A novel Shigella strain (Shigella flexneri G3) showing high cellulolytic activity under mesophilic, anaerobic conditions was isolated and characterized. The bacterium is Gram negative, short rod shaped, and nonmotile and displays effective production of glucose, cellobiose, and other oligosaccharides from cellulose (Avicel PH-101) under optimal conditions (40°C and pH 6.5). Approximately 75% of the cellulose was hydrolyzed in modified ATCC 1191 medium containing 0.3% cellulose, and the oligosaccharide production yield and specific production rate reached 375 mg g Avicel ؊1 and 6.25 mg g Avicel ؊1 h ؊1 , respectively, after a 60-hour incubation. To our knowledge, this represents the highest oligosaccharide yield and specific rate from cellulose for mesophilic bacterial monocultures reported so far. The results demonstrate that S. flexneri G3 is capable of rapid conversion of cellulose to oligosaccharides, with potential biofuel applications under mesophilic conditions.Lignocellulosic biomass is abundant in nature, as well as in agricultural, forestry, and municipal wastes, and can be used as an excellent bioconversion feedstock. Biomass-derived saccharides, such as glucose, cellobiose, and other minor sugars, can be readily fermented by appropriate microbes into bioenergy products, such as hydrogen, ethanol, biodiesel, and other commodity chemicals. However, the high cost of converting biomass to sugars is the primary factor impeding establishment of a cellulosic-biofuels industry (24, 40). Lignocellulosic biofuels can be competitive on an industrial scale if efficient technologies can be developed (29,39). Currently, the most efficient process for utilization of cellulose as a feed stock is either a three-step process (separate hydrolysis and fermentation [SHF]) involving separate pretreatment, cellulose hydrolysis (i.e., saccharification), and hexose and pentose fermentation steps or a two-step process (simultaneous saccharification and fermentation [SSF]) involving separate pretreatment and simultaneous saccharification of hexose and pentose fermentation (48, 64). Combining hydrolysis of cellulose with simultaneous fermentation of hexose and pentose in a single process, i.e., direct microbial conversion (DMC), is an ideal strategy for converting cellulosic biomass to ethanol. However, no single microorganism/community can implement DMC with high efficiency (67). In any of these configurations, rapid and efficient saccharification is critical for developing competitive biotechnologies for cellulosic-biofuel production.In nature, cellulose is hydrolyzed to oligosaccharides by microorganisms, mainly fungi (e.g., brown-, white-, and soft-rot fungi) and bacteria (e.g., Clostridium and Cellulomonas), which produce either free cellulolytic enzymes or extracellular enzyme complexes known as cellulosomes (12). Many white-rot Basidiomycetes and some Actinomycetes have been employed for hydrolyzing lignocellulosic materials. For example, Trichoderma reesei has shown the highest cellulolytic activity currently known (27)....
