Summary of strains used in the work and the key genes related to the use of 2'FL
one hundred and fifty-one strains of Bifidobacterium used in this study were deposited at Culture Collection of Food Microorganisms (CCFM), Jiangnan University, including B. bifidum, B. breve, B. longum subsp. longum, B. longum subsp. infantis, B. adolescentis, B. animalis, B. pseudocatenulatum, B. angulatum and B. dentium. An 2'FL ABC transporter coupled ATP hydrolysis to efficient internalization of sugars and appeared to represent the essential carbohydrate transportation frameworks for bifidobacteria (Davidson et al., 2008; Jojima et al., 2010; O'Callaghan et al., 2016). These systems were broadly found in numerous microbes and had also been identified in most, but not all, bifidobacterial genomes (O'Callaghan and van Sinderen, 2016). Despite B. bifidium, 2'FL specific ABC transporter was essential for 2'FL utilization in Bifidobacterium (Thomson et al., 2018; Sakanaka et al., 2019), which composed of nucleotide-binding domain(NBD), trans-membrane domain(TMD) and solute binding protein (SBP) (Verrier et al., 2008; Matsuki et al., 2016). Putative 2'FL ABC transporter (K02025, K02026, K02027) was identified to play a vital role in 2'FL utilization through genomic analysis and gene knockout approaches in a previous research (Matsuki et al., 2016). Through genetic determinants analysis, complete putative 2'FL ABC transporter were found in most B. longum subsp. longum (50%), B. longum subsp. infantis (90%) and B. angulatum (75%), which was significantly higher than that in B. bifidum (4.54%), B. breve (22.22%), B. adolescentis (5%), B. pseudocatenulatum (9.09%) and B. dentium (22.22%). While no complete putative 2'FL ABC transporter system was identified in any B. animalis strains (Table 1). In addition, no obvious relationship was found between the existence of carbohydrate utilization active enzyme and transporter system as well as the origin of those strains (infants, teenagers, adults, etc). Simultaneously, among those 2'FL utilizers, most strains were isolated from neonates and infants (67.57%).