Is Rhodococcus equi a soil organism?

A total of 189 isolates of Rhodococcus equi and related organisms and 16 marker strains representing the genera Rhodococcus and Corynebacterium were screened for 160 unit characters in a numerical taxonomic study. Analysis of the data indicated that R. equi forms a relatively homogeneous cluster distinctly separated from the recognized species of Rhodococcus and Corynebacterium (sensu stricto). Other members of the genus Rhodococcus are soil organisms and R. equi appears to fit into the genus on ecological as well as taxonomic grounds. It seems unlikely that R. equi could be a gastrointestinal tract commensal because unlike members of the latter group it is an obligate aerobe with an optimum temperature requirement of 28-30 degrees C. It is capable of utilizing simple organic compounds as sources of carbon or carbon and nitrogen and is sensitive to bile salts. Furthermore, it appears that isolation of R. equi from the gut contents of animals is dependent on those animals having access to grazing. Taxonomic studies (backed-up by ecological studies) support the concept that R. equi is a soil organism.