In the land race for niches in our guts, what we eat often determines who wins and who loses, and breast milk seems to have the added value of being both a vehicle for those initial colonizers as well as a nutrient source. We see it as a train that brings in new colonizers who then strip the train for essential supplies and food as they set off to claim their land. Infants do not have a fully mature GI system when they are born, so factors in breast milk, like epidermal growth factor (EGF), help speed up that maturity by stimulating an increase in mucus production and decreasing the permeability of intestinal epithelial cells[i]. The faster an infant’s gut matures, the less likely he will succumb to disease and dietary stress. Further, breast milk is not the sterile liquid we once thought it was. Based on culturing techniques and DNA analysis, scientists have found about 15 genera of bacteria present in breast milk that are also common bionts in the human GI tract. Not only does breast milk offer human factors to develop the infant gut, it appears to be a transmitter of beneficial bacteria as well. Pictures of certain immune cells and milk cells show bacterial structures associated with them, linking these cells as possible elements of microbial transfer from mother to infant[ii]. These bacteria most likely work with the young human system, educating the immune system[iii] and maturing the structure of the gut itself through the microbial version of terraforming.
This maturity is important because as soon as we take our first mouthful of milk, we begin controlling that second step of colonization (which is really the only factor we can overtly and daily control). With factors that seem unrelated to microbiota at first, like EGF, we are actually setting up niches for our inhabitants, delineating space and boundaries that will be modified some by our microbial bionts. EGF levels in breast milk respond to the changing needs of the infant’s digestive tract[iv]: first by being present in very high levels just post birth (and at even higher levels when the birth is premature) when it is vital that the infant establish a healthy gut topography, then to lower levels as the infant ages and her gut’s structure becomes more stable. In addition to EGF, a myriad of other human factors in breast milk confer protective and metabolic powers to our genetic offspring by directly affecting the colonizing microbiota. For example, cytokines, secretory immunoglobulin A, and lactoferrin offer protective influence from pathogenic microbes and possible allergens. Further, amylase, casein, and folate binding protein work to make nutrients in breast milk more accessible to both human and microbial cells[v].
Of these factors, human milk oligosaccharides (HMO), the third most prevalent component of breast milk next to lactose and lipids, have the important role in selecting and nurturing the types of microbes that colonize the gut, perhaps setting a baseline for microbial and intestinal health that will affect the infant throughout the rest of his life. Studies show that types of HMOs as well as other factors like cytokines in breast milk vary not only from woman to woman but also from early stages of breastfeeding to late, possibly responding to the individual human ecosystems[vi]. The two factors—EGF and HMO—work together to encourage microbial colonization and growth in the gut yet also keep them in their place: the lumen[vii].