SummaryFeatures of the interaction between cellulose and xyloglucen have been studied using the cellulose-producing bacterium Acetobacter aceti ssp. xylinum (ATCC 53524) and tamarind seed xyloglucen. Direct microscopic evidence is provided for the generation of cross-bridges between cellulose ribbons produced in the presence of xyloglucan but not cerboxymathyiceltulose. Cross-bridge lengths are very similar to those observed for de-pectinated onion cell walls. Similar cross-bridge lengths are observed following mixing of isolated A. xylinum cellulose and xyloglucan, showing that network formation can be an abiotic process. The level of incorporation of xyloglucan in an actively growing system (ca. 38% of cellulose) is an order of magnitude higher than that observed in mixtures of isolated polymers and is comparable with cell wall levels.NMR spectroscopy suggests that 80-85% of incorporated xyloglucen is segmentally rigid with the backbone adopting an extended 'cellulosic' conformation and probably aligned with cellulose chains. The remaining xyioglucan is more mobile and is assigned to cross-bridges with, on average, a twisted backbone conformation. No evidence for specific involvement of side-chain residues in binding is found, and the observation of cross-bridges with a non-fucosylated xyloglucan shows that fucose residues are not essential for network formation. Xyloglucen causes cellulose ribbons to become more amorphous end to have a decreased IoJl[~ crystallite ratio without any significant alteration in ribbon diameter.Based on the findings that levels of xyloglucan incorporation, the presence and lengths of cross-bridges, and the modification of cellulosic molecular organization are all similar to those found in plant cell walls, we suggest that