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 &stances 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.Gordon and Mihm (5) named some strains received as nocardiae "Mycobacterium" rhodochrous. Later, Gordon (4) regarded this taxon as an intermediate between the genera Mycobacteriurn and Nocardia. Tsukamura (16) isolated a number of slightly acid-fast organisms from sputa of patients with lung disease and from soil and classified them into three species of a new genus, Gordona. Thereafter, strains received as M . rhodochrous were shown to belong to the same genus, and the majority of the strains received were identified as belonging to the species Gordona rhodochroa of this genus (17). Independently of these studies, Goodfellow (2) and Goodfellow et al. (3) reported that strains of M . rhodochrous could be differentiated from nocardiae and rapidly growing mycobacteria, respectively.Previously, studies on the relationships between rapidly growing mycobacteria, Gordona (rhodochrous group), and Nocardia were carried out by Tsukamura (15,16,18). However, he did not consider the slowly growing mycobacteria. The present study is concerned with the relationships between Mycobacterium, including slowly growing mycobacteria, the rhodochrous group, and Nocardia.
MATERIALS AND METHODSA total of 1,054 strains of Mycobacterium, the rhodochrous group, and Nocardia was allocated to species by comparing their characters with those of reference strains (14).The characters tested are listed in Table 1. Most of the methods used were described previously (11-14, 16, 20). Amidase activity, catalase activity, Tween hydrolysis activity, esterase activity, P-galactosidase activity, and acid phosphatase activity were tested by the methods of Bonicke (l), Kubica...