The symbioses between cellulose-degrading flagellates and bacteria are one of the most fascinating phenomena in the complex micro-ecosystem found in the hindgut of lower termites. However, little is known about the identity of the symbionts. One example is the epibiotic bacteria colonizing the surface of hypermastigote protists of the genus Staurojoenina. By using scanning electron microscopy, it was shown that the whole surface of Staurojoenina sp. from the termite Neotermes cubanus is densely covered with long rod-shaped bacteria of uniform size and morphology. PCR amplification of 16S rRNA genes from isolated protozoa and subsequent cloning yielded a uniform collection of clones with virtually identical sequences. Phylogenetic analysis placed them as a new lineage among the Bacteroidales, only distantly related to other uncultivated bacteria in the hindgut of other termites, including an epibiont of the flagellate Mixotricha paradoxa. The closest cultivated relative was Tannerella forsythensis (<85 % sequence identity). Fluorescence in situ hybridization with a newly designed clone-specific oligonucleotide probe confirmed that these sequences belong to the rod-shaped epibionts of Staurojoenina sp. Transmission electron microscopy confirmed the presence of a Gram-negative cell wall and revealed special attachment sites for the symbionts on the cell envelope of the flagellate host. Based on the isolated phylogenetic position and the specific association with the surface of Staurojoenina sp., we propose to classify this new taxon of Bacteroidales under the provisional name 'Candidatus Vestibaculum illigatum'.
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