An enzyme of 16.8 +/- 0.5 kDa, designated the potential cellulose microfibril swelling enzyme (Cms), was purified from newly isolated Bacillus sp. AY8 culture filtrate. The Cms-treated cotton cellulose hydrogen (H) bond intensity at approximately 3000-4000 cm(-1) decreased, and consequently, the crystalline index declined by 2.07%, as shown by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD), respectively. In addition, carbon atoms shifted the resonance up- or downfield, as shown by nuclear magnetic resonance (NMR). These results indicate that the Cms caused swelling of the cotton by weakening the cellulose intra-chain, inter-chain, and inter-sheet H-bonds. The optimum pH and temperature of the Cms were 7 and 37 degrees C, respectively. The activity of the Cms was enhanced by 2.24-fold in the presence of Cu2+ ions. The Cms-treated avicel yielded a 1.58-fold greater reducing sugar than that of the untreated avicel after 48 h of saccharification. This study revealed a potential cellulose swelling enzyme found in nature that may have significant applications in the bioethanol industry. (C) 2015 Elsevier Ltd. All rights reserved.