Dormant bacteria may spawn infection.

Bacterial infections that seem to be cured can flare up again months or years later. Dormant bacteria may be to blame, according to a new laboratory study. Hadley Hartwell and Todd R. Steck of the University of North Carolina in Charlotte have been investigating the effect of antibiotics on Escherichia coli, a frequent cause of urinary tract infections.

Up to 50 percent of all women contract urinary tract infections, says Hartwell. E. coli is the most common pathogen associated with these infections. Symptoms include a frequent urge to urinate and burning or pain during urination. Even after antibiotic treatment, a third of these infections recur.

Hartwell and Steck tested ciprofloxacin and trimethoprim, two antibiotics typically prescribed for urinary tract infections. Hartwell added each antibiotic to a saline solution containing an E. coli strain known to cause urinary tract infections.

Over several weeks, she tested whether bacteria were still alive by placing samples on a solid culture medium. After a week, none of the solutions yielded bacterial growth. Clinics use such a test to detect E. coli in urine samples, so the solutions would have registered as bacteria-free.

The researchers found, however, that some of the bacteria were not dead but in a dormant state. Removing the antibiotics from the original solutions and adding nutrients revived the cells, which then grew on the solid culture.

Even though dormant bacteria remain alive, clinicians can't detect them using standard microbiological methods, says Steck. Dormancy could explain why a variety of diseases are as persistent as urinary tract infections, he says.