Numerical Analysis of the Relationship Between Mycobacterium, Rhodochrous Group, and Nocardia by Use of Hypothetical Median Organisms

A total of 1,054 strains was allocated to species of the genera Mycobacterium, Nocardia, and Gordona (the rhodochrous group) on the basis of 88 characters, and hypothetical median organisms (HMOs) were derived for each species. Numerical analysis was carried out, treating these HMOs as operational taxonomic units. The dendrogram obtained showed that strains received as members of the rhodochrous group, including “Mycobacterium” rhodochrous, and strains named previously by the present author as Gordona, constituted a taxon comparable to the genus Nocardia. The analysis showed two separable groups at a 70% M value (matching coefficient) level. One consisted of slowly growing mycobacteria (Mycobacterium subgenus Mycobacterium Tsukamura) and the second consisted of rapidly growing mycobacteria (Mycobacterium subgenus Mycomycobacterium Tsukamura), the genus Gordona (including the rhodochrous group) and Nocardia. Diagrammatic representation of the taxonomic relationships by taxonomic distances showed the same result. Four taxa, which might be regarded as separate genera, separated at higher M values. These included the slowly growing mycobacteria, the rapidly growing mycobacteria, the genus Gordona (rhodochrous group), and the genus Nocardia, respectively. The slowly growing mycobacteria seem to be only distantly related to the other three, which bear a close relationship with each other.