SYNOPSISA thin membrane of bacterial cellulose (BC) obtained from Acetobacter culture was tested for its performance as a dialysis membrane in aqueous systems. The BC membrane showed superior mechanical strength to that of a dialysis-grade regenerated cellulose membrane, allowing the use of a thinner membrane than the latter. As a result, the BC membrane gave higher permeation rates for poly(ethy1ene glycols) as probe solutes. The cutoff molecular weight of the original BC membrane, significantly greater than that of regenerated cellulose, could be modified by concentrated alkali treatments of the membrane. The nature of the change at the ultrastructural level caused by the alkali treatments was studied by X-ray diffraction and scanning electron microscopy. 0 1993 John Wiley & Sons, Inc.I NTRO D U CT 1 0 N Acetobacter xylinum, a Gram-negative bacterium, produces cellulose extracellularly. This cellulose is formed as gel-like mass (pellicle) at the surface of the medium and can be purified by proper chemical treatments. This material has high crystallinity and large surface area and has been attracting attention as a new form of cellulosic material. The proposed application includes an acoustic vibrator taking advantage of its high elastic modulus's2 and an insoluble thicker/binder for foods and sheetlike mater i a l~.~ When a purified pellicle is dried on a flat substrate, a thin translucent cellulose membrane is formed. This membrane is expected to have unique properties because it consists of fine and continuous crystalline microfibrils, not like paper sheets or regenerated cellulose films. One possible application is molecular filtration such as dialysis or ultrafiltration. It has been proposed to use the bacterial cellulose as a dialysis membrane in nonaqueous syst e m~.~ On the other hand, regenerated cellulose membranes have been widely used as a dialysis membrane in aqueous systems, where chemical stability and low toxicity of cellulose are preferable properties, especially in applications for labile biological systems. This study aimed at elucidating the basic characteristics of bacterial cellulose membrane as molecular separation medium in aqueous conditions, together with modifying the structure of the membrane by chemical treatments for controlling its molecular permeation characteristics. The tests were conducted in the dialyzing mode, i.e., without pressurizing the primary-side solution, by using a series of poly (ethylene glycols) [ poly (ethylene oxide) ] as probe solutes. EXPERIMENTAL Bacterial Cellulose (BC) MembraneThirty milliliters of sterilized Schramm-Hestrin medium4 was placed in a plastic Petri dish (inner diameter, 87 mm) and was inoculated with Acetobacter xylinum ( ATCC 23769 ) . Immediately after inoculation, the medium was gently but thoroughly mixed by swirling the dish so that the cells were distributed uniformly. The culture was statistically incubated at room temperature for 5-7 days, until the liquid medium was filled with cellulose pellicle.The harvested pellicle was rinsed with...

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