It reveals that bacteria are increasingly developing resistance to the drugs commonly prescribed to wipe them out.

Over a four-year period the proportion of antibiotic-resistant strains of bacteria causing such infections doubled.

As 80% of women who have them get another infection within about 18 months, effective antibiotics are essential to manage the problem.

Beating infections

The study looked at the performance of antibiotics in 4,000 women. Its findings are published in the Journal of the American Medical Association.

Kal Gupta, a scientist at the University of Washington in Seattle, conducted the research.

He said that bacteria such as Escherichia coli (E. coli), responsible for painful urinary tract infections (UTIs), have already proved resistant to antibiotics such as ampicillin and cephalothin.

More alarming, though, was the growing resistance of bacteria to the remaining effective treatments, he said.

"Of considerably greater concern is the increasing prevalence of resistance to trimethoprim and trimethoprim-sulphamethoxazole that we observed."

Increasing immunity

As bacterial infections become immune to treatment with existing antibiotics, more powerful drugs need to be developed.

The growing redundancy of antibiotics has been put down to intensive use of antibiotics, particularly in children.

While most infections are caused by E. coli, which normally live in the colon, microorganisms called Chlamydia and mycoplasma may also cause UTIs.

These infections usually remain in the urethra and reproductive system, and may be sexually transmitted.

Ineffective treatment

The study looked at 4,000 women with bladder infections.

While trimethoprim and trimethoprim-sulphamethoxazole were ineffective in 8% of the cases in 1992, by 1996 this had risen to 16%.

At this stage three other drugs were able to beat the infections - nitrofurantoin, gentamicin and ciprofloxacin hydrochloride.

However, in time it is possible that bacteria will also develop resistance to these drugs.

The researchers said doctors should monitor levels of levels of drug resistance, which varies by community.

They should consider changing antibiotics when local resistance rates exceed 15-20%.