The development of the microbiome in humans is closely related to the maturation of the human immune system and forms a crucial part of it. Vaginal delivery is one of the first steps in this process, as it promotes the transmission of beneficial bacterial species from mother to baby during labor that continues during labor and breastfeeding.
A recent study Current Opinion in Microbiology explores the sources and functional importance of certain key species in mother-infant transmission of the microbiota.
Study: Mother-infant transmission of human microbiota. Image credit: Alena Ozerova / Shutterstock.com
Introduction
The human body is typically sterile in the womb and receives a wide range of microorganisms from its local environment and from the mother that will eventually colonize various bodily sites within the baby. Over time, pioneer taxa give rise to a ‘climax community’, consisting of host-adapted microbial species, capable of resisting invasive organisms and preventing pathogenic overgrowth, while contributing to the overall health of the host. host.
Although bacterial transmission is an integral part of bacterial persistence, it ensures that microorganisms continue to exist by colonizing new hosts, common mechanisms such as spore formation, tolerance to air exposure, and walls. thickened cells do not function in intestinal bacteria.
Important maternal species
Several microbial species have been reported to be transmitted from mother to baby during vaginal delivery. These same species have been detected elsewhere in the mother’s body, such as breast milk and skin, as well as in the baby’s feces during the perinatal period.
Some of these early species include Bacteroidetes and Actinobacteria. In particular, several species of Bifidobacterium and Bacteroides break down human milk oligosaccharides (HMOs) to allow the baby to digest them.
Although bacteroids dominate the intestinal microbiome (GM) in adults, these species differ from those found in infants. These species are able to better adapt to intestinal immune and environmental changes as the baby becomes an adult.
In addition, Bacteroids found in the small intestine have unique characteristics, such as a number of enzymes that break down complex carbohydrates, gene changes that can promote the transcription of operons to regulate the products associated with surface of bacterial cells, as well as certain mutations that cause the cell envelope. favorable for living and proliferating in the intestine.
Phylum Firmicutes is also found in abundance in the adult gut. However, of the 11 species transmitted from mother to baby, only two persist after the first month of life. Spore formation is minimal, perhaps because an essential component is missing for these species to bloom in the baby’s gut.
In fact, breast milk can promote bacteria that metabolize HMOs. However, once breastfeeding stops, spore formers begin to replace these species, which can lead to the spread of other sporulating bacteria from family members or other environmental factors. Therefore, these bacteria are not particularly dependent on maternal transmission.
Phylogenetic distribution of maternal transmission intestinal bacteria in the neonatal period. A representative phylogeny of human intestinal bacteria found in infants and adults from cultured isolates [28]. The main bacterial threads of the human intestine are represented by blue tones. Red dots represent species that are reported as maternal transmission during the newborn period (<1 month) and persist beyond, unfilled red dots represent species that are transmitted maternally but do not persist beyond this. period. The absence of a red dot indicates that the persistence level is unknown. The determination of persistence is based on the absence of species based on sequencing or cultivation technologies and, therefore, we cannot rule out the possibility that they are still present at extremely low levels of abundance. The colored bars represent the maternal reservoir. There is a strong phylogenetic signal, as maternal transmission is dominated by Bacteroidetes and Actinobacteria bacteria derived from the maternal intestine.
Microbial transmission pathways
The main routes of microbial transmission are from the mother’s intestine, which provides almost 30 species, and from the skin and tongue, each of which provides seven and five species respectively. The vagina and breast milk also provide less than five microbial species each to the baby.
Most species of skin and tongue die soon, except Veillonella parvula. However, Veillonella is very common in the baby’s gut and feeds on lactate supplied by Streptococcus or Lactobacillus to generate short-chain fatty acids (SCFA) such as propionate and acetate. This suggests a relationship of interdependence, so that “its abundance in the intestinal microbiota of early life may be due to cross-feeding with lactic acid-producing bacteria that consume HMOs.”
