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Placental microbiome

Placenta and its tissue layers

The placental microbiome is the nonpathogenic, commensal bacteria present in a healthy human placenta and is distinct from bacteria that cause infection and preterm birth in chorioamnionitis.[1] Until recently, the healthy placenta was considered to be a sterile organ but now genera and species have been identified that reside in the basal layer.[1][2][3][4][5][6]

The placental microbiome more closely resembles that of the oral microbiome than either the vaginal or rectal microbiome.[1]

Bacterial species and genera

Culturable and non-culturable bacterial species in the placenta obtained following normal term pregnancy have been identified.

Binomial name Commensal Transient Potential
Prevotella tannerae x x [3][7][8]
Firmicutes spp x x [2][9]
Tenericutes ssp x x [2]
Fusobacterium nucleatum x x [10][11]
Prevotella tanerae x
Bacteroides spp x x [9]
Fusobacterium spp x x
Streptomyces avermitilis x [9]
Neisseria polysaccharea x
Neisseria lactamica x
Proteobacteria ssp x [2][9]
Bacteroidetes ssp x [2]
Escherichia coli x x [4][8]
Escherichia ssp x x [8]
Actinobacteria ssp x x [9]
Cyanobacteria ssp x [9]
Chloroflexi ssp x [9]
Aquificae ssp x [9]
Verrucomicrobia ssp x [9]
Vibrio ssp x [9]
Burkholderia ssp x [9]
Beijerinckia ssp x [9]

A change in the composition of the microbiota in the placenta is associated with excess gestational weight gain, pre-term birth, and decreased overall species richness and variant abundance.[9] The placental microbiota varies between low birth weights and normal birth weights.[12] While bacteria are often found in the amniotic fluid of failed pregnancies, they are also found in particulate matter that is found in about 1% of healthy pregnancies.[2]

In non-human animals, part of the microbiome is passed onto offspring even before the offspring are born. Bacteriologists assume that the same probably holds true for humans.[2]


Investigations into reproductive-associated microbiomes began around 1885 by Theodor Escherich. He wrote that meconium from the newborn was free of bacteria. This was interpreted as the uterine environment was sterile. Other investigations used sterile diapers for meconium collection. No bacteria were able to be cultured from the samples. Bacteria were detected and were directly proportional to the time between birth and the passage of meconium. A 1927 study demonstrated the presence of bacteria in the amniotic fluid of those that were in labor for longer than six hours.[6]


  1. ^ a b c Fox C, Eichelberger K (2015). "Maternal microbiome and pregnancy outcomes". Fertility and Sterility. 104 (6): 1358–63. doi:10.1016/j.fertnstert.2015.09.037. PMID 26493119. 
  2. ^ a b c d e f g Wassenaar TM, Panigrahi P (2014). "Is a foetus developing in a sterile environment?". Letters in Applied Microbiology. 59 (6): 572–9. doi:10.1111/lam.12334. PMID 25273890. 
  3. ^ a b Yarbrough VL, Winkle S, Herbst-Kralovetz MM (2015). "Antimicrobial peptides in the female reproductive tract: a critical component of the mucosal immune barrier with physiological and clinical implications". Human Reproduction Update. 21 (3): 353–77. doi:10.1093/humupd/dmu065. PMID 25547201. 
  4. ^ a b Stout MJ, Conlon B, Landeau M, Lee I, Bower C, Zhao Q, Roehl KA, Nelson DM, Macones GA, Mysorekar IU (2013). "Identification of intracellular bacteria in the basal plate of the human placenta in term and preterm gestations". American Journal of Obstetrics and Gynecology. 208 (3): 226.e1–7. doi:10.1016/j.ajog.2013.01.018. PMC 3740162Freely accessible. PMID 23333552. 
  5. ^ Schwiertz, Andreas; Rusch, Volker (2016). "A Short Definition of Terms". In Schwiertz, Andreas. Microbiota of the Human Body. Advances in Experimental Medicine and Biology. 902. pp. 1–3. doi:10.1007/978-3-319-31248-4_1. ISBN 978-3-319-31248-4. 
  6. ^ a b Perez-Muñoz ME, Arrieta MC, Ramer-Tait AE, Walter J (2017). "A critical assessment of the "sterile womb" and "in utero colonization" hypotheses: implications for research on the pioneer infant microbiome". Microbiome. 5 (1): 48. doi:10.1186/s40168-017-0268-4. PMC 5410102Freely accessible. PMID 28454555. 
  7. ^ Mor G, Kwon JY (2015). "Trophoblast-microbiome interaction: a new paradigm on immune regulation". American Journal of Obstetrics and Gynecology. 213 (4 Suppl): S131–7. doi:10.1016/j.ajog.2015.06.039. PMID 26428492. 
  8. ^ a b c Todar, K. "Pathogenic E. coli". Online Textbook of Bacteriology. University of Wisconsin–Madison Department of Bacteriology. Retrieved 2007-11-30. 
  9. ^ a b c d e f g h i j k l m Antony KM, Ma J, Mitchell KB, Racusin DA, Versalovic J, Aagaard K (2015). "The preterm placental microbiome varies in association with excess maternal gestational weight gain". American Journal of Obstetrics and Gynecology. 212 (5): 653.e1–16. doi:10.1016/j.ajog.2014.12.041. PMC 4892181Freely accessible. PMID 25557210. 
  10. ^ Prince AL, Antony KM, Chu DM, Aagaard KM (2014). "The microbiome, parturition, and timing of birth: more questions than answers". Journal of Reproductive Immunology. 104-105: 12–9. doi:10.1016/j.jri.2014.03.006. PMC 4157949Freely accessible. PMID 24793619. 
  11. ^ Hitti J, Hillier SL, Agnew KJ, Krohn MA, Reisner DP, Eschenbach DA (2001). "Vaginal indicators of amniotic fluid infection in preterm labor". Obstetrics and Gynecology. 97 (2): 211–9. doi:10.1016/s0029-7844(00)01146-7. PMID 11165584. 
  12. ^ Zheng J, Xiao X, Zhang Q, Mao L, Yu M, Xu J (2015). "The Placental Microbiome Varies in Association with Low Birth Weight in Full-Term Neonates". Nutrients. 7 (8): 6924–37. doi:10.3390/nu7085315. PMC 4555154Freely accessible. PMID 26287241. 

See also