Study shows antibiotics disrupt gut
November 18, 2008
THE CANADIAN PRESS
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
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
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
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
"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."