<i>Bifidobacterium infantis—</i> a key (late) colonizer of the infant gut?

Human milk oligosaccharide (HMO)-degrading <i>Bifidobacterium</i> species are key early colonizers of the gut and influence gut and immune maturation. Loss of these taxa, particularly <i>Bifidobacterium infantis</i>, in many industrialized populations has raised concern. O'Brien et al. showed that supplementation with <i>B. infantis</i> EVC001 in exclusively breastfed U.S. infants aged 2-4 months leads to rapid and abundant colonization that persists 1 month after supplementation, demonstrating effective colonization beyond the neonatal period (C. E. O'Brien, S. A. Frese, K. Cernioglo, K. Damian-Medina, et al., mSphere e00518-25, 2025, https://doi.org/10.1128/msphere.00518-25). These findings align with observational cohort data showing that <i>B. infantis</i> can overcome priority effects and dominate the gut microbiome in breastfed infants by 2-3 months of age. Key questions remain regarding colonization in mixed- or formula-fed infants, the HMO thresholds required to sustain dominance during milk- and complementary feeding, and the critical developmental windows of <i>B. infantis</i> colonization for beneficial immune effects. Ongoing clinical trials with <i>B. infantis</i> will further clarify its role in disease prevention.