Swimming motility in the gut microbiota is diverse and increased in inflammation.

This study presents a direct, functional analysis of gut bacterial motility in health and inflammation. Using phase contrast microscopy and high-throughput 3D tracking, motile bacteria were quantified and their swimming behaviours characterised in fresh gut content from healthy and inflamed mouse models. In health, less than 3% of gut bacteria were motile, exhibiting diverse swimming patterns rather than the run-tumble behaviour typical of model gut species. In all five inflammation models, the motile fraction increased 3.8- to 102-fold, correlating with elevated Lipocalin-2 where measured. Increased motility arose from both enrichment of motile taxa and rapid environmental modulation of motility expression. In vitro assays with human-derived isolates confirmed motility across several phyla, with variability down to strain level, and identified oxygen and viscosity as key modulators. These findings support increased motility as a hallmark of the inflamed gut and challenge established assumptions about gut bacterial motility.