The first representative of the genus, Azotobacter chroococcum, was discovered and described in 1901 by Dutch microbiologist and botanist Martinus Beijerinck.

Azotobacter species are Gram-negative bacteria found in neutral and alkaline soils,[1][2] in water, and in association with some plants.

In microscopic preparations, the cells can be dispersed or form irregular clusters or, occasionally, chains of varying lengths.

[8] The colored grains are composed of volutin, whereas the colorless inclusions are drops of fat, which act as energy reserves.

[10] The formation of cysts is induced by changes in the concentration of nutrients in the medium and the addition of some organic substances such as ethanol, n-butanol, or β-hydroxybutyrate.

[11] The formation of cysts is induced by chemical factors and is accompanied by metabolic shifts, changes in catabolism, respiration, and biosynthesis of macromolecules;[12] it is also affected by aldehyde dehydrogenase[13] and the response regulator AlgR.

[14] The cysts of Azotobacter are spherical and consist of the so-called "central body" – a reduced copy of vegetative cells with several vacuoles – and the "two-layer shell".

[18] The main constituents of the outer shell are alkylresorcinols composed of long aliphatic chains and aromatic rings.

When more favorable environmental conditions resume, which includes a certain value of pH, temperature, and source of carbon, the cysts germinate, and the newly formed vegetative cells multiply by a simple division.

Microscopically, the first manifestation of spore germination is the gradual decrease in light refractive by cysts, which is detected with phase contrast microscopy.

During germination, the central body grows and captures the granules of volutin, which are located in the intima (the innermost layer).

The synthesis of DNA and nitrogen fixation are initiated 5 hours after the addition of glucose to a nitrogen-free nutrient medium.

[24] While growing, Azotobacter produces flat, slimy, paste-like colonies with a diameter of 5–10 mm, which may form films in liquid nutrient media.

[25] Bacteria of the genus Azotobacter are also known to form intracellular inclusions of polyhydroxyalkanoates under certain environmental conditions (e.g. lack of elements such as phosphorus, nitrogen, or oxygen combined with an excessive supply of carbon sources).

This process occurs at high levels of metabolism during the fixation of nitrogen and is thought to protect the nitrogenase system from oxygen.

[33] Representatives of the genus Azotobacter are also found in aquatic habitats, including fresh water[34] and brackish marshes.

[43] An alternative type contains vanadium; it is independent of molybdenum ions[44][45][46] and is more active than the Mo-Fe nitrogenase at low temperatures.

Azotobacter also synthesizes some biologically active substances, including some phytohormones such as auxins,[52] thereby stimulating plant growth.

[55] Some kinds of Azotobacter can also biodegrade chlorine-containing aromatic compounds, such as 2,4,6-trichlorophenol, which was previously used as an insecticide, fungicide, and herbicide, but later was found to have mutagenic and carcinogenic effects.

[61] The genus Azotobacter was discovered in 1901 by Dutch microbiologist and botanist Martinus Beijerinck, who was one of the founders of environmental microbiology.