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Two novel anaerobic, moderately thermophilic and cellulose-/cellobiose-digesting bacteria, EBR45T and EBR596T, were isolated from anaerobic sludge of a cellulose-degrading methanogenic bioreactor. Phylogenetic analysis based on 16S rRNA gene sequences indicated that these strains belonged to cluster III within the low-G+C-content Gram-positive bacteria. The close relatives of EBR45T were Clostridium straminisolvens DSM 16021T (sequence identity, 94.6 %) and Clostridium thermocellum DSM 1237T (93.4 %). The closest relative of EBR596T was Clostridium stercorarium DSM 8532T (95.9 %). Both isolates were rod-shaped sporulators, growing optimally at 60 °C. EBR45T was Gram-staining-reaction-variable and non-motile, formed bright-yellow colonies on solid media, and grew on a relatively narrow range of carbohydrates including cellulose and cellobiose. EBR596T was Gram-staining-reaction-negative and motile, formed glossy white colonies and grew on cellobiose and various carbohydrates except cellulose. Major fatty acid compositions were 16 : 0 iso, 16 : 0 and 16 : 0 dimethylacetal (strain EBR45T) and 15 : 0 iso, 16 : 0 iso, 15 : 0 anteiso and 17 : 0 anteiso (strain EBR596T). The DNA G+C contents were 36.9 mol% (EBR45T) and 51.1 mol% (EBR596T). Based on the phenotypic and phylogenetic data and genomic distinctiveness, strains EBR45T and EBR596T represent two novel species, for which the names Clostridium clariflavum sp. nov. (type strain EBR45T =DSM 19732T =NBRC 101661T) and Clostridium caenicola sp. nov. (type strain EBR596T =DSM 19027T =NBRC 102590T) are proposed.