A numerical taxonomy of 604 strains of the genera Arthrobacter, Aureobacterium, Brevibacterium, Cellulomonas, Clavibacter, Corynebacterium, Curtobacterium, Microbacterium, Nocardia, Nocardioides, Rhodococcus, Terrabacter and Tsukamurella was undertaken based on 280 physiological characters with the aid of miniaturized tests. Clustering was by the unweighted pair group method (UPGMA) with 12 different measures of similarity. Test error, overlap between the phena and cophenetic correlation coefficients for the classifications obtained with the Jaccard coefficient (S(J)), the Pearson coefficient (S(P)), the simple-matching coefficient (S(SM)), and the Dice coefficient (S(D)) as measures of similarity were within acceptable limits. Clusters were defined at the 55.0 to 70.5% levels (S(J)). The compositions of clusters corresponded largely the delineations of 81.1 to 93.5% (S(SM)), 29.1 to 55.0% (S(P)), and 55.3 to 81.1% (S(D)). A total of 31 major clusters (containing five or more strains), 41 minor clusters and subclusters (containing less than five strains), and 54 single-member clusters were obtained in the UPGMA/S(J) study. The following conclusions were reached: (i) A high degree of similarity between the genera Aureobacterium, Cellulomonas, Clavibacter, Curtobacterium and Microbacterium found in phylogenetic-based studies could be shown also phenetically. Strains belonging to these genera were found in S(J) clusters 1 to 45, often representing closely related, or single species. (ii) Several strains of the plant pathogenic coryneform bacteria assigned to the genus Clavibacter and Curtobacterium flaccumfaciens were found to be within one S(J); cluster, indicating the high similarity between these genera. The current classification of species within the genera Curtobacterium and Clavibacter is unsatisfactory; a close relationship to Microbacterium suggests a reclassification into a redefined genus. Subspecies of Clavibacter and pathovars of Curtobacterium flaccumfaciens should only be retained for practical purposes. (iii) Bacteria belonging to the genus Corynebacterium, including Corynebacterium glutamicum, C. ammoniagenes, C. minutissimum, C. striatum, C. variabilis, C. kutscheri, C. diphtheriae, C. pseudotuberculosis, and 'C. ulcerans' formed a separate complex of the distinct, adjacent clusters 47 to 63. The physiologically inactive species C. mycetoides, C. pseudodiphtheriticum, C. xerosis, C. renale and C. pilosum were found in clusters 116 to 126. (iv) Differences between the Arthrobacter globiformis group and the Arthrobacter nicotianae group were reflected in the structure of the phenogram, species differentiation being based on only a few characters. (v) Strains assigned to the four species of the genus Brevibacterium were grouped into two clusters; the taxonomic implications are discussed. (vi) The results of the study are largely in line with a previously published numerical survey and with chemotaxonomic and genetic data. Suggestions for an improved classification for some species is given in addition to an extensive data-base on physiological reactions for differentiation purposes.
