The name "Actinomyces suis" was applied to each actinomycete isolate from swine actinomycosis by Grässer in 1962 and Franke in 1973. Nevertheless, this specific species was not included in the "Approved List of Bacterial Name" due to absence of the type cultures. Therefore, "Actinomyces suis" based on the description of Franke 1973 has been considered as "species incertae sedis". We isolated a number of Actinomyces strains from swine. The representative strains of them was designated as Chiba 101 that was closely similar to the description in "Actinomyces suis" reported by Franke in 1973. Interestingly, it was found that the biological characteristics of these strains were also very similar to those of Actinomyces denticolens. Furthermore, the average nucleotide identity (ANI) value between strain Chiba 101 and the type-strain of Actinomyces denticolens (=DSM 20671T) was found to be 99.95%. Sequences of the housekeeping genes and 16S rRNA gene showed 100% homology. These results strongly suggested that "Actinomyces suis" Franke 1973 is the same species as Actinomyces denticolens. Since actinomycosis caused by Actinomyces denticolens have been demonstrated in horses recently, it is necessary to recognize that Actinomyces denticolens is the pathogenic actinomycetes in broader range of animals.
Keywords: Actinomyces denticolens; actinomycosis; nomenclature; “Actinomyces suis” Franke 1973.