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A novel spirochete strain, SPN1, was isolated from the hindgut contents of the termite Neotermes castaneus. The highest similarities (about 90%) of the strain SPN1 16S rRNA gene sequence are with spirochetes belonging to the genus Spirochaeta, and thus, the isolate could not be assigned to the so-called termite clusters of the treponemes or to a known species of the genus Spirochaeta. Therefore, it represents a novel species, which was named Spirochaeta coccoides. In contrast to all other known validly described spirochete species, strain SPN1 shows a coccoid morphology and is immotile. The isolated strain is obligately anaerobic and ferments different mono-, di-, and oligosaccharides by forming formate, acetate, and ethanol as the main fermentation end products. Furthermore, strain SPN1 is able to grow anaerobically with yeast extract as the sole carbon and energy source. The fastest growth was obtained at 30°C, the temperature at which the termites were also grown. The cells possess different enzymatic activities that are involved in the degradation of lignocellulose in the termite hindgut, such as ␤-D-glucosidase, ␣-L-arabinosidase, and ␤-D-xylosidase. Therefore, they may play an important role in the digestion of breakdown products from cellulose and hemicellulose in the termite gut.Spirochetes are distinguished from all other bacteria by their unique morphology and mechanism of motility due to the helical shape of the cells and the location of the flagella (axial filaments). They form a coherent phylogenetic group at the phylum level. These axial filaments, ultrastructurally similar to bacterial flagella, are attached to the cell poles and wrapped around the protoplasmic cylinder, which consists of the cytoplasmic and nuclear regions. The flagella and the protoplasmic cylinder are surrounded by a multilayered membrane called the outer sheath or outer cell envelope (9). Comparisons of 16S rRNA sequences demonstrate that the spirochetes represent a monophyletic phylum within the bacteria (35). Spirochetes are widespread in several environments, either as freeliving cells, mainly in marine and limnic sediments, or host associated as commensals or parasites of animals and humans.One of the spirochetal habitats is the digestive tract of termites and wood-eating cockroaches. Termites have developed a unique hindgut flora consisting of bacteria, archaea, flagellates, and yeasts. This symbiotic microbial community supports the decomposition of complex organic compounds and thus enables the termites to feed on wood or soil (6, 13, 21, 44). Spirochetes are one of the most abundant bacteria present in the gut fluid of termites, (36). Spirochetes of several sizes (3 to 100 m in length; 0.2 to 1.0 m in width) are consistently present in the hindguts of all termites (5). Whereas most spirochetes usually exist free-living in the gut fluid, they have also been found as ectosymbionts attached to the surfaces of protists, such as Mixotricha paradoxa, that inhabit the gut of the lower wood-eating termite Mastotermes darwinien...

Over the course of several million years, the eukaryotic gut symbionts of lower termites have become adapted to a cellulolytic environment. Up to now it has been believed that they produce nutriments using their own cellulolytic enzymes for the benefit of their termite host. However, we have now isolated two endoglucanases with similar apparent molecular masses of approximately 36 kDa from the not yet culturable symbiotic Archaezoa living in the hindgut of the most primitive Australian termite, Mastotermes darwiniensis. The N-terminal sequences of these cellulases exhibited significant homology to cellulases of termite origin, which belong to glycosyl hydrolase family 9. The corresponding genes were detected not in the mRNA pool of the flagellates but in the salivary glands of M. darwiniensis. This showed that cellulases isolated from the flagellate cells originated from the termite host. By use of a PCR-based approach, DNAs encoding cellulases belonging to glycosyl hydrolase family 45 were obtained from micromanipulated nuclei of the flagellates Koruga bonita and Deltotrichonympha nana. These results indicated that the intestinal flagellates of M. darwiniensis take up the termite's cellulases from gut contents. K. bonita and D. nana possess at least their own endoglucanase genes, which are still expressed, but without significant enzyme activity in the nutritive vacuole. These findings give the impression that the gut Archaezoa are heading toward a secondary loss of their own endoglucanases and that they use exclusively termite cellulases.Cellulose is the major polysaccharide component of plant cell walls and the most abundant renewable energy source on earth. In the biological conversion of cellulose to glucose, at least three distinct types of glycolytic enzymes are involved. Endoglucanases (endo-1,4-␤-glucanase, EC 3.2.1.4) randomly hydrolyze 1,4-␤ bonds of the cellulose chains. Cellobiohydrolases (exo-1,4-␤-glucanase, EC 3.2.1.91) cleave cellobiosyl units from nonreducing ends of the cellullose chains. ␤-Glucosidases (EC 3.2.1.21) cleave glucosyl units from nonreducing ends of cello-oligosaccharides. The diverse spectra of cellulases are classified into 12 of the 57 glycosyl hydrolase families based on amino acid sequence similarities (5).Termites are among the most important lignocellulose-digesting insects and possess a great variety of symbiotic microorganisms in their hindguts, including Bacteria, Archaea and Eukarya, i.e., protozoa and yeasts (7). In the digestive tracts of lower termites, cellulose seems to be synergistically degraded by flagellates, bacteria, and yeasts (3, 7, 15) as well as by the termiteЈs own cellulases (14, 17). Cellulases of termite origin belong to glycosyl hydrolase family 9 (GHF9). Flagellated protozoa belonging to the Archaezoa are unique symbionts in the phylogenetically lower termites. They are cellulolytic and produce acetate from cellulose for the benefit of their hosts (10). The termite Mastotermes darwiniensis is the only species of the most primitive termite family, M...

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