Bifidobacteria are among the first colonizers of the infant gut, due to their specialized ability to metabolize human milk oligosaccharides (HMOs) found in breastmilk. However, little is known about the assembly of bifidobacterial communities. Using a community of four representative infant-type Bifidobacterium species that employ varied strategies for HMO consumption, we show that arrival order and sugar utilization phenotypes significantly affected community structure. Bifidobacterium bifidum and Bifidobacterium longum subsp. infantis, two avid HMO consumers, dominate through inhibitory priority effects. Bifidobacterium breve, a species with limited HMO-utilization ability, can benefit from facilitative priority effects and dominates by utilizing fucose, an HMO degradant left unconsumed by the other bifidobacterial species. Analysis of publicly available breastfed infant fecal metagenome data showed that the observed trends for B. breve were consistent with our in vitro data, suggesting that priority effects may have contributed to its dominance. Our study provides evidence of facilitative interactions between bifidobacterial species. It also highlights the importance of the early stages of
community assembly and its implications for the maturation trajectory of the infant gut microbiota.