Previous studies have also reported that lactobacilli are mainly supplied to the baby from the mother’s vagina during childbirth. However, in the current study only three vaginally derived species were found, all of which did not survive beyond the neonatal period.
Three species of Bifidobacterium were located in breast milk and breast intestine. It is not clear how intestinal bacteria get into breast milk; however, researchers have proposed the existence of an enteromammary pathway that can transport gut-derived bacteria from the mother’s gut to breast milk through immune cells. Interestingly, some of the Bifidobacterium species found in breast milk, such as B. infantis and B. breve, are never found in the adult gut.
The maternal intestine remains the source of all persistent species for the baby, indicating that these microorganisms are important for the development of the infant’s intestine. The time of transmission may be at birth; however, transmission of these microorganisms is more likely to occur after delivery, probably via the fecal-oral route. Over time, the profile of the child’s intestinal microbiome will become more similar to that of the adult gut.
Early life microbiota assembly trajectories: Acquisition of the intestinal microbiota begins at birth and rapidly increases composition and functional capacity. This process is marked by different stages, (1) an early stage that lasts up to 6-12 months when weaning occurs (exact times are unknown) and acts as a window of development, important for immune and physiological development. (2) a transitional stage associated with cessation of breastfeeding and the introduction of solid foods leading to colonization by various spore-forming species and a reduction in the maturation stage of Bifidobacterium species (3 ) which resembles an adult-like composition abundant in Bacteroidetes and Firmicutes species. Microbiotic disturbances affect this assembly path by preventing the transmission and colonization of beneficial species and, in turn, lead to the colonization of opportunistic pathogens. Therapeutic interventions using Live Bacterial Therapeutics (LBT) or through diet modulation promise to restore and select age-appropriate beneficial species.
Immunological impact of infant colonization
It is well recognized that cesarean section and the use of antibiotics are sometimes necessary. Unfortunately, these events are also associated with a severe disruption of the natural processes that lead to the establishment of a healthy intestinal microbiome.
In the absence of normal mother-to-child transmission, there is an increased risk that opportunistic pathogens, including proteobacteria such as hospital Enterococcus and Klebsiella or other environmental sources after a cesarean section, may harm the baby.
Comparatively, the use of antibiotics reduces the rate of transmission of maternal microbes to the baby, thus affecting the development of the infant intestinal microbiome. At any age, babies who have received antibiotics are less likely to have the optimal diversity of intestinal microbes compared to those who have not.
Interestingly, asthma is more common after cesarean section and antibiotics in childhood. This suggests that early interventions that disrupt microbiome development may also have an impact on immune system development.
During childhood, innate immunity, including dendritic cells and natural killer cells, matures. This is related to microbial colonization of the gut during the first three months of life. Exposure to these antigens induces immune tolerance, as long as this exposure occurs within the first few weeks of life.
If after this period there is exposure to intestinal immunomodulatory bacteria such as Bifidobacterium and Bacteroides, a proinflammatory response is initiated instead of tolerance. “This suggests that there is a window of development in humans, during which key interactions between the immune system and microbes lead to normal immune development.”
B. infantis promotes normal immune development by subsequently promoting the expansion of T (Treg) regulatory cells; however, this species decreases the frequency of type 2 proinflammatory Th2 cells. With breastfeeding, B. infantis and other Bifidobacterium species generate metabolites such as aromatic lactic acids. These activate the aryl hydrocarbon receptor, thus causing immunomodulation.
In experiments with sterile mice, Bacteroides fragilis from the mother’s intestine produces capsular polysaccharides on the cell surface. This leads to the development of lymphoid and CD4 T cells, which help cytotoxic CD8 T cells to eliminate pathogens.
Implications
Maternal transmission of intestinal bacteria provides a microbial “starter kit” for babies that promotes healthy growth and disease resistance.
The current study suggests that both Bifidobacterium and Bacteroides species probably play an important role in the development of …