Classification of 'Anaerocellum thermophilum' strain DSM 6725 as Caldicellulosiruptor bescii sp. nov.

The thermophilic, cellulolytic, anaerobic bacterium 'Anaerocellum thermophilum' strain Z-1320 was isolated from a hot spring almost two decades ago and deposited in the German Collection of Microorganisms and Cell Cultures (DSMZ) as DSM 6725. The organism was classified as representing a new genus, 'Anaerocellum', primarily on its growth physiology, cell-wall type and morphology. The results of recent physiological studies and of phylogenetic and genome sequence analyses of strain DSM 6725 of 'A. thermophilum' obtained from the DSMZ showed that its properties differed from those originally described for strain Z-1320. In particular, when compared with strain Z-1320, strain DSM 6725 grew at higher temperatures and had an expanded range of growth substrates. Moreover, the 16S rRNA gene sequence of strain DSM 6725 fell within the Caldicellulosiruptor clade. It is therefore suggested that 'Anaerocellum thermophilum' should be classified as a member of the genus Caldicellulosiruptor, for which the name Caldicellulosiruptor bescii sp. nov. is proposed (type strain DSM 6725(T)=ATCC BAA-1888(T)). C. bescii sp. nov. DSM 6725(T) is the most thermophilic cellulose-degrading organism known. The strain was able to grow up to 90 degrees C (pH 7.2) and degraded crystalline cellulose and xylan as well as untreated plant biomass, including potential bioenergy plants such as poplar and switchgrass.