Xanthobacter

Abstract

Xan.tho.bac' ter . Gr. adj. xanthos yellow; M.L. masc. n. bacter the equivalent of Gr. neut. n. bacterion rod, staff; M.L. masc. n. Xanthobacter yellow rod. Proteobacteria / Alphaproteobacteria / Rhizobiales / Hyphomicrobiaceae / Xanthobacter Cells are rod shaped , sometimes twisted , 0 . 4–1 . 0 × 0 . 8–6 . 0 µm . Pleomorphic cells are produced on media containing tricarboxylic cycle‐intermediates (especially succinate), whereas coccoid cells as well as cells up to 10 µm long are produced on media containing an alcohol as the sole carbon source. Refractile (polyphosphate) and lipid (poly‐β‐hydroxybutyrate) bodies are evenly distributed in the cells. Resting stages are unknown. Key sporulation genes are absent. Depending on the species and growth conditions, the cells are nonmotile or motile (by peritrichous flagella). The Gram reaction frequently appears falsely to be positive or variable due to polyphosphate granules ; however , ultrastructurally and biochemically , the cell wall is of the negative Gram‐type . 1 Aerobic , having a strictly respiratory type of metabolism with oxygen as the terminal electron acceptor. Optimal temperature, 25–30°C. Optimal pH, 5.8–9.0. Colonies are opaque and slimy (although “slimeless” strains exist) and are yellow due to a water‐insoluble carotenoid pigment ( zeaxanthin dirhamnoside ). The color intensity depends on the amount of slime produced by individual strains. Catalase positive . All strains can grow chemolithoautotrophically in mineral media under an atmosphere of H 2 , O 2 , and CO 2 (7:2:1, v/v) as well as chemoorganoheterotrophically on methanol, ethanol, n‐propanol, n‐butanol, and various organic acids as sole carbon sources. The carbohydrate utilization spectrum is limited, and neither volatile/ nonvolatile fatty acids nor gas is produced from carbohydrates such as fructose, glucose, or mannose. Some strains require vitamins. Some strains can utilize substituted thiophenes as sole carbon, energy, and sulfur sources. When degrading aliphatic epoxides , tested strains contain coenzyme M , which otherwise is a typical coenzyme of the obligate anaerobic methanogenic archaea . N 2 is fixed in nitrogen‐deficient media under heterotrophic or chemolithoautotrophic growth conditions, but by most strains only under a decreased O 2 pressure. The bacteria occur free‐living in freshwater (mainly X . agilis ), wet soil containing decaying organic material ( X . autotrophicus , X . flavus ), marine sediments ( X . flavus ), compost of root balls of Tagetes (thiophenutilizing X . tagetidis ), and associated with the roots of plants including wetland rice ( X . flavus ). Xanthobacter can be regarded an associative N 2 ‐fixing bacterium (rice, tagetis, coconut palm). The induction of root or stem nodules has not been observed. 16S rDNA sequence analysis places the members into the class Alphaproteobacteria ; however, the presently recognized species of Xanthobacter are intermixed with the single‐species genera Aquabacter and Azorhizobium , and together they form a distinct cluster. The mol % G + C of the DNA is : 65–70 ( T m ) and 66–68 (Bd). Type species : Xanthobacter autotrophicus (Baumgarten, Reh and Schlegel 1974) Wiegel, Wilke, Baumgarten, Opitz and Schlegel 1978, 580 (“ Corynebacterium autotrophicum ” Baumgarten, Reh and Schlegel 1974, 214.)