Characteristics of S Organism Isolated from Methanobacillus omelianskii

Abstract

Previous work showed that Methanobacillus omelianskii was a mixed culture of an ethanol-oxidizing organism called S organism and a hydrogen-utilizing methane bacterium, strain MOH. S organism grows poorly on ethanol unless a hydrogen-utilizing methanogenic bacterium is included to utilize the H 2 produced during growth. Further studies have shown that, among many substrates tested, only ethanol, n -propanol, n -butanol, isobutanol, n -pentanol, acetaldehyde, oxalacetate, and pyruvate are fermented by S organism, either alone or in combination with Methanobacterium ruminantium . It grew better in pure culture with pyruvate than with alcohols. H 2 gas phase inhibited growth on pyruvate as well as on alcohol. When grown alone on pyruvate, S organism produced mainly acetate, ethanol, and CO 2 , in addition to a small amount of H 2 . When combined with M. ruminantium , no H 2 and very little ethanol were produced and acetate production was increased. When M. ruminantium was present, electrons from pyruvate oxidation by S organism were channeled almost entirely to H 2 and hence to methane formation rather than ethanol. Also, S organism utilized more pyruvate when grown with M. ruminantium . Attempts to obtain better growth of S organism on ethanol by addition of many possible electron acceptors were unsuccessful. It grew best between 32 and 45 C, had a per cent guanine plus cytosine content of deoxyribonucleic acid bases of 47.27 ± 0.1, contained no cytochrome, and could be grown on a defined medium with pyruvate as the energy and carbon source and with (NH 4 ) 2 SO 4 as the main nitrogen source. These and other results suggest that S organism belongs in a new genus, but assignment of a definite taxonomic status should await isolation and characterization of more strains.