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Lutispora thermophila gen. nov., sp. nov., a thermophilic, spore-forming bacterium isolated from a thermophilic methanogenic bioreactor digesting municipal solid wastes The genus Clostridium, comprising Gram-positive species, was first proposed by Prazmowski in 1880. At the time of writing, the names of 188 species of the genus have been validly published (http://www.bacterio.cict.fr). Most members of the genus are chemo-organotrophs that can utilize carbohydrates and/or proteinaceous compounds as an energy source. In classical taxonomy, an organism only had to meet four criteria to be classified as a member of the genus Clostridium, i.e. be endospore-forming, be obligately anaerobic, have a Gram-positive cell wall and lack the ability to carry out dissimilatory reduction of sulfate. The genus was thus highly divergent and included psychrophilic, mesophilic and thermophilic species with a wide range of DNA G+C content (21-54 mol%).To clarify the phylogenetic incoherence within the genus Clostridium, Collins et al. In the present study, we investigated the taxonomy of a novel member of the family Clostridiaceae (strain EBR46 T ), isolated from thermophilic methanogenic sludge. In a previous study, we successfully determined the 16S rRNA gene sequence of this organism but failed to isolate it owing, to a large part, to its inefficient recovery from long-term storage at 280 u C (Shiratori et al., 2006). Following longterm successive cultivations, we succeeded in isolating and stably culturing strain EBR46 T . The distinct phylogenetic lineage represented by this isolate fell within a position close to members of the genus Gracilibacter outside the cluster of members of Clostridium sensu stricto. Both phylogenetic and Abbreviation: DMA, dimethyl acetal.