Because a natural entity “species” cannot be recognized as a group of strains that is genetically well separated from its phylogenetic neighbors, a pragmatic approach was taken to define a species by a polyphasic approach (L. G. Wayne, D. J. Brenner, R. R. Colwell, P. A. D. Grimont, O. Kandler, M. I. Krichevsky, L. H. Moore, W. E. C. Moore, R. G. E. Murray, E. Stackebrandt, M. P. Starr, and H. G. Trüper, Int. J. Syst. Bacteriol. 37:463-464, 1987), in which a DNA reassociation value of about 70% plays a dominant role. With the establishment of rapid sequence analysis of 16S rRNA and the recognition of its potential to determine the phylogenetic position of any prokaryotic organism, the role of 16S rRNA similarities in the present species definition in bacteriology needs to be clarified. Comparative studies clearly reveal the limitations of the sequence analysis of this conserved gene and gene product in the determination of relationships at the strain level for which DNA-DNA reassociation experiments still constitute the superior method. Since today the primary structure of 16S rRNA is easier to determine than hybridization between DNA strands, the strength of the sequence analysis is to recognize the level at which DNA pairing studies need to be performed, which certainly applies to similarities of 97% and higher.

For the first time a single experimental approach, 16 S ribosomal RNA sequence characterization, has been used to develop an overview of phylogenetic relationships in the bacterial world. The technique permits the tracing of relationships back to the common ancestor of all extant life. This first glimpse of bacterial phylogeny reveals a world whose roots appear to span more than 3 billion years. A deep phylogenetic split exists among the bacteria, which necessitates their division into two major lines of descent, the archaebacteria and the true bacteria (or eubacteria). It is a general finding that the most ancient bacterial phenotypes are anaerobic, and that aerobic phenotypes have arisen a number of times. Photosynthetic phenotypes are also extremely ancient. Many nonphotosynthetic groups appear to have arisen from photosynthetic ancestry, which is reason to question the generally held belief that the first bacteria were anaerobic heterotrophs. The two ultimate lines of bacterial descent are no more closely related to one another than either is to the cytoplasmic aspect of the eukaryotic cell. However, in that the eukaryotic cell is a phylogenetic chimera, it itself cannot be seen as a line of descent comparable to the two bacterial lines—although some of its individual parts can be so viewed. In this way, the chloroplast and perhaps the mitochondrion are each eubacterial, and at least one ribosomal protein is archaebacterial. A third line of descent that is neither eubacterial nor archaebacterial is represented in the 18 S ribosomal RNA.

A Gram-positive, motile, short-rod-shaped strain, designated YIM 004^T, was isolated from a forest-soil sample collected from Lijiang, Yunnan Province, China, and was investigated using a polyphasic taxonomic approach. The isolate contained chemotaxonomic markers that corresponded to those of its phylogenetic neighbour,Georgenia muralis, i.e. it possessed peptidoglycan type A4αwith lysine as the diagnostic cell-wall diamino acid, the predominant menaquinone was MK-8(H₄) and the major fatty acid was ai-C_15 : 0. The G+C content of the genomic DNA was 72.9 mol%. Strain YIM 004^Texhibited a 16S rRNA gene sequence similarity of 97.3 % and a DNA–DNA relatedness value of 18 % with respect toG. muralisDSM 14418^T. On the basis of the phenotypic and genotypic differences between the isolate andG. muralis, strain YIM 004^Trepresents a novel species of the genusGeorgenia, for which the nameGeorgenia ruaniisp. nov. is proposed. The type strain is YIM 004^T(=CCTCC AB 204065^T=DSM 17458^T=KCTC 19029^T). In addition, an emended description of the genusGeorgeniais presented.

The higher ranks of the class Actinobacteria were proposed and described in 1997. At each rank, the taxa were delineated from each other solely on the basis of 16S rRNA gene sequence phylogenetic clustering and taxon-specific 16S rRNA signature nucleotides. In the past 10 years, many novel members have been assigned to this class while, at the same time, some members have been reclassified. The new 16S rRNA gene sequence information and the changes in phylogenetic positions of some taxa influence decisions about which 16S rRNA nucleotides to define as taxon-specific. As a consequence, the phylogenetic relationships of Actinobacteria at higher levels may need to be reconstructed. Here, we present new 16S rRNA signature nucleotide patterns of taxa above the family level and indicate the affiliation of genera to families. These sets replace the signatures published in 1997. In addition, Actinopolysporineae subord. nov. and Actinopolysporaceae fam. nov. are proposed to accommodate the genus Actinopolyspora, Kineosporiineae subord. nov. and Kineosporiaceae fam. nov. are proposed to accommodate the genera Kineococcus, Kineosporia and Quadrisphaera, Beutenbergiaceae fam. nov. is proposed to accommodate the genera Beutenbergia, Georgenia and Salana and Cryptosporangiaceae fam. nov. is proposed to accommodate the genus Cryptosporangium. The families Nocardiaceae and Gordoniaceae are proposed to be combined in an emended family Nocardiaceae. Emended descriptions are also proposed for most of the other higher taxa.