At 2 to 4 months of age, infants share on average just over a third of their gut microbiome – an ecosystem seeded at birth that plays a role in shaping their future health – with their mother’s gut or vaginal microbiome. That microbial sharing is notably higher when mom has a vaginal birth as opposed to a C-section delivery.
The findings, from a recent observational study led by the University of Colorado Anschutz, could offer more insight into why a child’s risk of developing some diseases is more heavily weighted by a mother’s predisposition to the disorders than a father’s.
“If a mother has obesity, her child has a much higher risk of also having obesity,” said Noel T. Mueller, PhD, a CU Anschutz School of Medicine professor of pediatrics. That’s true of other metabolic and immune-mediated diseases, such as asthma and eczema, and the shared risk is not all attributable to genetics, said Mueller, lead author of the study published last month in Cell Reports.
“That led us to think about the microbiome.” Understanding how this biological ecosystem of trillions of bacteria, viruses and other microbes develops and plays a role in human health could lead to evidence-based preventive measures.
How much of a baby's gut microbiome is shared with the mother?
The researchers’ main goals were to understand how much and from where babies’ microbiome came from mom, and if those factors differed with delivery mode.
“We found that the infant gut microbiome from 2 to 4 months of age shared about 35% of its species with the mother's gut or vaginal microbiome,” said Mueller, who has a secondary appointment in the Department of Epidemiology at the Colorado School of Public Health.
The dominant source of the shared microorganisms was the maternal gut microbiome at 33.7% compared to the maternal vaginal microbiome at 1.8%. “We also found that infants that were born vaginally shared more species with their mother (38%) than infants that were born by C-section (24.7%).”
The research team, which also included colleagues from Johns Hopkins, Michigan and Michigan State universities, studied 68 mother-infant pairs. A fecal and vaginal sample was collected from each mother during her third trimester of pregnancy, and at least one stool sample was taken at 2 to 4 months for each newborn.
By using advanced technology (shotgun metagenomic sequencing, which analyzes DNA from across the microbial community, and StrainPhlAn, a strain-tracking tool), the researchers were able to differentiate both species and strains of microbes. That extra step is important because gut bacteria health effects are often strain-specific.
Glossary:
Gut microbiome: An ecosystem including bacteria, viruses and other microorganisms living in the digestive tract.
Microbial species vs. strain: A species is a group of related microorganisms; a strain is a more specific genetic variation within that species.
Shotgun metagenomic sequencing: A method that analyzes genetic material in a sample to identify microorganisms and study their genetic differences.
Human milk oligosaccharides (HMOs): Complex carbohydrates in breast milk that help nourish beneficial gut bacteria.
Why are Bacteroides and Bifidobacterium important for infants?
The team evaluated all species and strains but focused on two genera: Bacteroides and Bifidobacterium. Both bacteria have been identified in previous studies as being:
- shared between mother and child
- important in early infant immune training
- linked to delayed colonization after C-section
“These two bacteria are typical early life colonizers of the infant gut microbiome and may play important roles in determining different outcomes, including obesity and other metabolic and immune-mediated outcomes,” Mueller said.
Some species within these two groups of bacteria help break down human milk oligosaccharides (HMOs), specialized carbohydrates found in breast milk. “So we were also interested in the sharing of these microbes because of their importance for utilizing breast milk.”
Can an infant's microbiome change after a C-section?
Especially for mothers who have C-sections, it’s important to note that 1) not every C-section delivered baby is going to have a negatively impacted microbiome and 2) the microbiome is highly driven by environmental factors, Mueller said.
“That's particularly true in the very beginning of life where it's very modifiable. And so these interruptions in maternal infant sharing of microbes, such as surgical delivery, can be potentially restored through different postnatal factors, such as breastfeeding and a prebiotic-rich infant diet.”
Another important study finding was that breastfeeding was associated with a higher ratio of Bifidobacterium infantis to Bifidobacterium longum in the infant, Mueller said.
“Bifidabacterium infantis is the species of Bifidobacteria that is most adept at breaking down human milk oligosaccharides found in breast milk. Introducing B infantis with human milk oligosaccharides could help this beneficial microbe engraft in the intestinal tract of infants that were not able to acquire it from their mother or other sources,” Mueller said.
For mothers who choose not to or who cannot breastfeed, they could opt for alternatives to breast milk that have HMOs, which feed microbiota, and be prudent with the use of antibiotics, which can alter the microbiome, he said.
By the numbers:
- 35% – average proportion of infant gut microbial species (at 2 to 4 months) shared with their mother's gut or vaginal microbiome
- 33.7% – shared with the maternal gut microbiome
- 1.8% – shared with the maternal vaginal microbiome
- 38% – maternal species sharing among vaginally born infants
- 24.7% – maternal species sharing among C-section-delivered infants
- 68 – mother-infant pairs in study population
More studies are needed to determine health implications
Mueller stressed that because the study was small and observational, it alone cannot establish causality. “We can't randomize women to have C-section delivery versus vaginal delivery, obviously, so we need to use observational studies to really try to assess the association of delivery mode with differences in mother-to-child microbiome sharing and, ultimately, with health outcomes.”
Future studies should continue to define what species are most impacted by perinatal exposures such as C-section, antibiotics and lack of breast milk, as well as other environmental factors, he said.
“Next would be experimental trials, including randomized controlled trials, to test interventions that can restore the infant microbiome and microbiome-associated health outcomes.”
Key points:
- Over a third of microbial species in infants' gut microbiomes at 2 to 4 months were shared with their mothers' gut or vaginal microbiomes.
- Most shared microbial species were also detected in the maternal gut microbiome rather than the vaginal microbiome.
- Vaginally born infants shared more microbial species with their mothers than C-section-delivered infants.
- Researchers are particularly interested in Bacteroides and Bifidobacterium because of their potential roles in early immune development and breast milk utilization.
- The study was small and observational, so it identifies associations rather than proving that delivery mode causes later microbiome or health differences.
