Human Placenta Contains a Community of Microbes

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The human microbiome is vast, accounting for 90 percent of our cells. Microbial composition varies from site to site across a range of niches in and on the body. Some niches—such as the colon—are colonized by a very high number of microbes. Other niches—such as the stomach—are colonized by lower amount of microbes. There are yet other areas of the body that are thought to be sterile. One such site—until recently—is the placenta that develops in the uterus during pregnancy.

Previously, it was thought that a healthy placenta is free of microbes. A recent study published in the journal Science Translational Medicine found that placenta does, however, contain an array of microbes. They analyzed the placenta of 320 women who had given birth and found that 10 percent of the placenta is made up of nonpathogenic microbes from the Firmicutes, Tenericutes, Proteobacteria, Bacteroidetes, and Fusobacteria phyla, or groups of bacteria. Most interestingly, they found that the bacteria in placenta is made up of a unique community that most resembles bacteria from the mouth, which may help explain the connection between periodontal disease and preterm birth. Only one participant of the study had periodontal disease, however, so further studies will be needed to determine whether periodontal pathogens are transmitted to placenta.

The study also found that women who had urinary tract infections during early pregnancy were at higher risk of premature birth, and the infectious bacteria turned up in the placenta even when the infection was cured. The researchers are not sure whether it was the infection or if it was the antibiotic treatment of the infection that had an effect on preterm birth.

It appeared as though vaginal gut bacterial colonization, maternal obesity, or mode of delivery were not linked to the composition of placental bacteria. More studies will be needed to determine just what role these bacteria play, how they are acquired, and whether they contribute to the development of gut bacteria in the infants.

It is not a surprise to me that placenta contains bacteria. Studies have previously found that commensal bacteria exist in umbilical cord blood of healthy neonates,2 and I have long suspected that infants receive the benefit of their mother’s bacteria even before birth.  Another recent study found DNA from Lactobacillus and Bifidobacterium in the placenta of newborn infants. These studies will continue to elaborate our understanding of how microbes are an integral component to each and every phase of our lives. I would bet that women who eat a diet high in plant-based foods with pre- and probiotics as well as some fermented foods will have healthier babies with highly educated gut and immune systems trained in utero by mom’s beneficial bacteria.

References

  1. Aagaard K, Ma J, Antony K, et al., “The placenta harbors a unique microbiome.” Sci Transl Med. 2014 May 21;237(6):ra65.
  2. Jimenez E, Fernandez S, and Marin ML, et al. “Isolation of commensal bacteria from umbilical cord blood of healthy neonates born by cesarean section.” Curr Microbiol. 2005 Oct;51(4):270-4.