Maternal microbiota in pregnancy and early life

These GEITP pages have often discussed the microbiome, because of its direct relationship to our gene-environment interactions theme. Molded by both host biology and the physical relationship between mother and child, an important microbial connection is formed at the earliest moments of life when the newborn’s skin and mucosal surfaces are seeded with microorganisms derived from the mother’s body — referred to as the maternal microbiota. This initial microbial exposure establishes an early-life microbiota that reflects a mutualistic relationship with the host, and leaves a lasting impression on childhood development that can control the balance between health and disease. As described in the attached 2-page editorial ‘perspectives’, the quest to understand this microbial bond has uncovered exciting new discoveries about host–microbial mutualism and immune development in early life, while simultaneously revolutionizing our understanding of how certain traits and diseases are passed down through generations.

In humans and other mammals, the first microbes encountered in early life are those from the maternal microbiota. Despite the foundational nature of this event in human development, uncertainty persists about the precise timing of this first contact. The dogmatic belief — that fetal development occurs within a sterile intrauterine environment — has been challenged by evidence of bacterial genomic DNA within placental and chorioamniotic tissues and culturable microbes in newborn meconium (i.e. the first feces passed after birth), raising the controversial possibility that microbial colonization may begin in utero. However, these observations stop short of providing definitive evidence of colonization during fetal development with a bona fide microbiota (i.e. a live, persistent, and functional community of microorganisms); therefore, the concept of a fetal microbiota remains the subject of debate. In support of this, a recent study (on 11 Sept 2019, these GEITP pages reported this) found that the human placenta was devoid of a microbiome — although it was found to contain potential pathogens in a small proportion of samples.

On the contrary, a wealth of evidence demonstrates that the early-life microbiota is seeded at the time of delivery through contact with maternal commensal (i.e. natural, nonpathogenic) bacteria that inhabit the birth canal. The microbial inoculum of vaginally-delivered neonates is dominated by maternal cervicovaginal and fecal microbes, whereas delivery by caesarian section imparts distinct microbial consortia that are often dominated by skin microorganisms. These findings — together with observational studies suggesting an association between caesarian delivery and increased risk of adverse childhood health outcomes (e.g. obesity, asthma, and others), have led some to hypothesize that the distinctive early-life microbiota associated with caesarian delivery may have lasting effects on childhood health.

Although this does not constitute a causal relationship, this provocative association has contributed to the increasingly popular practice of “vaginal seeding” of infants born by caesarian section (i.e. exposing neonates to maternal vaginal content in the first minutes of life) — in an attempt to recapitulate the microbial exposure of a vaginal delivery. However, observational studies of maternal–infant pairs have challenged the lasting impact of delivery route on the early-life microbiota — with data showing evolution of microbial communities over the first 6 weeks of life that culminates in a diversified microbiota, regardless of the original route of delivery. Hmm. Some interesting concepts to ponder… See the attached 2-page perspectives article for more. 😊



· Science 6 Sept 2019; 365: 984-985

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