TABLE 2.

Various mechanisms that members from four infant-associated bifidobacterial species employ for HMO, GOS, and FOS/inulin degradation^a

	HMOs	GOS	FOS/inulin	References
B. infantis	ABC transporters, various GHs present in 43 kb “HMO island,” intracellular Bga42A hydrolyze LNT and Bga2A for LNnT. GlcNAc-6-p deacetylase upregulated in the presence of LNT and LNnT to breakdown HMO-bound GlcNAc. GH29 1,3/1,4-α-L-fucosidase hydrolyzes the 3-fucosyl and 4-fucosyl structures of fucosylated HMOs. Predicted intracellular GH33 2,3-2,6-α-sialidases present.	ABC transporter, intracellular GH2 and GH42 β-galactosidases (Bga42A, Bga42B, and Bga42C) for GOS hydrolysis. Bg42A is encoded for 3′-GL hydrolysis.	ABC transporter, and GH13 (Blon_0128) and GH32 enzymes (Blon_2056) are expressed for FOS and inulin. Blon_0787 is encoded for FOS hydrolysis. An exoinulinase with β-fructofuranosidase and invertase for inulin.	(149, 224, 244–249)
B. bifidum	Extracellular LnbB (GH20) liberates LNB from HMOs. Intracellular GLNBP for LNB degradation. Extracellular GH95 1,2-α-L-fucosidase for 2′-FL, GH29 1,3-1,4-α-L-fucosidase for 3-FL, and GH33 exo-α-sialidase for 6′-SL degradation.	GH42 for hydrolysis of GOS GH2 β-1,4-galactosidase and BIF3 for extracellular hydrolysis of GOS.	Highly variable. GH13 potentially involved.	(7, 159, 189, 244, 247, 250–255)
B. breve	ABC transporters, intracellular GH95 and GH29 α-fucosidases, GH33 α-sialidase encoded for fucosylated and sialylated HMO degradation. β-Galactosidases involved in LN(n)T metabolism.	GL-BG transports 6′-GL into the cells for degradation by GH42 β-galactosidase (e.g., BbrY_0422). Endo-β-galactanase (GH53) encoded for 4′-GL degradation. ABC transporter for internalization.	MFS transporter (lacy), ABC transporter, intracellular β-fructofuranosidase encoded by GH32 family for FOS and inulin hydrolysis. Variable growth observed.	(103, 105, 245, 256, 257)
B. longum	GH136 lacto-N-biosidase LnbX-positive, extracellular GH95 and GH29 α-fucosidases. LnbX-negative intracellular degradation of HMOs. GH112 family, GLNBP. GH33 2,3-2,6-α-sialidase.	Extracellular β-galactosidase I (BL0259), endo-β-galactanase (GH53).	Intracellular GH32 β-fructofuranosidase, ABC transporters (BL1164-BL1165), and putative GHs (BL0529 and BL0544) for inulin.	(129, 244, 247, 249, 258–260)

^a 3′-GL, 3’ galactosyllactose; LN(n)T, lacto-N-(neo)tetraose; LnbX, Lacto-N-biosidase; GH, glycoside hydrolase; GlcNac-6-p, N-acetyl-glucosamine-6-phosphate; SA-C2, sialic acid utilization cluster 2; GLNBP, GNB/LNB phosphorylase.