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J Reprod Immunol. Author manuscript; available in PMC 2015 October 01.
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J Reprod Immunol. 2014 October ; 0: 12–19. doi:10.1016/j.jri.2014.03.006.
The Microbiome, Parturition, and Timing of Birth: More
questions than answers
Amanda L. Prince, PhD1, Kathleen M. Antony, MD1, Derrick M. Chu1, and Kjersti M.
Aagaard, MD PhD1,2,3
1Departments of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, Baylor College
of Medicine, Houston, Texas
2Bioinformatics Research Lab, Baylor College of Medicine, Houston, Texas
3Molecular and Cell Biology, Baylor College of Medicine, Houston, Texas
Abstract
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The causes of preterm birth are multifactorial, but its association with infection has been well-
established. The predominant paradigm describes an ascending infection from the lower genital
tract through the cervix and into the presumably sterile fetal membranes and placenta. Thus, an
evaluation of the role of the vaginal microbiome in preterm birth is implicated. However,
emerging fields of data described in this review suggest that the placenta might not be sterile, even
in the absence of clinical infection. We thus propose an additional mechanism for placental
colonization and infection: hematogenous spread. When considered in the context of decades of
evidence demonstrating a strong risk of recurrence for preterm birth, studies on parturition are
ideal for applying the rapidly expanding field of metagenomics and analytic pipelines. The
translational implications toward identification of innovative treatments for the prevention of
preterm birth are further discussed. In sum, exciting advances in understanding the role of both
host and microbiota in parturition and preterm birth are on the horizon.
Introduction
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In 2005, the World Health Organization estimated that 12.9 million births worldwide
occurred preterm; up to 42% of these resulted in mortality (Beck et al., 2010). Preterm birth
is the leading cause of neonatal morbidity and mortality, yet little is understood regarding
the underlying etiology ( Kilpatrick, 2013). It is traditionally thought that an ascending
infection from the vagina causes preterm premature rupture of membranes (PPROM), which
initiates preterm labor and ultimately birth. However, more recent studies have shown that
© 2014 Elsevier Ireland Ltd. All rights reserved.
Address for correspondence: Kjersti Aagaard, MD, PhD Department of Obstetrics and Gynecology Baylor College of Medicine
Houston, TX 77030 (aagaardt@bcm.edu)..
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, Prince et al. Page 2
bacteria from the oral cavity are most often found in the amniotic fluid of patients with
preterm labor (Figure 1) (Madianos et al., 2013). Additionally, studies are now
demonstrating that bacteria are naturally found in placental tissue, and our own lab has
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shown that the placenta harbors its own unique microbiome (Aagaard et al., 2014). Here we
will discuss how the microbiome changes during pregnancy and how these changes may
influence preterm birth (Table 1). Further research in this area will lead to a greater
understanding of the etiologies of preterm birth and may result in innovative treatments to
prevent preterm birth.
The microbiome during pregnancy
During a normal pregnancy, the gravidae undergoes a spectrum of anatomical,
physiological, and biochemical changes. These functional alterations result from the
influences of hormonal and physical fluctuations, and they affect every organ of the body.
These are accompanied by concomitant changes in the microbiome, at least in the vagina
and gut, which are the only sites that have been specifically examined in pregnancy to date
(Aagaard et al., 2012; Koren et al., 2012; Romero et al., 2014).
During pregnancy, hormonal changes result in increased thickness of the vaginal mucosa,
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hypertrophy of the smooth muscle cells, and relaxation of the connective tissues. Recently,
we cataloged the “normal” microbiota signature during pregnancy in a cross-sectional study
sampling women at a variety of gestational ages (Aagaard et al., 2012). Using 454
pyrosequencing technology, we deep sequenced the V3–V5 region of 16S rRNA from
samples obtained from the vaginal introitus, midvagina, and posterior fornix. Interestingly,
we found that the vaginal microbial community differed by gestational age and proximity to
the cervix (Aagaard et al., 2012). Furthermore, the microbial community structure resembled
a non-pregnant state in late gestation, and we saw a decrease in alpha diversity, or within-
sample diversity, with a corresponding increase in Lactobacillus species in gravid patients
compared with nonpregnant subjects (Aagaard et al., 2012). Recently, Romero et al. took
these studies further by examining the vaginal microbiome longitudinally during pregnancy
at the posterior fornix (Romero et al., 2014). While the vaginal microbiome of gravid
women could still be classified into distinct community state types, as previously described
in nonpregnant women (Ravel et al., 2011), the vaginal microbiome became more stable and
less diverse throughout pregnancy, as we previously described (Aagaard et al., 2012;
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Romero et al., 2014).
One such species that was discriminately and specifically enriched in our study was L.
johnsonii. This species encodes enzymes and transporters that are essential for the release of
bile salt hydrolase and is primarily found in the upper gastrointestinal tract (Pridmore et al.,
2004). L. johnsonii also produces Lactacin F, which limits other lactobacillus and
Enterococcus species in the gastrointestinal tract (Abee et al., 1994). Thus, the increase in L.
johnsonii may be important for the inoculation of neonates in order to promote the digestion
of breast milk postpartum.
While these alterations in the microbiome may serve to inoculate the neonatal gut, they may
also contribute to pregnancy maintenance. In addition to the aforementioned enrichment in
J Reprod Immunol. Author manuscript; available in PMC 2015 October 01.
