The Genus Derxia

Abstract

In the genus Derxia only a single species is recognized, D. gummosa. This species was originally described by Jensen et al. (1960) from an isolate obtained from Indian soil, but was later found to be widely distributed in soils of South America (Brazil), Southern Africa, Indonesia, and China, but not in temperate regions. Although Roy and Sen (1962) described a second species of Derxia (D. indica sp. nov.) from a sample of partially retted jute plant (Corchorus olitorius L.) from Uttar Pradesh, India, they did not present cultural and physiological data showing that their isolate was sufficiently different from Derxia gummosa to warrant the designation of a new species. The genus is named for H. G. Derx (1894–1953), a Dutch microbiologist. Using rRNA cistron similarity as a criterion for genetic relatedness, De Smedt et al. (1980) found that D. gummosa is quite different from species of the genera Azotobacter, Azomonas, and Beijerinckia. From hybridization studies with DNAs from three D. gummosa strains using the 14C-labeled rRNAs from a great variety of other organisms, it was quite obvious that the Derxia rRNA cistrons most closely resemble those of Pseudomonas acidovorans, P. solanacearum, Chromobacterium violaceum, Janthinobacterium lividum, and Alcaligenes faecalis. These taxa, together with a few others, constitute the third rRNA superfamily of De Ley (see The Proteobacteria: Ribosomal RNA Cistron Similarities and Bacterial Taxonomy in the second edition). Members of these taxa have in common that they consist of Gram-negative rods, usually 1.0–4.0 µm in length and 0.5–1.0 µm in diameter, do not possess resting stages, have a GC content of the DNA of 57–72 mol%, are chemoheterotrophic, exhibit respiratory (oxidative) metabolism, and occur in soil and water habitats. The third rRNA superfamily of De Ley (De Smedt et al., 1980) is identical with the beta subclass of the Proteobacteria, according to the phylogenetic taxon nomenclature of Stackebrandt et al. 1988. Based on numerical analysis of a large number of attributes, Thompson and Skerman (1979) showed fusion of their five Derxia strains tested with “Azotomonas” (“Azotomonas insolita” Stapp, 1940, sometimes termed “Pseudomonas insolita” [Stapp] Brisou, 1961) at group 1296 in their dendrograms of hierarchical interrelations (see also Krieg and Holt, 1984). This indicates that Derxia is not related to the genera Azotobacter or Azomonas and thus confirms the conclusions of De Smedt et al. (1980).