Rhodococcus equi, two species within a new genus?

Rhodococcus equi is a facultative intracellular pathogen that can infect a wide range of animals, but which is best known as a casual agent of bronchopneumonia in 3-5 month old foals [1]. The organism is being increasingly recognised as an agent of necrotizing pneumonia and extrapulmonary infections in humans, especially in immunocompromised patients such as those with aids [2]. Initially, the organism was assigned to the genus Corynebacterium but was subsequently transferred to the genus Rhodococcus primarily on the basis of chemotaxonomic and numerical phenetic data [3]. The genus currently encompasses 30 validly described species which can be assigned to three 16S rRNA groups, the R. equi, R. erythropolis and R. rhodochrous subclades [4]. In the present study, 32 R. equi strains, composed of the type strain and isolates from clinical, environmental and veterinary sources, were the subject of a polyphasic taxonomic study designed to determine whether the organisms merited generic status. The strains formed a well circumscribed monophyletic branch in the 16S rRNA Corynebacterineae gene tree and were most closely related to R. kunmingensis. The taxonomic integrity of the two subclades that formed the monophyletic clade was underpinned by corresponding molecular fingerprinting (ARDRA and Rep-PCR), fatty acid and phenotypic data. The results provide good grounds for classifying R. equi strains into a new genus containing two species.