SUMMARY: rRNA homologies have been determined on reference strains representing 56 species of Clostridium. Competition experiments using tritium-labelled 23 S rRNA were employed. The majority of the species had DNA with 27 to 28% guanine plus cytosine (%GC). These fell into rRNA homology groups I and II, which were well defined, and a third group which consisted of species which did not belong in groups I and II. Species whose DNA was 41 to 45% GC comprised a fourth group. Thirty species were placed into rRNA homology group I on the basis of having 50% or greater homology with or Ten subgroups were delineated in homology group I. Species in each subgroup either had high homology with a particular reference species or a similar pattern of homologies to all of the reference organisms. The eleven species in rRNA homology group II had 69% or greater homology to Species in groups I and II had intergroup homologies of 20 to 40%. The six species in group III had very low homologies with groups I and II. Negligible homology also resulted when five of the species were tested against the sixth, The five species having DNA with 41 to 45% GC were and Little rRNA homology was apparent between and the other high % GC species or with several Bacillus species having similar % GC DNA. Correlations between homology results and phenotypic characteristics are discussed.


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