Difference between Azotobacter and Azospirillum

Azotobacter and Azospirillum are two genera of bacteria that are important for nitrogen fixation. They are both gram-negative, free-living bacteria that promote plant growth. The chief difference between the two bacteria genera is that Azotobacter is an aerobic, soil-dwelling bacteria, whereas Azospirillum is microaerophilic and surface colonising bacteria.


Domain: Bacteria

Phylum: Proteobacteria

Class: Gammaproteobacteria

Order: Pseudomonadales

Family: Pseudomonadaceae

Genus: Azotobacter

Azotobacter is free-living, motile, spherical bacteria that form cysts. They are aerobic and play a large role in nitrogen fixation.

They are used as model organisms in the study of diazotrophs, and also for the production of food additives, biopolymers and some biofertilisers.

They are mostly found in neutral and alkaline soils, in association with plants. They are oval, 2-4 μm in diameter and can form clusters or chains of varying lengths. They are mobile due to the presence of numerous flagella. The cells of Azotobacter are resistant to environmental stresses because they secrete a thick mucus-like layer, forming a cyst.

Some of the species produce pigments that range from yellow-green to purple colours.


Kingdom: Bacteria

Phylum: Proteobacteria

Class: Alphaproteobacteria

Order: Rhodospirillales

Family: Azospirillaceae

Genus: Azospirillum

Azospirillum is a plant growth-promoting diazotroph. It is a free-living, gram-negative bacteria. They are oblong-rod shaped and do not produce spores. They are microaerophilic, i.e., they are aerobic but can survive under low oxygen conditions as well.

They are found in freshwater and in close association with plant roots in soil habitats. It is the plant growth-promoting bacteria (PGPB) and does so in the following ways:

  • They secrete plant hormones that make the root grow more branches and root hairs.
  • It makes nutrients such as nitrogen and phosphorus more available to the plants.
  • It also produces antioxidants that protect the plants in stressful conditions.
  • It competes with disease-causing microbes, making the plant less susceptible to diseases.

Azotobacter vs Azospirillum

Azotobacter is an aerobic, soil-dwelling bacteria. Azospirillum is microaerophilic and surface colonising bacteria.
Oxygen Requirement
They are aerobic bacteria and require high oxygen concentration for them to function. They are aerobic, but are also microaerophilic and can survive in low oxygen conditions.
Shape of Bacteria
Oval or spherical shape. Oblong-rod shaped.
Formation of Cysts
They form cysts to protect themselves from harsh environmental conditions. No formation of cysts.
It functions mainly as a nitrogen fixer, also useful in the production of biofertilisers and biopolymers. It is mainly useful as a plant growth-promoting bacteria.
Family of the Bacteria
Pseudomonadaceae Azospirillaceae
Temperature for Growth
20°C to 30°C 5°C to 42°C

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Frequently Asked Questions


What do you mean by diazotrophs?

Diazotrophs are species of bacteria and archaea that convert atmospheric nitrogen into a more usable form, such as ammonia.


What is the difference between Azotobacter and Rhizobium?

Azotobacter is a free-living bacteria that fix atmospheric nitrogen in the soil, whereas Rhizobium forms symbiotic relationships with roots of the plant for nitrogen fixation.


What are the similarities between Azotobacter and Azospirillum?

Following are the points of similarities between Azotobacter and Azospirillum:

  • Both are nitrogen-fixing bacteria.
  • Both are motile.
  • Both are gram-negative and free living.

What is the difference between Azotobacter and Acetobacter?

Azotobacter is a nitrogen-fixing bacteria. On the other hand, Acetobacter is an acetic acid bacteria that are able to convert sugars and alcohols to acetic acid.


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