3. Diversity Discovery - The Spirochete Model.
Studies of termite gut spirochetes continue to reveal impressive species diversity and previously unrecognized physiological novelty. Our comprehensive analysis of their phylogenetic diversity was summarized in the last annual report and has since been published (Lilburn, et. al., 1999), as has our report on the first isolation of termite gut spirochetes (Treponema sp. strains ZAS-1 & ZAS-2) in pure culture and recognition of them as H2-utilizing CO2-reducing homoacetogens (Leadbetter, et. al., 1999). The latter is the first demonstration of H2/CO2-acetogenesis in the spirochete phylum of bacteria. Ongoing nutritional studies revealed that these strains will also utilize various sugars as fermentable energy sources, but each requires a source of folic acid (or a folate derivative) for growth. This was surprising, as tetrahydrofolate is a critical C1-carrier in the Wood-Ljungdahl pathway of acetogenesis from CO2, but it implies that growth and acetogenesis by these forms may depend on folate-compound secretion by other members of the gut microbiota. A new termite gut isolate, Treponema sp. strain ZAS-9, which is closely related to strains ZAS-1 and ZAS-2 phylogenetically, grows by fermentation of sugars to acetate, H2 and other products, but is apparently incapable of H2/CO2-acetogenesis. Thus, H2-based spirochete-spirochete trophic interactions may be another important aspect of the biology of termite gut spirochetes.
Research planned for the coming year includes: (i) a search for folate compound secretors among the termite gut microbiota as a prelude to studies of their interaction with homoacetogenic spirochetes; and (ii) an estimate of the in situ contribution of termite gut spirochetes to H2/CO2-acetogenesis by using a combined rRNA probe-14CO2 autoradiography approach (STARFISH).
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