Comparative Analysis of gen. nov., sp. nov. and gen. nov., comb. nov. () Reveals the Importance of Interspecies Hydrogen Transfer in the Energy Metabolism of Spirochetes.

Most members of the family () are associated with vertebrate hosts. However, a diverse clade of uncultured, putatively free-living treponemes comprising several genus-level lineages is present in other anoxic environments. The only cultivated representative to date is Treponema zuelzerae, isolated from freshwater mud. Here, we describe the isolation of strain RmG11 from the intestinal tract of cockroaches. The strain represents a novel genus-level lineage of and is metabolically distinct from T. zuelzerae. While T. zuelzerae grows well on various sugars, forming acetate and H as major fermentation products, strain RmG11 grew poorly on glucose, maltose, and starch, forming mainly ethanol and only small amounts of acetate and H. In contrast to the growth of T. zuelzerae, that of strain RmG11 was strongly inhibited at high H partial pressures but improved considerably when H was removed from the headspace. Cocultures of strain RmG11 with the H-consuming Methanospirillum hungatei produced acetate and methane but no ethanol. Comparative genomic analysis revealed that strain RmG11 possesses only a single, electron-confurcating hydrogenase that forms H from NADH and reduced ferredoxin, whereas T. zuelzerae also possesses a second, ferredoxin-dependent hydrogenase that allows the thermodynamically more favorable formation of H from ferredoxin via the Rnf complex. In addition, we found that T. zuelzerae utilizes xylan and possesses the genomic potential to degrade other plant polysaccharides. Based on phenotypic and phylogenomic evidence, we describe strain RmG11 as Brucepastera parasyntrophica gen. nov., sp. nov. and Treponema zuelzerae as Teretinema zuelzerae gen. nov., comb. nov. Spirochetes are widely distributed in various anoxic environments and commonly form molecular hydrogen as a major fermentation product. Here, we show that two closely related members of the family differ strongly in their sensitivity to high hydrogen partial pressure, and we explain the metabolic mechanisms that cause these differences by comparative genome analysis. We demonstrate a strong boost in the growth of the hydrogen-sensitive strain and a shift in its fermentation products to acetate during cocultivation with a H-utilizing methanogen. Our results add a hitherto unrecognized facet to the fermentative metabolism of spirochetes and also underscore the importance of interspecies hydrogen transfer in not-obligately-syntrophic interactions among fermentative and hydrogenotrophic guilds in anoxic environments.