Study shows antibiotics disrupt gut

November 18, 2008
Helen Branswell
THE CANADIAN PRESS
<http://www.healthzone.ca/health/article/539233>http://www.healthzone.ca/health/a
rticle/539233

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."