Structures of cellulose assemblies produced by Acetobacter xylinum under various conditions have been studied mainly by transmission electron microscopy. Native cellulose crystals are composites of cellulose Iα and Iβ . Twisted-ribbn cellulose assemblies produced in the HS medium at 28 °C were rich in cellulose Iα . On the contrary, splayed microfibrils produced in the presence of CMC at 28 °C were rich in Iβ . Not only the ribbon assembly but also the bundle of splayed microfibrils was determined to twist in the right-handed manner. When the bacteria were incubated at 4 °C, two kinds of band-like assemblies, “dense” and “coarse”, were extruded perpendicularly to the long axis of bacterial cells. The number of cellulose chains produced by one bacterium was different between “dense” and “coarse” assemblies. The “dense” assembly gave the electron diffraction pattern of cellulose II. In certain cases the transition region from dense to coarse portions was observed in one assembly. Initially a “dense” portion was produced and thereafter a “coarse” portion was produced. The number of cellulose synthesis sites seems to decrease, because a bacterium becomes less active after a certain period of time at 4 °C
