Pregnancy Alters Resident Gut Microbes

By Monya Baker and Nature magazine
From Nature magazine
August 2, 2012
http://www.scientificamerican.com/article.cfm?id=pregnancy-alters-resident-gut-microbes


     Women's third-trimester microbiota resembles that
     of people at risk of diabetes

     Changes to gut microbes seen in pregnancy prepare a
     woman to nourish her child.

Women's gut microbe populations change as pregnancy
advances, becoming more like those of people who might
develop diabetes. These changes, which do not seem to
damage maternal health, correspond with increases in
blood glucose and fat deposition thought to help a
mother nourish her child.

Although scientists have profiled microbial communities
around the world and throughout the human body, this is
the first time they have tracked the gut microbiome
during pregnancy, says Ruth Ley, a microbiologist at
Cornell University in Ithaca, New York, who led the
work1.

Ley had previously explored the interplay of gut
microbiota with metabolic syndrome2, a precursor to
diabetes that is characterised by high levels of
inflammatory markers, blood sugar and fats. Because
similar changes occur during pregnancy, she wondered
whether the gut microbiota might reflect this. She and
her colleagues sequenced microbial DNA from stool
samples collected early and late in pregnancy, and found
consistent shifts in the bacterial communities.

Overall, the diversity of gut bacteria declined between
the first and third trimesters, but the abundance of
certain types, such as the Proteobacteria and
Actinobacteria, increased. These are also more common in
people who are obese or have metabolic syndrome, says
Ley. "Proteobacteria in particular are often the bad
guys in these studies. They are associated with
inflammation."

"This is the next step in describing how completely
normal states of health can have profound changes," says
Kjersti Aagaard, an obstetrician at Baylor College of
Medicine in Houston, Texas, who was not involved in the
research. The fact that the observed shift occurred by
the third trimester of pregnancy makes sense, she says.
"That's when babies start packing on the pounds."
Earlier this year, Aagaard and her co-workers published
work comparing vaginal microbiomes in pregnant and non-
pregnant women; those in the pregnant women were
dominated by Lactobacillus species, which are thought to
prevent the growth of harmful bacteria and aid human
digestion3.

One size fits all

Although the shifts in microbial diversity did not
affect mothers' health, stools collected during the
third trimester contained more inflammatory markers than
those collected during the first trimester. What's more,
the trends held true regardless of whether women were of
normal weight or overweight before falling pregnant, had
actually developed diabetes, or had taken antibiotics or
probiotics (supplements taken to provide or boost
populations of 'healthy' bacteria) during pregnancy.
Meanwhile, after birth, the children's microbiotas
resembled those of the mothers' first trimester samples.

When Ley and her colleagues transplanted bacteria from
stool samples into mice that had been raised under
sterile conditions, they found that mice receiving
microbiota from third-trimester samples became fatter
and less sensitive to insulin than mice that were given
first-trimester samples.

"That is pretty suggestive that the microbiome is at
least contributing to the change, or maybe driving it,"
says David Relman, a microbiologist at Stanford
University in California who is looking for associations
between pregnancy microbiomes and pre-term birth.

Ley speculates that physiological changes that occur
during pregnancy alter the microbial community, which,
in turn, creates a positive-feedback loop sustaining
conditions seen in metabolic syndrome. "The body might
be using the microbes as a tool," she says. "You alter
the microbiota, and they give you the changes in
metabolism that you want."

Distinguishing cause and effect will take a lot of work,
says Relman. But such studies will be valuable
regardless of the outcome. "It may be a mirror or
monitor of the changes in the human host, and for that
reason alone, it could provide novel insight into the
physiology of pregnancy," he says.

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