Mesophilic Cellulolytic Clostridia from Freshwater Environments

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Abstract
Eight strains of obligately anaeroic, mesophilic, cellulolytic bacteria were isolated from mud of fresh-water environments. The isolates (C strains) were rod-shaped, gram negative and formed terminal spherical to oval spores that swelled the sporangium. The G + C content of the DNA of the C strains ranged from 30.7-33.2 mol% (midpoint of thermal denaturation). The C strains fermented cellulose with formation primarily of acetate, ethanol, CO2 and H2. Reducing sugars accumulated in the supernatant fluid of cultures which initially contained .gtoreq. 0.4% (wt/vol) cellulose. The C strains resembled Clostridium cellobioparum in some phenotypic characteristics and C. papyrosolvens in others, but they were not identical to either species. The C strains differed from thermophilic cellulolytic clostridia (e.g., C. thermocellum) in growth temperature range and because they fermented xylan and C-5 products of plant polysaccharide hydrolysis such as D-xylose and L-arabinose. At 40.degree. C, cellulose was degraded by cellulolytic mesophilic cells (strain C7) at a rate comparable to that at which C. thermocellum degrades cellulose at 60.degree. C. Substrate utilization and growth temperature data indicate that the C strains contribute to the anaerobic breakdown of plant polymers in the environments they inhibit.