Co-migration of hundreds of microbial species over metres drives selection and promotes non-motile hitchhikers.

Bacteria dominate the biosphere and assemble into highly diverse communities, yet the mechanisms by which these communities migrate remain poorly understood. Here, we used a meso-tube chemotaxis assay to track taxonomic, functional and genomic shifts within sewage-derived microbial communities that migrate in self-organized bands over metre scales. Chemotactic bands accelerated during migration and incorporated non-motile bacterial hitchhikers as well as up to 10⁶ viruses/mL. Approximately 500 species co-migrated, with relative abundances fluctuating by orders of magnitude over time. The final communities exhibited enrichment of functional genes linked to motility and chemotaxis, consistent with adaptation to migration. Despite this functional convergence, replicate communities differed in taxonomic composition, reflecting environmental filtering that selects for functionally equivalent species. By revealing how chemotaxis governs large-scale microbial migration, this work provides a framework for understanding microbial spread in natural ecosystems and host-associated environments.