Figure 1. Representation of abundant HMOs present in milk and strategies employed by infant gut microbiota for their degradation.

(A) Secretor status and HMO composition. HMO structures that are most abundant in secretors are indicated by the blue arrow, while those that are most abundant in the breast milk of non-secretors are indicated by the red arrow. Structures at the intersection are found in both secretor and non-secretor mothers in similar abundances. The structures of these HMOs, along with their glycosidic linkages, are described by the inset key. (B) Most strains of Bifidobacterium use an ‘internalize, then degrade’ strategy where HMO structures are first imported using ABC transporters and degraded by intracellular glycoside hydrolases (GH). (C) Strains of Bacteroides typically employ an ‘external degradation’ strategy for HMO structures that involves cell surface associated carbohydrate binding proteins and secreted glycoside hydrolases encoded by polysaccharide utilization loci (PULs) that have features similar the prototypic starch utilization system (Sus) of Bacteroides thetaiotaomicron. This external degradation can result in ‘cross-feeding’ of secondary consumers in the infant gut microbiota.