Rhodococcal systematics: problems and developments

Various approaches that have been used in the development of a system of cl assification for the genus Rhodococcus are discussed. The application of ch emotaxonomic, molecular systematic and numerical phenetic methods have grea tly contributed to improvements in the systematics of rhodococci and relate d mycolic-acid containing actinomycetes. The genus currently encompasses tw elve validly described species but improved diagnostic methods are needed t o distinguish between them. In addition, evidence from 16S ribosomal RNA se quencing suggests that the genus is still heterogeneous.