In 2014, studies done by the Human Microbiome Project (HMP), documented that human development and physiology is associated with the human microbiota. Studies describes that newborn child have complex microbiota in their gut within the first week of their life, and it composition become increases and microbiota become mature when child aged around 3 years. The species present in the gut of neonates are largely colonized by Actinobacteria, Bacteriosides, Proteobacteria and small colonies of Firmicutes including Lactobacillus species which is mostly present in vagina. The weight of neonates also correlates with presence of microbiota, newborn weight <1200 have high abundance colonies of phyla Tenericutes and Formicates and low abundance of phyla Actinobacteria. Researcher though that it is the probability that these microbes are passing by placenta to newborn, and low and high abundance of microbes depend upon the duration of pregnancy. (Aagaard, et al., 2014). For justifying their view, they set up ha cross sectional study of 195 patients, gram positive and gram negative bacteria intracellular bacteria are detected in basal plate which collected from maternal-fetal interface tissue layer. It is detected only in one third placental sample, mostly present in women with preterm pregnancy but is not show any evidence against chorioamniotis. The recent model show that the intrauterine infections are correlated with preterm deliveries, originated in the lower genital tract and move into intrauterine environment, most microbiota found in placenta by the DNA-based technology are not similar with genitourinary tract but they have close association with oral microbiota. For example, oral microbe like F. nucleatum help in hematogenous transmission (blood production) during placentation because it has capacity to bind with endothelium, changes penetrability as well as act as enabler for other commensals like E.coli. So they believed that placenta harbor microbiome similar as oral microbiome. To confirm their finding they have done Meta genomic Rapid Annotation using Subsystem Technology for taxonomic classification of placenta microbiome with help of Whole Genome Shotgun. They compare the microbes identified placenta with the microbes residing in other body parts including vagina, skin, gut, oral cavity and airways. They used Bray-Curtis distance of these niches at phylum level to reflect the similarity between the different niches. The placenta microbiome gives abundant similarity for oral microbiome including tongue, tonsils and gingival plaques. So their result said that placenta harbor low abundance but metabolically rich microbiome. (Aagaard, et al., 2014)
Figure 1: The taxonomic profiling of placenta microbiome similar to oral microbiome. (The thicker connecting lines show more similarity, greater association within the taxonomic phylum between different niches.) (Aagaard, et al., 2014)
Traditionally, it was thought that placenta is sterile for healthy pregnancy, but new studies and research explain that placenta harbors the microbiota and it challenging the sterile womb model. Later studies showed that placenta have bacteria that present in the basal plate at the maternal fetal interference. It is also explaining that microbes live and grow in the term of placenta. The culture dependent and independent methods isolates the low abundance of microbes, which their genomes mostly occur in healthy placenta. It is believed that oral microbiome is source of microbiome in the placenta. (Aagaard, et al., 2014). In 2017 another studies take place and explains the presence of spatially viable profile of bacteria in the placenta in normal pregnancy. They work on placenta, like placenta is made up of outer layer of blastocysts stage. It has three layers, first the fetal amniotic membrane, their function is to develop a protective shield around fetus and the amniotic cavity. Second layer, placenta villi (PV) is the tree like projection, multinucleated, syncytial trophoblast layer. Their function is to directly contact to the mother blood during pregnancy. Third layer is called basal plate, present on maternal side of placenta, it functions as defense providence, immune response, and cross talk between mother and fetus. Different type of cells is also present in these layer, including the endothelial cell of maternal uterine, immune cells, extra villous trophoblast (EVT) derived from fetus, these cell eject from placenta villi and move to basal plate and responsible for the spiral artery remodeling to help to flow blood into placenta fetal side. Different studies demonstrated that placental layer have barrier capabilities. For example, in the basal plate the (EVT) consist of colonies of L. monocytogens and E.coli and in placental villi have less vulnerability to L. monocytogens. This different vulnerability against different microbes shows that placental villi exhibits high basal autophagy as compare to basal plate (EVT) have low autophagy capability.(Parnell, et al., 2017). They worked on 57 women patients, and employed DNA sequencing method multiple variable region in three layers of bacterial 16s ribosomal gene. The three layers are analyzed by bar-coded primers closest to 16Srna gene of bacteria by using Fluidigm Access Array System and sequencing is done by Illumina Miseq Platform.
Figure2: A) Diagrammatic view of three layer of placenta. B) copy number on basis of 16s C) Shannon diversity index of basal plate, placental villi and fetal membrane. (Parnell, et al., 2017)
With help of 16srna copy number and specific location of sample, it gives median copy number, copy number (number of genome or microbes) is higher in basal plate, than in fetal membrane and placental villi have medium range copy number between basal plate and fetal membrane. They also done alpha and beta diversity with help of Shannon Diversity Index to evaluate variation within the sample of basal plate, placental villi and fetal membrane. It gives result that median Shannon variation was less in fetal membrane as compare to basal plate, whereas fetal membrane and basal plate have same microbes’ diversity. Taking all account, it is demonstrated that microbes abundance and variation vary with specific location and it also support our argument that placenta contains microbiome. (Parnell, et al., 2017)
Over thousands years, the function of microbiome (composition of microorganism and its genes on the body) in regulating the immune responses, metabolism and human behavior has more deceptive. The microbiota is located in various part of body, usually the gut microbiota exhibits the great importance, but microbiota is also present in skin, oral cavity, lungs, urogenital system and in amniotic fluid and placenta during pregnancy. The composition of microbiome depends upon two main factors, firstly the body site and secondly the host-dependent factors including genetic diversity, nutritional consumption, state of disease, geographical locality and bacterium species present. Any disturbance in the composition of microbiota will lead to serious infectious disease called as dsybiosis. The presence of bacteria in placenta was confirmed by culture dependent techniques, presence of microbes in tissue of placenta has describe the presence and absence of infection by using culture dependent and culture independent techniques. The DNA based analysis provide evidence that placenta have low biomass endogenous microbiota in the placenta. The species present in placenta include Lactobacillus species, Propionibacteria speciesand member of Enterobacteriaceae in healthy term pregnancy. The lactobacillus is relatively low in the tissue of placenta in preterm deliveries, gives a perspective of positive outcomes of pregnancy. (Pelzer, et al., 2017)
Researcher have evidence that transfer of bacteria from mother to infants, which is independent of delivery, for example vaginal or C-section. It is also believed that fetus delivery either by vagina or C-section, it contains common commensal bacteria like species that produced lactic acid. This said that fetus is not sterile, and different cocci gram positive bacteria also culture from umbilical cord of infant delivery through C-section. (Pelzer, et al., 2017)
The studies give a literature review about the placental microbiome and is association with preterm labor. A cross sectional study take place, including the sample from 14 patients of preterm while 10patient of term labor. The sample is taken from amnion and chorion. They don’t give exact procedure from which sample are collected. The tissue of preterm are diagnosed with the histological chorioamnionitics (infection of fetal membrane due to bacteria). The 16s RNA sequencing done for extracted DNA and then pyrosequencing. Both type of labor including preterm and term deliveries contain genera Streptococcus, Micro bacteria, and Rhodobacteria. (Singh, Bhuchitra, & Xia., 2019).