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

ABSTRACT Human milk oligosaccharide (HMO)-degrading Bifidobacterium species are key early colonizers of the gut and influence gut and immune maturation. Loss of these taxa, particularly Bifidobacterium infantis , in many industrialized populations has raised concern. O’Brien et al. showed that supplementation with B. infantis 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 B. infantis 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 B. infantis colonization for beneficial immune effects. Ongoing clinical trials with B. infantis will further clarify its role in disease prevention.