A study of diversity in anaerobic cellulolytic thermophilic bacteria

Abstract

Enrichments were set up using anaerobic bicarbonate buffered medium with cellulose and various other polysaccharides as the enrichment substrate. The inocula were water and sediment samples from freshwater and brackish thermal springs. These enrichments yielded anaerobic, cellulolytic, thermophilic bacteria with three distinct morphologies. Two of these isolates, a spirochete and a Thermotoga-like organism, were easily distinguished on the basis of their characteristic morphologies. The remaining isolates were rod-shaped and not easily distinguishable. The spirochete and the Thermotoga-like organism were fully characterised and their taxonomic position determined. The majority of the rod-shaped isolates had temperature optima of 70°C, in contrast to the previously described cellulolytic, thermophilic species of the genus Clostridium, which have temperature optima of 60-65°C. The degree of diversity within this group of rod-shaped bacteria and previously described isolates from various geographic locations was determined using three methods, numerical taxonomy, protein electrophoresis and phylogenetic analysis of the 16S ribosomal RNA. The results from these three studies showed the rod-shaped organisms to be a diverse group, not closely related to the cellulolytic, thermophilic members of the genus Clostridium. The phylogenetic study of representative isolates demonstrated this group of organisms to be a new sub-line of descent within the Clostridium/Bacillus subdivision of the gram positive bacteria. The results of this polyphasic approach allowed suggestions to be made as to the taxonomic position of these organisms. The isolation of the thermophilic spirochete and its designation to the genus Spirochaeta has extended the phenotypic and phylogenetic diversity of the organisms classified within the Spirochaetales. The phylogenetic study demonstrated the incoherence of the present taxonomy of the genus Spirochaeta. The Thermotoga-like isolate was shown to be a member of the genus Thermosipho on the basis of both phenotypic and phylogenetic characteristics. The isolation of this organism extended the geographical locations from which the members of the Thermotogales have been isolated.