Figure 5.

HMO utilization strategies adopted by B. bifidum, B. longum subspecies infantis, and B. breve, representative infant-type species. B. bifidum extracellularly degrades HMOs into mono- and disaccharides to assimilate them by itself and also to cross-feed the degradants to other bifidobacteria, especially B. breve. B. longum subspecies infantis import almost all HMOs as intact forms and degrades them inside the cells. During the process, a part of the liberated monosaccharides is expelled from the cell, which is then preferentially utilized by B. breve to outcompete B. longum subspecies infantis. The ability of B. breve to rapidly consume mono- and disaccharides in the environment could partly explain the dominance and prevalence of B. breve in the intestine of breastfed infants living the industrialized lifestyle (Ojima et al., 2022a; Olm et al., 2022).