The 16s rRNA gene sequences of 34 named and unnamed clostridial strains were determined by PCR direct sequencing and were compared with more than 80 previously determined clostridial sequences and the previously published sequences of representative species of other low-G+C-content gram-positive genera, thereby providing an almost complete picture of the genealogical interrelationships of the clostridia. The results of our phylogenetic analysis corroborate and extend previous findings in showing that the genus Clostridium is extremely heterogeneous, with many species phylogenetically intermixed with other sporeforming and non-spore-forming genera. The genus Clostridium is clearly in need of major revision, and the rRNA structures defined in this and previous studies may provide a sound basis for future taxonomic restructuring. The problems and different possibilities for restructuring are discussed in light of the phenotypic and phylogenetic data, and a possible hierarchical structure for the clostridia and their close relatives is presented. On the basis of phenotypic criteria and the results of phylogenetic analyses the following five new genera and 11 new combinations are proposed: Caloramator gen. nov., with Caloramator fervidus comb. nov.; Filifactor gen. nov., with Filifactor villosus comb. nov.; Moorella gen. nov., with Moorella thermoacetica comb. nov. and Moorella thermoautotrophica comb. nov.; Oxobacter gen. nov., with Oxobacter pfennigii comb. nov.; Oxalophagus gen. nov., with Oxalophagus oxalicus comb. nov.; Eubacterium barkeri comb. nov.; Paenibacillus durum comb. nov.; Thermoanaerobacter kivui comb. nov.; Thermoanaerobacter thermocopriae comb. nov.; and Thermoanerobacterium thermosaccharolyticum comb. nov. ~ ~~Great advances, have been made over the past decade in unravelling the phylogenetic complexities within the grampositive endospore-forming bacteria. For example, 16s rRNA oligonucleotide cataloging and, more recently, almost complete rRNA (or gene) sequencing have shown that the aerobic endospore-forming bacilli are phylogenetically very heterogeneous, consisting of at least six highly divergent lines (1,11,51). As a result of these studies, taxonomic reorganization of the genus Bacillus was initiated with the introduction of the genus Alicyclobacillus for some acidophilic species (51) and the genus Paenibacillus (2) for Bacillus polymyxa and its close relatives (rRNA group 3 [l]). Although the remainder of the genus is still in need of taxonomic revision, the phylogenetic groups established by rRNA analysis are already forming the foundation for a new molecular data-based taxonomy for this group of organisms. Knowledge of the natural interrelationships within the anaerobic genus Clostridium is more fragmented than knowledge of the interrelationships among the aerobic bacilli. The earliest, and until recently the most comprehensive, phylogenetic study of the genus Clostridium was published by Johnson and Francis (22), who demonstrated that there is considerable diversity within the genus by u...

A number of N-methyl compounds, including several methylamines, creatine, sarcosine, choline, and betaine,

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