The levels of relatedness among strains of (serovars 1 through 23 and type N) were estimated by performing DNA-DNA hybridization experiments with the type strains of and , which are the two species that have been described. Two distinct DNA relatedness groups were identified. The group 1 strains, representing serovars 1, 2, 4 through 6, 8, 9, 11, 12, 15 through 17, 19, and 21 and type N, exhibited more than 73% hybridization with the type strain of but less than 24% hybridization with the type strain of . Group 2 included serovar 3, 7, 10, 14, 20, 22, and 23 strains, and these strains exhibited more than 66% hybridization with the type strain of but less than 27% hybridization with the type strain of . Strains representing serovars 13 and 18 exhibited low levels of hybridization (16 to 47%) with both of the type strains, indicating that these serovars may be members of a new genomic species. The members of the and groups resembled each other in many phenotypic characteristics, but differed in their ability to produce acid from saccharose and in their pathogenicity for swine. Our results confirm that the genus contains two main genomic species, and , which can be differentiated into serovars.


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