Gut Microbiome Cross-Feeding: Axial Fermentation Front

The colon is not a stirred tank — it is a flow tube. This 2D companion to the 3D gut cross-feeding reactor resolves the same syntrophic chain (fiber → primary fermenters → lactate/acetate → cross-feeders → butyrate) along a spatial axis instead of a single well-mixed pool. Fiber, lactate/acetate and butyrate advect and diffuse down the tube while fermenter and cross-feeder biofilms react locally on the wall, producing a real axial concentration and pH gradient that a batch model cannot show. The same four controls — fiber supply, primary fermenter growth, cross-feeder efficiency and epithelial absorption — reshape where along the colon fermentation peaks and how acidic it gets before SCFAs are cleared.