Human milk oligosaccharide mediates mutualism between Escherichia coli and Bifidobacterium bifidum.

Infant gut microbiota development involves frequent colonization by Enterobacteriaceae, particularly Escherichia coli, yet their ecological role in healthy infants is unclear. Here, we analyse longitudinal stool samples from healthy, term-born, breastfed infants (n = 41) and related mothers (n = 30) using shotgun metagenomics and novel computational approaches. Strain-resolved profiling indicates that Bifidobacterium species are frequently shared within families, whereas E. coli derive from external sources, but often persist within individuals. Despite differing ecological strategies, these genera co-exist and share evolutionary adaptations related to lactose acquisition in the infant gut. In vitro, we demonstrate that interactions between E. coli and Bifidobacterium bifidum are mutualistic in co-culture, where E. coli supplies cysteine to its auxotrophic partner, facilitating cooperative degradation of 2'-fucosyllactose, the predominant human milk oligosaccharide. In turn, the liberated monosaccharides sustain E. coli growth, highlighting a cooperative cross-feeding interaction that may contribute to regulating E. coli abundance within the infant host.