The taxonomic positions of strains previously assigned to Pseudomonas diminuta and Pseudomonas vesicularis were investigated by a polyphasic approach. The results of DNA-rRNA hybridization studies indicated that these two species belong to a separate genus in the alpha subclass (rRNA superfamily IV) of the Proteobacteria, for which the name Brevundimonas is proposed. Genus delineation and species delineation were determined by comparing the results of numerical analyses of whole-cell protein patterns, fatty acid compositions, and phenotypic characteristics and by measuring DNA base ratios and degrees of DNA relatedness. Taxonomic characteristics of Brevundimonas diminuta and Brevundimonas vesicularis strains were compared with characteristics of reference strains belonging to the following phylogenetically related taxa: a group of organisms gathered in Enevold Falsen group 21, the genera Sphingomonas and Rhizomonas, and the generically misclassified organisms [Pseudomonas] echinoides and "[Pseudomonas] riboflavina."

The broad and vague phenotypic definition allowed the genus Pseudomonas to become a dumping ground for incompletely characterized polarly flagellated, gram-negative, rod-shaped, aerobic bacteria, and a large number of species have been accommodated in the genus Pseudomonas. The 16S rRNA sequences of 128 valid and invalid Pseudomonas species, which included almost valid species of the genus Pseudomonas listed in the Approved Lists of Bacterial Names, were obtained: sequences of 59 species were determined and those of 69 species were obtained from the GenBank/EMBL/DDBJ databases. These sequences were compared with the sequences of other species of the Proteobacteria. Fifty-seven valid or invalid species including Pseudomonas aeruginosa (type species of the genus Pseudomonas Migula 1894) belonged to the genus Pseudomonas (sensu stricto). Seven subclusters were formed in the cluster of the genus Pseudomonas (sensu stricto), and the resulting clusters conformed well to the rRNA-DNA hybridization study by Palleroni (1984). The other species did not belong to the genus Pseudomonas (sensu stricto) and were related to other genera, which were placed in four subclasses of the Proteobacteria (alpha, beta, gamma and gamma-beta subclasses). Twenty-six examined species, which were not included in the cluster of the Pseudomonas (sensu stricto) and have not been transferred to other genera as yet, are listed alphabetically: 'Pseudomonas abikonensis', Pseudomonas antimicrobica, Pseudomonas beijerinckii, Pseudomonas beteli, Pseudomonas boreopolis, 'Pseudomonas butanovora', Pseudomonas carboxydohydrogena, Pseudomonas cissicola, Pseudomonas doudoroffii, Pseudomonas echinoides, Pseudomonas elongata, Pseudomonas flectens, Pseudomonas geniculata, Pseudomonas halophila, Pseudomonas hibiscicola, Pseudomonas huttiensis, Pseudomonas iners, Pseudomonas lanceolata, Pseudomonas lemoignei, Pseudomonas mephitica, Pseudomonas pictorum, Pseudomonas saccharophila, Pseudomonas spinosa, Pseudomonas stanier, Pseudomonas syzygii and Pseudomonas woodsii. The phylogenetic affiliations of these 26 pseudomonads species are shown.

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