibiotics which we still use."

The findings come as a blow to those clinicians who believed that cutting back antibiotic prescribing radically could reduce resistance overall, allowing antibiotics which had been becoming defunct to be phased back into service.

A spokesman for the Public Health Laboratory Service, where Dr Livermore works, told BBC News Online it was still important to use antibiotics sparingly to prevent future resistance, even if past problems could not be undone.

He added: "Once we have got resistance in place, it may be very hard to get rid of it."