Early diet and the infant gut microbiome: how breastfeeding and solid foods shape the microbiome

Upon delivery, diet is one of the major factors impacting the maturation and diversification of the microbiome in early life. Both the type of milk given to the suckling infant and the subsequent introduction of solid foods appear to be involved in shaping the microbiome. In contrast to infant formulas, breast milk contains a wide variety of bioactive factors that support the development and maturation of the infant gut. Amongst others a complex mixture of human milk oligosaccharides, lactoferrin, lysozyme, immunoglobulins and lipids all contribute to an intestinal microbiome in breastfed infants that is clearly distinct from that of formula-fed infants. Moreover, breast milk contains its own microbiome and may as such directly seed the infant gut with bacteria. The intestinal microbiome of breastfed children is less diverse and characterised by a high abundance of bifidobacteria and lactobacilli. The indigenous microbiome of formulafed children is, on the other hand, characterised by a higher diversity, with increased numbers of bacteroides, clostridia and Enterobacteriaceae including opportunistic pathogens such as Clostridium difficile and Escherichia coli. The introduction of solid foods exposes the infant gut microbiota to a whole new range of nutrients. Consequently, the microorganisms capable of degrading complex sugars and starch flourish, intra-individual variations in the microbiota become less pronounced and the composition starts to resemble the adult microbiome. It is however still debated whether the major driving force in the development of an adult-like microbiome is the actual timing of the introduction of these new food substrates, the cessation of breastfeeding or a combination of both.