November 19 (HealthDay News) -- Antibiotic treatment, especially when prolonged or repeated, may have a negative impact on beneficial bacteria that live in the gut, according to a new study.

Gut bacteria play helpful roles in various aspects of human nutrition, metabolism and immune responses, experts note.

Researchers focused on the widely-used antibiotic ciprofloxacin, prescribed for a number of bacteria-caused conditions, including urinary tract infections. It has been believed that ciprofloxacin causes only modest harm to beneficial bacteria in the body.

In this study, Stanford University's Dr. David Relman and colleagues catalogued bacteria in the feces of volunteers being treated with ciprofloxacin and identified more than 5,600 different bacterial species and strains. However, while the patients were taking the antibiotic, the overall abundance of about 30 percent of the bacterial species and strains was significantly affected, the researchers found.

The effects of the antibiotic on gut bacteria varied greatly between individuals, with two volunteers showing a strong reduction in bacteria diversity. The study also found that once antibiotic treatment was completed, it took up to four weeks for most strains of gut bacteria to return to pre-treatment levels.

None of the study participants reported signs of gut-related problems during fluctuation of their gut bacteria populations.

While the findings, published online this week in the journal PLoS Biology, reveal aspects of resiliency in gut bacteria, they also suggest that antibiotic treatment may have long-lasting effects on overall health that could go unnoticed, the researchers concluded.

SOURCE: PLoS Biology, news release, Nov. 18, 2008