Phenotypic characterization of bacteria from diseased and healthy horses identified 18 isolates as Bisgaard taxon 9 and 11 isolates as Actinobacillus lignieresii. All strains of taxon 9 were alpha-galactosidase- and raffinose-positive and showed variable fermentation of (+)L-arabinose and (-)D-sorbitol. Strains of A. lignieresii were negative for these characteristics, with the exception of raffinose. Two strains from the (-)D-sorbitol-negative group of taxon 9 showed a 16S rRNA similarity of 99-6%, while 99.5% similarity was found between two strains of the (-)D-sorbitol-positive group. DNA-DNA hybridization between the two strains representing the (-)D-sorbitol-negative group showed 98% binding, and their closest relationship was to a strain of A. lignieresii (64%). The two strains of the (-)D-sorbitol-positive group showed 83% binding and were related to the (-)D-sorbitol-negative group at a 76% DNA binding level. Actinobacillus arthritidis sp. nov. is proposed for 12 strains of the (-)D-sorbitol-positive group. Actinobacillus genomospecies 2 is suggested for the six strains of the (-)D-sorbitol-negative group. Phenotypically, strains of A. arthritidis and Actinobacillus genomospecies 2 differ in (-)D-sorbitol fermentation and can be separated from Actinobacillus equuli by being trehalose-negative, while a positive reaction for alpha-galactosidase separates the taxa from A. lignieresii. The type strain of A. arthritidis, CCUG 24862T, was isolated from a joint of a horse. Three equine isolates of A. lignieresii that could not be separated from the type strain by means of phenotypic characteristics showed 98.6-100% 16S rRNA similarity, but only 96.4-96.7% similarity to the type strain. DNA-DNA hybridization between two strains of this group showed 92% binding but only 70% binding to the type strain of A. lignieresii. Consequently, these equine isolates of A. lignieresii represent a new genomospecies of Actinobacillus, suggested as genomospecies 1 because phenotypic characteristics are not presently available to separate it from the type strain of A. lignieresii.


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