Gut Microbiota Through the Lifespan

Until recently, scientist have assumed that the introduction between infants and beneficial bacteria species occurs only once the infant is conceived. However, current research indicates that the relationship begins far sooner, when the infant is still in utero. This exchange occurs through the fetal dependence of the amniotic fluid, in addition to the umbilical cord and exposure to meconium. During delivery, several influential species of commensal bacteria are transferred to the infant from the mother. It is important to note that the mode of delivery plays a critical role in the establishment and growth of the gut microbiota. For instance, surgical delivery via Cesarean section (C-section) is associated with altered colonization of the infant gut at birth, ostensibly because the infant is not exposed to the maternal microbiota in the same manner as in vaginal conception. 

            Mode of feeding- breast milk versus formula- has a significant impact on the composition of the microbiota in early infancy. Human breast milk contains, in addition to live bacteria, a wide variety of complex carbohydrates called human milk oligosaccharides (HMOs). HMOs are known for modulating intestinal microbiota of breast-fed infants and act as prebiotics by enriching certain beneficial bacteria and stimulating the release of short-chained fatty acids that nourish the infant gut. 

            Childhood represents a unique transitional stage in regard to the colonization and composition human gut microbiota. Once thought that the human gut becomes adult like by the age of two, recent studies suggest that this maturation continues well beyond two years of age. Compared to adults, children have gut microbial communities that are enriched in functions that have the potential to support ongoing physical development. Conversely, the microbiota of adults is enriched in functions associated with inflammation and increased risk of adiposity (or obesity). The most notable differences include the enrichment of genes involved in the syntheses of vitamin B-12 and folate. 

            As individuals age, the composition of their gut microbiota undergoes substantial changes. Interestingly enough, the gut ecology of older adults displays greater inter-individual variation than that of younger adults. Potentially the most notable shift is observed in the decrease in Bifidobacterial strains. Recent research has alluded to the inverse correlation between the decreases observed in Bifidobacteria and the increased risk of pathogenic infection.

Bacterial Composition of the Human Gut over the Lifespan

  

·     Fetus: 

o  Relatively sterile 

·     Baby: 

o  Breast-fed: Usually Bifidobacteria dominant  

o  Bottle-fed: More diverse, with more Bacteroidetes, and less Bifidobacteria

·     Child: 

o  Increase in microbial diversity following weaning and intake of solid foods

·     Adults: 

o  Dominant Phyla: 

§ Firmicutes

§ Bacteroidetes

§ Actinobacteria

o  Less Dominant Phyla: 

§ Proteobacteria

§ Verrucomicrobia

·     Elderly:

o  Compared to healthy adults: 

§ Reduction in Firmicutes and Bifidobacteria

§ Increase in Bacteroidetes and Proteobacteria

MC

Hollister, E. B., Riehle, K., Luna, R. A., Weidler, E. M., Rubio-Gonzales, M., Mistretta, T. A., ... & Gibbs, R. A. (2015). Structure and function of the healthy pre-adolescent pediatric gut microbiome. Microbiome, 3(1), 36.

Ishiguro, E., Haskey, N., & Campbell, K. (2018). Gut Microbiota: Interactive Effects on Nutrition and Health. Academic Press.