A facultatively anaerobic, marine spirochaete, designated strain SIP1(T), was isolated from interstitial water from a cyanobacteria-containing microbial mat. Cells of strain SIP1(T) were 0.3-0.4×10-12 mum in size, helical with a body pitch of approximately 1.4 mum and motile by means of two to four periplasmic flagella (one, or occasionally two, being inserted near each end of the cell). Cells were catalase-negative and used a variety of monosaccharides and disaccharides and pectin as energy sources, growing especially well on cellobiose. Neither organic acids nor amino acids were utilized as energy sources. One or more amino acids in tryptone and one or more components of yeast extract were required for growth. Growth was observed at 9-37 degrees C (optimally at or near 37 degrees C), at initial pH 5-8 (optimally at initial pH 7.5) and in media prepared with 20-100 % (v/v) seawater (optimally at 60-80 %) or 0.10-1.00 M NaCl (optimally at 0.30-0.40 M). The products of cellobiose fermentation were acetate, ethanol, CO(2), H(2) and small amounts of formate. Aerated cultures performed incomplete oxidation of cellobiose to acetate (and, presumably, CO(2)) plus small amounts of ethanol and formate, but exhibited a Y(cellobiose) that was only slightly greater than that of cellobiose-fermenting anoxic cultures. The G+C content of the genomic DNA of strain SIP1(T) was 41.4 mol%, the lowest among known spirochaetas. On the basis of its 16S rRNA gene sequence, strain SIP1(T) was grouped among other members of the genus Spirochaeta, but it bore only 89 % similarity with respect to its closest known relatives, Spirochaeta litoralis and Spirochaeta isovalerica, two marine obligate anaerobes. On the basis of its phenotypic properties and phylogenetic position, strain SIP1(T) represents a novel species of the genus Spirochaeta, for which the name Spirochaeta cellobiosiphila sp. nov. is proposed. The type strain is SIP1(T) (=ATCC BAA-1285(T) =DSM 17781(T)).