Rhodococcus equi, a new genus?

Since the recognition of Corynebacterium equi, as the causal agent of equine pneumonia by Magnusson in 1923, the taxonomic status of this microorganism has not been straight forward. Now known as Rhodococcus equi, this species resides within the genus Rhodococcus, which is made up of 3 distinct subclades, namely; R. equi, R. erythropolis and R. rhodochrous. It has become increasingly evident that the subclade R. equi deserves genus status though further work is required to determine whether this is so. In the present study, thirty one R. equi isolates including the type strains of the genus Rhodococcus, were the subject of 16S rRNA gene sequence studies along with establishing their chemotaxonomic, molecular fingerprinting and phenetic profiles. The R. equi isolates, including the type strain of the species, formed two distinct groups based on the molecular fingerprinting and numerical taxonomic data. The results of the chemotaxonomic studies confirmed these data and also provided additional characters for the differentiation of the two groups. These data add weight to the view that the species R. equi should be given genus status and that this prospective new taxon contains two species.