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 is extremely heterogeneous, with many species phylogenetically intermixed with other sporeforming and non-sporeforming genera. The genus 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: gen. nov., with comb. nov.; gen. nov., with comb. nov.; gen. nov., with comb. nov. and comb. nov.; gen. nov., with comb. nov.; gen. nov., with comb. nov.; comb. nov.; comb. nov.; comb. nov.; comb. nov.; and comb. nov.


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