The infant’s gut microbiota begins to develop at birth, via contact with the mother’s vaginal and intestinal microbiota, through skin contact and during breast-feeding. 1,2
After birth, the infant’s skin, oral cavity and respiratory tract come into direct contact with the external world, leading to continuous exposure to microorganisms.1 This increase in environmental exposure means the infant must rapidly develop a healthy and resilient immune system.3
Human milk is best for infants
Human milk is the best source of nutrition for infants, as it provides a complex and diverse matrix of nutritional and bioactive compounds in an optimal balance of nutrients and other components that are specifically tailored to the infant’s needs.4
Human milk is one of the first and most important factors in establishing the infant’s immune system. It acts as an “inoculum” for the infant’s gut, priming the immune cells to respond appropriately to food, antigens, pathogens, and commensal bacteria.5
Human milk helps establish a healthy gut and immune system
The beneficial effects of human milk are due to its many bioactive compounds. These include:6
- Human milk oligosaccharides (HMOS)
- Bacteria and their metabolites (such as bifidobacteria and lactobacilli)
- Immune cells
Of the many compounds of human milk, oligosaccharides, bacteria and bioactive compounds produced by bacteria play a key role in the development of a healthy gut microbiota and immune system.
Oligosaccharides in human milk have a “prebiotic” effect
Human milk contains a relatively large number of oligosaccharides (Human Milk Oligosaccharides, HMOS).7 HMOS represent the third largest component of milk solids after lactose and lipids8.
More than 200 different oligosaccharides have been identified in human milk and they have an overall “prebiotic” effect. This means that they pass undigested into the lower part of the gastrointestinal tract where they promote the growth of beneficial microorganisms, such as bifidobacteria and lactobacilli.9 The International Scientific Association for Probiotics and Prebiotics (ISAPP) has published the definition for prebiotics in a consensus statement in June 2017, stating “a prebiotic is a substrate that is selectively utilized by host microorganisms conferring a health benefit”.
The total pool of HMOS as a collective group supports immune health, as they prevent pathogen to cause infection, and increase the number and activity of immune cells to fight against10.
Bacteria in human milk and their metabolites
Human milk contains viable and non-viable bacterial cells and fragments, e.g. Streptococcus, Staphylococcus, Lactobacillus and Bifidobacterium.11,12,13
There is a high variability in composition and number of bacteria per millilitre human milk among mothers and in some cases even within mothers at different time points14. Moreover, the total number of bacteria in human milk significantly differs according to detection methods. It has been estimated that human milk contains between 103 and 106 bacteria per millilitre14,15. The difference in number due to different detection methods, could be due to the fact that in molecular based methods, also DNA form non-viable bacteria and extracellular DNA can be amplified, suggesting not only live bacteria but significant amounts of non-viable bacteria in human milk14.
Although the composition and numbers of bacteria vary widely among individuals, human milk is now considered to be an important source of beneficial “probiotic” bacteria in the crucial first days and weeks of an infant’s life. The milk microbiome may thus be regarded as an inoculum for the infant’s gut, being essential in programming the immune system to respond appropriately to (food-)antigens, pathogens and commensal bacteria.16
Interestingly, the immunomodulatory activities of bacteria are also associated to the bioactive compounds produced by these bacteria. Postbiotics refer to these products or by-products secreted by bacteria. During fermentation bacteria metabolize food and produce various bacterial compounds and metabolites17. Each bacterial strain used during specific fermentation processes produce unique postbiotics and therefore postbiotics from other bacterial strains or produced with another fermentation process cannot be compared.
The various compounds produced as postbiotics are included in the Figure below:
Although the importance of postbiotics has relatively been overlooked, scientific evidence of their beneficial health effects is progressively increasing.