Perhaps the most famous of the microbiomes is the gut microbiome, as we are forever reminded to feed our “good bacteria”. Indeed each of the microbiomes found in the human body are unique. The various populations of bacteria in the gut are not identical to those found in the airways or in the vagina as both the abundance of bacteria and even the species of bacteria vary. These can also vary between individuals, it has been suggested that a gut microbiome is as unique as a fingerprint.
Intestinal cells are lined with bacterial flora (or a "microbiome")
which is specific to each individual.
Courtesy of Pacific Northwest National Laboratory
Where though, do all these bacteria come from? At what point in our lives are we accumulating these vast numbers of bacterial populations?
At the moment the answer is: it’s not certain. However, it’s easy to see with these vast microbiomes that perform so many functions within us how essential it is to understand how and when microbiomes are established. This importance is only amplified when we consider that microbial communities can cause oral disease and may be involved in some digestive disorders and potentially involved in obesity.
There are a few clues though that perhaps gives some insight into where and when these microbial communities are appearing. Babies possess complex microbiomes within their first of life, albeit taking several years for these to mature and settle into equilibrium. Until this time the bacterial populations can demonstrate quite dramatic fluctuations. The populations and percentages of bacteria present do display variation between babies, particularly noticeable in those who weigh less than 1200g (2lb 10 oz). This would suggest infants are being exposed to bacteria within the uterus, and babies born prematurely are failing to develop the microbiome patterns expected due to a reduced exposure time.
Previously it was believed the uterine environment was sterile but, it has been presented on several occasions that bacteria are present intrauterine and that the placenta contains bacteria, even in a healthy pregnancy. It also would appear the placenta may aid in the establishment of microbial communities. In a landmark study, published earlier this year, an American group of scientists have begun to characterise the placental microbiome and give a more detailed insight into what bacteria are present. The study published in Science Translational Medicine looks at placental tissue from 320 subjects using DNA sequencing techniques to examine the bacteria.
The exact species and percentages present varied between individuals nevertheless in the majority of cases, E. coli was found to be in highest abundance. Definitely noteworthy is the fact that the placenta possessed a low abundance of bacteria, quoted as being 0.002mg of bacteria per 1g of placenta demonstrating that although the uterus is not sterile it certainly does not contain the vast amounts of bacteria we’re used to seeing in organs. For comparison there are approximately 104 to 107 bacteria per gram of contents in the jejunum and ileum and 1011 to 1012 cells per gram in the colon.
The group then go on to compare the microbiome of the placenta to other microbiomes found within the body. Perhaps surprisingly, the microbiome did not register as similar to that found in the vagina nor in the gut with both the bactieral species and metabolic processes involved differing from both these body sites. In fact, the placental microbiome demonstrated most similarity with the oral microbiome. This garnered much interest with news outlets as a link between periodontal disease and premature birth has been suspected for many years, a quick search reveals many papers looking at both epidemiological and biological data in order to provide a link. However, the authors did not expect this finding and so the study was not designed in order to examine this similarity any further. Although there is much speculation around this fact, very little concrete evidence for the link between periodontal disease and preterm delivery can be drawn from this study.
The authors do demonstrate a variation between placental microbiomes from women who carried full term and those who did not. Both the bacterial species present in the placenta and the metabolic profile of the bacteria differed between these two cases. One must bear in mind though that the authors could not examine the placenta over the course of a pregnancy, as this would be rather invasive, so it might well be that the placental microbiome simply changes during pregnancy. Given the fact we know microbiomes of babies undergo such a radical change within the first few years of life, this does not seem such a far-fetched notion.
This study provides an overview of the microbiome in the placenta for the first time giving insight into the various bacteria and their functions. Additionally, the authors examine the influence of preterm delivery on the placental microbiome. What this study does not do is prove that bacteria are passed through the placenta from the mother to the infant, as no studies of the infant microbiome from the cases presented here were performed. The primary technique used here was DNA sequencing and statistical analysis based on the sequencing information generated and not any analysis into live bacteria and whether or not these were transferred. Nevertheless it provides further evidence the uterine environment is key in establishing the microbiomes present in the human body.
Kjersti Aagaard et al. 2014. The Placenta Harbors a Unique Microbiome