BREZNAK, J.A.: Phylogenetic Diversity and Physiology of Termite Gut Spirochetes
Spirochetes are major members of the termite gut microbiota, accounting for up to 50% of all the prokaryotes present. However, our understanding of them has been meager, as none had ever been isolated in pure culture since first being observed in termite guts over a century ago. By analyzing SSU rDNA clones (obtained after PCR amplification from termite gut DNA), we and others have found that termite gut spirochetes represent novel treponemes. As many as 26 new and different species of Treponema were detected in guts of various individual termite species, implying that earth’s termites constitute a rich reservoir of novel spirochetal diversity. Physiological properties that enable spirochetes to assume such abundance in termite guts are not yet clear, but probable roles for spirochetes in termite nutrition are now beginning to emerge from studies of the first pure cultures recently isolated in our laboratory [J. R. Leadbetter et al. (1999) Science 283:686-689]. All strains obtained so far produce acetate as a major fermentation product, with some capable of doing so from H2 + CO2. This latter process, previously unknown in spirochetes, is a significant H2 “sink” and source of acetate in termite hindguts. In addition, all strains possessed at least two homologues of nifH in their genomic DNA and catalyzed N2 fixation, another process heretofore unknown in spirochetes. Microbially-produced acetate is known to be a major carbon and energy source for termites, and N2 fixation by gut microbes can supply up to 60% of the N requirement of some termite species. Hence, with the discovery of acetogenesis and N2 fixation by termite gut spirochetes, we are finally beginning to reconcile the old observation that elimination of spirochetes from the gut results in decreased survival of termites.
