Genetic relatedness among 179 strains representing 13 named species and several unnamed taxa of the genus from three main habitats, i.e., (i) feces of man, (ii) feces of various other animals, and (iii) the bovine rumen, was assessed by means of deoxyribonucleic acid (DNA)—DNA hybridization by using a filter-paper technique in competition experiments. Assignment of these strains to the genus was based on the production of lactic and acetic acids as chief products from glucose, lack of gas production, fructose-6-phosphate phosphoketolase activity, and on morphology. About 180 DNA competitors were tested with 23 reference systems. Several genetically distinct groups were recognized. Little or no DNA homology was demonstrated between some of the groups, suggesting large evolutionary divergence in this genus. , and form one of these groups; the ecological significance of this relatedness was discussed. The DNA of the following pairs are homologous: (i) and , (ii) and , and (iii) and . Within a number of strains assigned to , many of which were isolated from waste waters, a large genetic heterogeneity was demonstrated: in addition to , at least three unrelated groups were recognized and are provisionally referred to as “,” and “.” Since these groups are not related genetically to any species of the genus and are phenotypically distinct, they may represent new species. The validity of the species , and was confirmed at the genetic level.


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