Microbiota of an individual is acquired at birth, or in womb of mother? Whether the first encounter of microbes and fetus occur before, during or after delivery? Can the micro-organism reach the fetus by crossing the placental barrier or by hematogenous spread? Placental microbiota is a hot debating topic for over 150 years. Meconium has been described as sterile by many researches. Meconium is the earliest stool of baby it contain the material from the uterus, amniotic fluid epithelial cell, mucous and water. So, it can give an idea about the presence of microbes in placenta and uterus (Escherich 1988). Later studies have shown that meconium has diversity of microbes (Hu et al. 2013). Meconium examination can give and idea about microbes in the fetus food before delivery. Infant gut colonization by bacteria can occur during delivery of the baby, from the placental colonization and also from the uterine colonization of microbes in such case the placenta is thought to be sterile but the uterine walls and structures are not.
Placenta has less but very metabolically rich community. Non-pathogenic organism Firmicutes. Tenericutes, Proteobacter, Neisseria polysaccharea, Rhodococcus erythropolis and Fusobacter are shown to be housing the placenta using the Illumina sequencing technique. The placental microbiota was shown to be similar to mother’s oral microbiotaby Bray-Curtis analysis. It is speculated that bacteria are transferred by hematogenous spread and colonize the fetus. Preterm birth involves the metabolic pathways of placental microbiota which is also affected by Gastational weight gain. 16 S taxonomic unit analysis has shown that placental microbiome is associated with antenatal infections and also with the PTM (Aagaard et al. 2014). The paper has rejected the previous data about sterile placental environment but the absence of positive control makes the results unclear moreover the method used cannot distinguish between living and dead bacteria and samples were collected from intervillous space so it must contain the maternal blood (Kliman and H.J 2014).
Placenta has a very diverse microbial community and it is shown to mediate various metabolic pathways. It is shown that the vaginal and c section deliveries donot impact the microbial diversity. But the diversity lies in the extent of Gestational weight gain. Excess of GWG can lead to change in microbial composition of placental microbiome that could be a risk for PTB. Individuals with high GWG have decreased diversity and this decrease in diversity of microbes can lead to PTM and affect the bacterial associated metabolic pathways of folate biosynthesis, butanoate metabolism and siderophore biosynthesis (Anthony et al. 2015). These papers argue that the pregnancy related complications like pre term birth can be affected by the metabolic pathways of microbes present in the placenta and the gestational weight gain.
The association between different pregnancy related complications like pre-eclampsia, small for gestational age infant (SGA) and preterm birth (PTB) and presence of different bacterial DNA in placenta was determined by using deep metagenomic sequencing of total DNA and 16S rRNA gene amplicon sequencing. To determine if placental microbiome has any role in the onset of these complications can give an idea about the presence and type of microbes present in placenta. Vaginal organismswere shown to be more abundant than S. agalactiae in intrapartum and pre labour c section deliveries. But vaginal microbes were shown to be less abundant than S. agalactiae in pre labour C section delivery. This shows that there is a link between the types of delivery and microbe present. Vaginal lactobacillus is shown frequently in vaginal route delivered placenta which shows that microbes are acquired during the delivery and labour. Only S. agalactiae was shown to be assosiaed with placenta and not a result of contamination but presence of S. agalactiae and pregnancy related complications are not related. Only 5 percent of samplesa show the presence of S. agalactiae. Delivery bacteria like L. iners is shown to be associated with pre eclampsia and Streptococcus angiosus. PTM can be related to L. monocytogenes. it is evident that the placental samples are contaminated from laboratory procedures and before the onset of labour. Only S. agalcatiae was shown before labour, it can cause fatal sepsis in baby. The data shows that human placenta has no microbiome. The presence of vaginal microbes in placenta after C section could result from ascending or haematogenous spread (de Goffau et al. 2019).
The transfer of pathogens from mother to fetus is another dimension. There are cases where mother is infected and fetus is not. Placenta provides a strong barrier against the pathogens to cross. The syncytiotrophoblast prevent many pathogens from entering the fetus (Robbins et al. 2012).
Pathogenic micro-organisms in placenta can cause complications in the neurodevelopment of fetus and can result in adverse neurocognitive outcomes. Pathogenic microbes can cause inflammation and DNA methylation of placenta which is thought to affect the neurodevelopment. The placental inflammatory response generated by pathogenic microbes can influence the development in utero or even the infants. These non-commensal microbes have the ability to affect the epigenetic machinery. DNA methylation adverse outcome are studied on other tissues but not very deeply on placenta. As placental methylome can influence the brain health of fetus and it is caused by various pathogens it should be studied extensively (Tomlinson et al. 2019).
The problem with placental sample is that the bacterial biomass is very low and the contamination of collected samples during collection or laboratory procedures. Quantification of 16S rRNA gene copies by qPCR has shown that the placental samples contain vague and unclear copy number that cannot be differentiated from the negative control. 16S rRNA gene sequencing and community analysis give no separation between communities of placenta and contamination control. DNA purification method with different contamination background give similar results. These points support the absence of placental microbiota and contamination in otherwise positive results (Lauder et al. 2016). Comparison between the placenta of term and preterm deliveries and control sample by using the 16S rRNA marker gene sequencing and shotgun metagenomic sequencing did not give promising results as the shot gun metagenomic sequencing, marker gene sequencing and DNA sequencing of placental microbiome cannot distinguish from negative controls. Mycoplasma and Ureaplasma were detected in preterm birth samples but the same microbes were found in vagina of same women (Lieby et al. 2018).