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Vaginal microbiome naturally less diverse, rich during pregnancy

Within the vagina of the pregnant woman, the microbiome – the population of microbes that cohabit with human cells – is naturally very different. During pregnancy, the vaginal microbiome becomes less diverse and less rich or abundant, with one particular type of bacteria dominating, said researchers from Baylor College of Medicine and Texas Children’s Hospital, Harvard School of Public Health and the Broad Institute at MIT and Harvard, in a report that appears today in the open access journal PLoS One.

"We anticipated that the vaginal microbiome would alter throughout pregnancy, but did not anticipate as significant and profound a difference between pregnant and non-pregnant women," said Dr. Kjersti Aagaard, associate professor of obstetrics and gynecology at BCM, and Texas Children’s Hospital and first author of the report. "We predicted far less subtle changes with pregnancy than we observed. The fact that vaginal microbial communities significantly differed by virtue of pregnancy speaks to the dynamic state of the microbiome throughout a woman’s reproductive life. Moreover, it challenges the notion that communities which are less diverse or less rich are indicative of a disease state."

The microbiome refers to the bacteria, viruses and fungi that inhabit our bodies along with us. There are 10 times as many of these microbial cells as there are human cells on our body, and they play an important role in our existence. A recent effort to identify and determine the function of this "other organ" is called the Human Microbiome Project. In an opening salvo of information, the Human Microbiome Consortium today has published a host of articles dealing with the first coordinated information coming from that effort.

Role of microbial colonies

Understanding the microbiome of the vagina in pregnant women is the first and key step to answering the question of how diverse microbial colonies affect the microbiome of the pregnant woman, her child and change during life, including the reproductive period.

To sample the vaginal microbiome in healthy pregnant women (from 18 to 40 weeks), Aagaard and her colleagues took 68 samples from 24 healthy women at various stages of pregnancy. They compared the 301 samples taken from 60 non-pregnant women. They followed the procedures they had used as key investigators in the Human Microbiome Project in order to have a comparable set of samples.

In addition to the fact that the microbiome was less diverse, they found that bacteria of the Lactobacillus species (L. iners crispatus, jensenii and johnsonii), and the orders Lactobacillales (and Lactobacillaceae family), Clostridiales, Bacteroidales, and Actinomycetale were differentially abundant in the pregnant vaginal microbiome.

Distinct change in communities

"When we looked carefully at the different areas of the vagina, we saw distinct changes in these bacterial communities. Some Lactobacillus species increase in abundance and others decrease," she said.

These changes in diversity and richness are most obvious about the time in a pregnancy when an infant could potentially begin to survive outside the womb – about 24 to 32 weeks, said Aagaard. This was especially true of the area closest to the uterus, she said. Closer analysis showed that these changes to a lesser or greater degree occurred throughout pregnancy.

She does not yet know why the microbiome changes during pregnancy, but she and her colleagues suspect it may have something to do with minimizing more harmful bacterial and viral infections during pregnancy, and establishing the healthy microbiome of the infant during birth and upon delivery.

Next steps will include comparing the vaginal microbiomes of healthy women to that of women who have complicated pregnancies and determining how the newborn acquires his or her microbiome, said Aagaard.

Others who took part in this work include: Kevin Riehle, Jun Ma, Toni-Ann Mistretta, Cristian Coarfa, Sabeen Raza, Sean Rosenbaum, Ignatia Van Den Veyver, Aleksandar Milosavljevic, Joseph Petrosino and James Versalovic, all of BCM; Nicola Segata and Curtis Huttenhower of the Harvard School of Public Health and Dirk Gevers of the Broad Institute, all in Cambridge, Mass.

Funding for this work came from the National Institutes of Health and the Burroughs Welcome Fund Preterm Birth Initiative.