Study shows antibiotics disrupt gut

November 18, 2008

Helen Branswell


This is your gut. This is your gut on drugs.

A new study reveals that a common antibiotic disrupts normal bacterial levels in the digestive tract of healthy adults for longer than previously thought. Six months later, in fact, some beneficial types of bacteria were still wiped out or remained at levels lower than before the drugs were taken.

"You don't want to be giving readers the impression that we shouldn't be using antibiotics (when needed)," says Dr. David Relman, senior author of the study, which was published Tuesday in the journal PLoS Biology.

"But it's the flip side. It's the trade-off part. . . . Because we do overuse antibiotics."

Relman, an infectious diseases specialist at Stanford University and the Veteran Affairs Hospital at Palo Alto, Calif., conducted the study with a team of colleagues. Funding for the work came from the Doris Duke Charitable Foundation and the U.S. National Institutes of Health.

Antibiotics aren't a targeted treatment. The drugs don't zero in on the bacteria you want to kill and leave intact the rest of the body's normal and healthy bacteria. That's why taking antibiotics to cure one problem can give rise to another for instance yeast infections or C. difficile diarrhea.

But it's not clear just how much damage the drugs wreak on the body's bacterial "flora" the beneficial bacteria that inhabit places like the gut, helping to keep us healthy and safe from bugs that would make us ill.

To try to quantify the effect, Relman and his colleagues gave three healthy volunteers two men and a woman a single course of the antibiotic ciprofloxacin.

While all antibiotics will knock out a range of bacteria, cipro is believed to be among the least disruptive of the drugs.

In fact, more than 30,000 U.S. postal workers were prescribed ciprofloxacin in 2001 after letters containing anthrax were processed through several sorting stations. There were few reports of postal workers suffering side-effects from the drug, Relman said.

In this study, the researchers collected stool samples from their volunteers before they started the five-day course of cipro, during treatment and for months after.

They are actually still studying these individuals plus four others and have samples going out a year after the first dose of the drug. But in this paper they report on results for the first six months only.

They mined the stool samples for traces of bacteria using a technique called polymerase chain reaction or PCR, identifying DNA from between 3,300 and 5,700 different types in the samples collected before treatment. Most of the bacteria in fact 93 per cent haven't yet been identified, Relman said.

The diversity in bacterial types was cut by about a third after the volunteers took the antibiotics. Relman said the size of the reduction came as a surprise.

"We find that cipro was more disruptive than we had thought.... About 30 per cent of all of the strains and species that we could see were disrupted. And most of them were ... either knocked out or knocked down."

By four weeks post-treatment, most of the bacterial populations seemed to have recovered, though some were still at depressed levels. And some were not evident at all.

Given that so little is known about most of the bacteria, the researchers can only hazard guesses at whether that effect would have any long-term impact on the health of their volunteers. In the short term, none reported feeling ill.

But these volunteers were healthy. If they were people already fighting some infection or illness, the results might have been different. And even with the healthy people, Relman said it's too soon to say there is no health cost.

"The things that we see getting knocked out or knocked down are typically associated with (good) health," he said.

"We can't say that each and every one of these individual organisms is necessary or important somehow or contributing to health ... But the overall communities are associated with a lot of beneficial features for the host. So the net effect could potentially be harmful."



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