Respiratory quinones and the ability to use fumarate as a terminal electron acceptor in anaerobic respiration were investigated in 49 bacterial strains representing a variety of conventional or species. The organisms examined were subdivided into two categories according to their quinones. (i) Ubiquinones are used by the neotype strain of and by cultures representing , and some unnamed species. (ii) Menaquinones are produced by both typical strains and many so-called or “” cultures. Several members of category ii exhibited low to medium reduced nicotinamide adenine dinucleotide-fumarate reductase activities when grown in unaerated complex media supplemented with fumarate. In addition, with , “group IIb” organisms, and a strain of , the yields of oxygen-limited growth were markedly increased by fumarate, indicating an energetic use of fumarate respiration. On the basis of these findings, restriction of the genus to “low-guanine-plus-cytosine” organisms containing ubiquinones and resembling is proposed. The incorporation of some former “flavobacteria” into a natural group of organisms containing menaquinones and placement in the vicinity of the guanine-plus-cytosine ratio are discussed.


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