Sep 13, 2010 4:10 PM by Melissa Canone
WASHINGTON (AP) - An antibiotic can temporarily upset your
stomach, but now it turns out that repeatedly taking them might
have lingering ill effects - by triggering changes in all those
good germs that live in your gut.
Nobody yet knows if that leads to later health problems. But the
finding is the latest in a flurry of research raising questions
about how the customized bacterial zoo that thrives in our
intestines forms - and whether the wrong type or amount plays a
role in ailments from obesity to inflammatory bowel disease to
Don't be grossed out: This is a story in part about, well, poop.
Three healthy adults collected weeks of stool samples so that
scientists could count exactly how two separate rounds of a fairly
mild antibiotic caused a surprising population shift in their
microbial netherworld - as some original families of germs
plummeted and other types moved in to fill the gap.
It's also a story of how we coexist with trillions of bacteria,
fungi and other microbes in the skin, the nose, the digestive
tract, what scientists call the human microbiome. Many are
beneficial, even indispensable, especially the gut bacteria that
play an underappreciated role in overall health.
"Gut communities are fundamentally important in the development
of our immune system," explains Dr. David Relman of Stanford
University, who led the antibiotic study published Monday in
Proceedings of the National Academy of Sciences. "Let's not take
them for granted."
Next, Relman plans to track if antibiotics during the first or
two year of life, when youngsters form what will become their
unique set of gut bacteria, seem to predispose children to later
Antibiotics already should be used cautiously because they can
spur infection-causing bacteria to become drug-resistant. The new
research raises different questions about effects on beneficial
bacteria - and if abnormalities in the microbiome really are linked
to health problems, how those changes might begin.
"We should start paying attention to this," says Dr. Martin
Blaser, a microbiome specialist at New York University Langone
Medical Center, who wasn't involved with Relman's work but also is
planning to study the issue in children. "The main point is that
antibiotic use is not free in a biological sense."
Everyone is born with an essentially sterile digestive tract,
but within days the gut is overrun with bacteria from mom and dad,
the environment, first foods. Ultimately, a healthy person's
intestinal tract teems with hundreds of species of microbes, the
body's biggest concentration, with many involved in such things as
digestion and immune reaction.
In the not-so-healthy, scientists have discovered that
overweight people harbor different types and amounts of gut
bacteria than lean people, and that losing weight can change that
bacterial makeup. They've also found links to other digestive
diseases, precancerous colon polyps - and even are pursuing a
theory that early use of antibiotics disrupts the developing
microbiome in ways that spur autoimmune disorders like asthma or
Antibiotics aren't choosey and can kill off good germs as well
as bad ones. But Relman and fellow research scientist Les
Dethlesfsen wondered how hardy gut bacteria are, how well they
bounce back. So they recruited healthy volunteers who hadn't used
antibiotics in at least the past year to take two five-day courses
of the antibiotic Cipro, six months apart.
The volunteers reported no diarrhea or upset stomach, yet their
fecal samples showed a lot going on beneath the surface. Bacterial
diversity plummeted as a third to half of the volunteers' original
germ species were nearly wiped out, although some other species
moved in. Yet about a week after stopping the drug, two of the
three volunteers had their bacterial levels largely return to
normal. The third still had altered gut bacteria six months later.
The surprise: Another die-off and shift happened with the second
round of Cipro, but this time no one's gut bacteria had returned to
the pre-antibiotic state by the time the study ended two months
"History matters," concludes Relman, whose next is testing
what jobs the most affected bacteria performed - such as helping to
maintain intestinal barriers against infection - and if the new
bugs fully replaced them. "We may have to be more careful" about
Of course, antibiotics aren't the only means of disrupting our
natural flora. Other research recently found that babies born by
cesarean harbor quite different first bacteria than babies born
vaginally, offering a possible explanation for why C-section babies
are at higher risk for some infections. Likewise, the gut bacteria
of premature infants contains more hospital-style germs than a
The big issue is when such differences will matter, something so
far, "we're not really smart enough to know," Relman says.