Morphology[edit]
Cells of the genus Azotobacter are relatively large for bacteria (1–2 micrometres in diameter). They are usually oval, but may take various forms from rods to spheres. In microscopic preparations, the cells can be dispersed or form irregular clusters or occasionally chains of varying lengths. In fresh cultures, cells are mobile due to the numerous flagella.[5] Later, the cells lose their mobility, become almost spherical and produce a thick layer of mucus, forming the cell capsule. The shape of the cell is affected by the amino acid glycine which is present in the nutrient medium peptone.[6]
Under microscope, the cells show inclusions, some of which are colored. In the early 1900s, the colored inclusions were regarded as "reproductive grains", or gonidia – a kind of embryo cells.[7] However, it was later demonstrated that the granules do not participate in the cell division.[8] The colored grains are composed of volutin whereas the colorless inclusions are drops of fat, which act as energy reserves.[9]