Microbiota plays a central role for the development and health of the future adult, intervening in key host metabolic and immunological functions.
The first 1000 days after conception (including the pregnancy period and the first two years of life), are crucial for the establishment of the infant intestinal microbiota which occur in parallel with immune system development throughout this short time space.
Microbiota plays a central role in neonates and infants health as it provides a powerful and efficient protection against pathogens while at the same time sustaining the tolerance against the normal commensals and participate to immune system maturation.
It has been suggested that the establishment of microbiota may start before birth, although this hypothesis is still controversial. Studies have shown a strong correlation between intra-uterine infections and preterm deliveries. Interestingly it has been reported that, most of the bacteria detected in these infections are common habitants of the vaginal tract of the mother.
The microbiota of vaginally delivered newborns resembles the maternal vaginal microbiota, with Lactobacillus and Prevotella dominant species. Whereas, neonates delivered by C-section appear to contain microbiota similar to the maternal skin, including Staphylococcus, Corynebacterium, and Propionibacterium, as well as environmental microbes.
Interestingly, detection of viable bacteria in the meconium such as Enterococcus and Escherichia, as well as the presence of bacteria and bacterial DNA in the maternal-fetal interphase indicate the existence of a potential utero microbial environment.
However, the mechanisms by which bacteria could reach the gestational cavity and whether they may have beneficial effects for pregnancy outcomes and future health status for neonates need further investigation.
Neonatal colonization is a step-wise process that may be affected by several maternal and neonatal factors as well as by the environment. The mode of delivery, breastfeeding, and antibiotics use early in life, clearly influence the microbiota composition.
Altered microbial composition, observed in C-section-delivered infants, may affect the immune system maturation process and contribute to diseases related to an immune imbalance, including asthma, atopic and allergic disorders.
The early intestinal microbiota of neonates is characterized by low diversity and a relative dominance of the phyla Proteobacteria and Actinobacteria. In a step-wise process, microbiota becomes more diverse to resembles an adult composition by 2-5 years of age.
Breastmilk which is considered the gold standard for the infant nutrition, supports an adequate microbial colonization. Breastfeeding has been associated with decreased morbidity and mortality in infants, and to lower incidence of gastrointestinal infections and inflammatory, respiratory, and allergic diseases.
Breast milk contains a wide variety of prebiotic compounds, as well as its own microbiota that supports adequate microbiota establishment and growth in the infant gut.
The prebiotic compounds present in breastmilk are essential as they play a key role in facilitating the seeding of good bacteria in the infant gut, facilitating infant digestion, offering protection by competing with pathogens, and improving intestinal barrier functions by enhancing mucine production and reducing intestinal permeability.
Amongst the prebiotics present in human breast milk are the human milk oligosaccharides (HMOs), which are complex carbohydrates present in high concentrations in human milk (5-20 g/L).
They are the third most abundant class of biomolecules found in human milk after lactose and lipids.
Human milk oligosaccharides are essential as they promote the growth of beneficial bacteria in the gut, such as Bifidobacterium and Lactobacillus, which is reflected in differences observed in the intestinal microbiota of breast-fed and formula-fed infants.
Human milk oligosaccharides reach the colon where they serve as substrate for fermentation to “good” bacteria, such as bifidobacteria and lactobacilli and also Bacteroides and Staphylococcus. Compounds resulting from this fermentation, such as lactate and Short-chain fatty acids (SCFAs), including acetate and butyrate, and other metabolites represent the main energy source for colonocytes and provide important immune protective functions.
Specific prebiotic oligosaccharides, like short chain galacto- and long chain fructo-oligosaccharides (scGOS/lcFOS), have been shown to reduce the development of atopic eczema and allergies as well as reduce the impact of pediatric infections.
The diversity and abundance and complexity of HMOs is unique in humans compared to other mammals. The concentration of HMOS varies with the lactational stage, maternal nutrition, genetic predisposition or even geographic localization and socioeconomic environment of the mother.
Ongoing research is currently being performed to identify new HMOs and characterize their roles in infant microbiota and health.