As the sole nutrition provided to infants, bioactive molecules dissolved in milk influence the development of our gut microbiota. Accordingly, human milk oligosaccharides (HMOs) are minimally digested by the infant and persist to negatively and positively regulate gut microbiota. Infant-type bifidobacteria utilize these soluble carbohydrate oligomers by convergent mechanisms. Bifidobacterium longum subsp. infantis efficiently consumes several small mass HMOs and possesses a large gene cluster and other loci dedicated to HMO metabolism. In contrast, adult-associated bifidobacteria such as the closely related B. longum subsp. longum are deficient for HMO utilization, although they retain the capacity to ferment plant oligosaccharides and constituent pentose sugars. Thus, the ability to subsist on HMO could demark infant-associated ecotypes potentially adapted to colonize the nursing infant.