The levels of relatedness among strains of Erysipefothrix rhusiopathiae (serovars 1 through 23 and type N) were estimated by performing DNA-DNA hybridization experiments with the type strains of E. rhusiopathiae and Erysipefothrix tonsiffarum, which are the two Erysipefothrix 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 E. rhusiopathiae but less than 24% hybridization with the type strain of E. tonsillarum. 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 E. tonsiffarum but less than 27% hybridization with the type strain of E. rhusiopathiue. 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 E. rhusiopathiae and E. tonsiffarum 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 Erysipefothrix contains two main genomic species, E. rhusiopathiue and E. tonsillarum, which can be differentiated into serovars.Erysipelothrix rhusiopathiae is an organism which has a major economic impact, causing a variety of diseases in animals and birds (ranging from septicemia to urticaria), as well as erysipeloid, a skin disease of humans (23). This organism has also been isolated from the tonsils of apparently healthy slaughter swine, from slime on fish, and from the environment. At the present time, the strains of E. rhusiopathiae are classified into 23 serovars and the type N strains, which do not produce any precipitating antibody against homologous and heterologous heat-stable extracts in rabbits (9, 17). Heat-stable antigens that are derived from cells by hot aqueous extraction are the basis for dividing the species into serovars (3).Most isolates obtained from diseased swine belong to serovars 1 and 2 (22), and the genus Erysipelothrix was once thought to be monospecific (12).A cluster of avirulent serovar 7 strains that were isolated from porcine tonsils were found to be genetically distinct from the other E. rhusiopathiae strains. We have previously described these strains as new species, Erysipelothrix tonsillarum (13, 14). However, whether genotypic E. tonsillarum strains occur in serovars other than serovar 7 is still unclear. The purposes of this study were to examine the levels of relatedness among organisms belonging to serovars 1 through 23 and type N by performing DNA-DNA hybridization experiments with the type strains of E. rhusiopathiae and E. tonsillarum and to clarify the taxonomic relationships among these serovars and the species in the genus Erysipelothrix.
