A.zo.to.bac' ter . Fr. n. azote nitrogen; M.L. masc. n. bacter the equivalent of Gr. neut. n. bactrum a rod or staff; M.L. masc. n. Azotobacter a nitrogen rod. Proteobacteria / Gammaproteobacteria / Pseudomonadales / Pseudomonadaceae / Azotobacter Cells range from straight rods with rounded ends to more ellipsoidal or coccoid , depending on the culture medium and age. Cells are up to 2 μm or more in diameter and 4 μm in length . A. paspali cells are usually longer, 5–10 µm in length, and can be filamentous, up to 60 µm long. Cells are usually single but may occur in pairs, irregular clumps (especially with A. paspali ), or, more rarely, in chains of varying length. Encystment occurs during late stationary phase at low frequency or at high frequency after culturing on butanol. Motile with peritrichous flagella or nonmotile. Aerobic, having a strictly respiratory type of metabolism with oxygen as the terminal electron acceptor. Nitrogen is fixed under microaerobic conditions (2% oxygen), under full aerobiosis, or after adaptation in hyperbaric oxygen. N 2 fixation uses Mo‐, V‐, or Fe‐containing nitrogenase enzymes, depending on the environmental metal supply. Water‐soluble and water‐insoluble pigments are produced by some strains of all species . Growth is heterotrophic; sugars, alcohols, and salts of organic acids are used as carbon sources. Ammonium salts, nitrate, and urea are used as sources of fixed nitrogen. Very few amino acids are used, probably due to a general deficiency in amino acid transport. The minimum pH for growth in the presence of fixed nitrogen sources ranges from 4.8 to 6.0 with maximum pH 8.5. The optimum pH for diazotrophic growth is 7.0–7.5. Most isolates are from soil, but a few are from water . One species ( A. paspali ) has been isolated only from roots of the tropical grass Paspalum notatum . The mol % G + C of the DNA is : 63.2–67.5. Type species : Azotobacter chroococcum Beijerinck 1901, 567.