Dual human milk oligosaccharide-fibre utilisation is a selection cue for the weaning gut microbiome.

Gut microbiome (GM) maturation in early life follows organised taxonomic successions, yet how the weaning diet impacts these trajectories remains underexplored. Here, we collected faecal samples at pre-, early and late weaning from seven mother-infant dyads forming the Milkome cohort, designed to evaluate the contribution of human milk oligosaccharides (HMOs) to GM maturation during weaning (NCT07026526). Surprisingly, all preweaning infant faecal consortia grew on multiple dietary fibres, consistent with the prevalence of fibre-degradation genes in their metagenomes. Utilisation of both HMOs and dietary fibres was discovered as a metabolic hallmark of the weaning GM, as supported by metagenomics and the growth of faecal consortia on HMOs, following their enrichment on fibres. The growth of a defined consortium on weaning-mimic substrates, further showed that distinct Clostridia simultaneously deploy HMO and fibre utilisation pathways, which confers competitive growth against HMO- or fibre-utilising bifidobacteria. Metagenomics, culturomics and HMO-utilisation profiles of 137 maternal isolates were concordant with retention of the HMO-utilisation capacity by the adult GM. Our findings highlight dual HMO-fibre utilisation as an unrecognised selection cue of core adult GM species during weaning, which outlines a plausible mechanism of GM maturation in early life and extends the importance of HMOs to the weaning transition.