Conference Coverage

Study challenges role of birth canal exposure in newborn microbiome establishment



During parturient transmission of gut bacteria from mothers to infants, the dominant maternal source of bacteria is rectal, according to investigators.

This challenges the hypothesis that exposure to the birth canal explains major differences in gut bacteria between infants born vaginally and those born via C-section, reported Moran Yassour, PhD, of Hebrew University in Jerusalem.

“It’s not how and if you entered the birth canal, but rather how you exited it,” Dr. Yassour said during a presentation at the annual Gut Microbiota for Health World Summit.

According to Dr. Yassour, a number of investigators have evaluated vertical transmission of gut bacteria from mothers to newborns, but most began collecting data a week or more after birth, potentially missing critical information.

“We wanted to generate large-scale, paired, longitudinal data, which means that we had [samples from] both mothers and children, and we wanted to start at birth,” Dr. Yassour said at the meeting, sponsored by the American Gastroenterological Association and the European Society for Neurogastroenterology and Motility.

Dr. Yassour explained that newborns delivered vaginally often exhibit Bacteroides in their gut, whereas babies born via C-section do not exhibit these bacteria until 6-18 months of age; however, the vaginal microbiome typically lacks Bacteroides, making the birth canal an unlikely source. This disconnect served as the impetus for the present investigation, Dr. Yassour said.

The study, which is available as a preprint, involved 73 mothers and their infants. To determine the impact of birth canal exposure, the investigators compared gut bacteria of infants born vaginally with those born via pre-labor C-section (no exposure to the birth canal), and those born via post-labor C-section (exposure to the birth canal).

Initial results were surprising, Dr. Yassour said, as 54% of babies delivered via C-section had Bacteroides in their stool during the first week. But in the second week, 94% of the C-section group lacked Bacteroides, which aligns with characteristic findings and suggests failure of colonization, rather than complete lack of exposure.

Out of the 24 infants with persistent Bacteroides colonization, 22 (92%) were born vaginally, compared with 2 (8%) born via pre-labor C-section, and none born via post-labor C-section. This pattern was maintained in a multivariate analysis that accounted for antibiotic use and exposure to formula, both of which are more common among mothers that give birth via C-section.

The investigators also conducted a strain-level analysis of mothers and infants using metagenomic sequencing. Across all time points, 90% of matched maternal-infant strains were detected in babies delivered vaginally.

“[W]e found evidence for mother-to-child transmission of rectal rather than vaginal strains,” the investigators wrote. “These results challenge birth canal exposure as the dominant factor in infant gut microbiome establishment and implicate colonization efficiency rather than exposure as a dictating factor of the newborn gut microbiome composition.”

Dr. Yassour said that these findings may have an immediate effect on clinical practice.

“People have reported the practice of smearing babies that were born by C-section with vaginal fluids in the sense of trying to recapitulate the microbial signature that we find in kids born vaginally,” Dr. Yassour said. “But it’s probably not the vaginal fluid that we need to smear; it’s probably the proximity to the rectum and the bowel movements that happen during delivery ... and that is what’s causing this initial seeding from mother to child.”

Dr. Yassour disclosed no conflicts of interest.

SOURCE: Yassour M et al. GMFH 2020.

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