PHYLOGENETIC ANALYSIS OF ANAEROBIC THERMOPHILIC BACTERIA - AID FOR THEIR RECLASSIFICATION

Small subunit rDNA sequences were determined for 20 species of the gen era Acetogenium, Clostridium, Thermoanaerobacter, Thermoanaerobacteriu m, Thermoanaerobium, and Thermobacteroides, 3 non-validly described sp ecies, and 5 isolates of anaerobic thermophilic bacteria, providing a basis for a phylogenetic analysis of these organisms. Several species contain a version of the molecule significantly longer than that of Es cherichia coli because of the presence of inserts. On the basis of nor mal evolutionary distances, the phylogenetic tree indicates that all b acteria investigated in this study with a maximum growth temperature a bove 65-degrees-C form a supercluster within the subphylum of gram-pos itive bacteria that also contains Clostridium thermosaccharolyticum an d Clostridium thermoaceticum, which have been previously sequenced. Th is supercluster appears to be equivalent in its phylogenetic depth to the supercluster of mesophilic clostridia and their nonspore-forming r elatives. Several phylogenetically and phenotypically coherent cluster s that are defined by sets of signature nucleotides emerge within the supercluster of thermophiles. Clostridium thermobutyricum and Clostrid ium thermopalmarium are members of Clostridium group I. A phylogenetic tree derived from transversion distances demonstrated the artificial clustering of some organisms with high rDNA G+C moles percent, i.e., C lostridium fervidus and the thermophilic, cellulolytic members of the genus Clostridium. The results of this study can be used as an aid for future taxonomic restructuring of anaerobic sporogenous and asporogen ous thermophilic, gram-positive bacteria.