RANK: Genus

TAXONOMY: cellular organisms -> Bacteria -> Proteobacteria -> Alphaproteobacteria -> Rhodospirillales -> Rhodospirillaceae -> Azospirillum


Plump, slightly-curved and straight rods, 0.6–1.7 × 2.1–3.8 µm, often with pointed ends. Intracellular granules of poly-β-hydroxybutyrate are present. Enlarged, pleomorphic forms may occur in old, alkaline cultures, under conditions of excess oxygen or other stress. Gram negative to Gram variable. Motile in liquid media by a single polar flagellum; on solid media at 30°C, numerous lateral flagella of shorter wavelength may also be formed. Nitrogen fixers, exhibiting N2-dependent growth under microaerobic conditions. Grow well under an air atmosphere in the presence of a source of fixed nitrogen such as an ammonium or glutamate salts. Cells previously grown in presence of an inorganic nitrogen source may fix nitrogen in air provided that all added nitrogen is exhausted and nitrogenase is derepressed. Possess mainly a respiratory type of metabolism with oxygen and, with some strains, nitrate or nitrite as the terminal electron acceptor. Fermentative metabolism may also occur. Under severe oxygen limitation, some strains may dissimilate nitrate to nitrite or to nitrous oxide and nitrogen gas. Optimal temperature for growth varies from 33 to 41°C and pH from 5.5 to 7.5. Some strains may grow and form light or dark pink colonies, often wrinkled and non-slimy, on potato agar. Oxidase positive. Chemoorganotrophic; some strains are facultative hydrogen autotrophs. Grow well on salts of organic acids such as malate, succinate, lactate or pyruvate. d-fructose and certain carbohydrates may also serve as carbon sources. Some species require biotin. Growth in presence of 3% NaCl has been observed for some species. Occur free-living in the soil or associated with the roots, stems, leaves, and seeds mainly of cereals and forage grasses, although they have also been isolated from coconut plants, vegetables, fruits, legume, and tuber plants. May also be found in freshwater lakes. Root nodules are not induced.

The mol% G + C of the DNA is: 64–71.

Type species: Azospirillum lipoferum

Fecal distribution