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''Azospirillum'' is a
Gram-negative Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), do ...
,
microaerophilic A microaerophile is a microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical struc ...
, non-
fermentative Fermentation is a metabolism, metabolic process that produces chemical changes in organic Substrate (chemistry), substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in ...
and
nitrogen-fixing Nitrogen fixation is a chemical process by which molecular nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific st ...
bacterial genus from the family of
Rhodospirillaceae The Rhodospirillaceae are a family of Proteobacteria Proteobacteria is a major phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the ...
. ''Azospirillum'' bacteria can promote
plant growth Important structures in plant development are buds In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialise ...
.


Characteristics

The genus ''Azospirillum'' belongs in the alpha-Proteobacteria class of bacteria. ''Azospirillum'' are
gram-negative Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), do ...
, do not form spores, and have a slightly-twisted oblong-rod shape.  ''Azospirillum'' have at least one
flagellum A flagellum (; ) is a hairlike appendage that protrudes from a wide range of microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and ...
and sometimes multiple flagella, which they use to move rapidly. ''Azospirillum'' are
aerobic Aerobic means "requiring Earth's atmosphere, air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Cellular respiration#Aerobic r ...
, but many can also function as microaerobic Diazotroph, diazotrophs, meaning, under low oxygen conditions, they can change inert nitrogen from the air into biologically useable forms. At least three species, ''A. melinis'', ''A. thiophilum'', and ''A. humicireducens'' are Facultative anaerobic organism, facultative anaerobes, and can live, if necessary, without oxygen. Growth of ''Azospirillum'' is possible between 5°C and 42°C and in substrates with a pH of 5 to 9, with optimal growth occurring around 30°C and 7 pH. Microbiologists use nitrogen-free semi-solid media to isolate ''Azospirillum'' from samples. The most commonly used media is called "NFb".


Discovery and reclassification

The first species described in the genus was originally named ''Spirillum lipoferum'' in 1925 by M.W. Beijerinck. In Brazil, during the 1970s, similar strains of this species were found associated with the roots of grain plants by scientists lead by Dr. Johanna Döbereiner. Her group discovered that these bacteria had the ability to fix nitrogen. Due to this discovery, ''Spirillum lipoferum'' was reclassified in 1978 as ''Azospirillum lipoferum'' by Jeffery Tarrand, Noel Krieg, and Döbereiner, who also added ''Azospirillum'' ''brasilense'' to the genus. By 2020, twenty-one species of ''Azospirillum'' had been described, most of which had been discovered after the year 2000.


Origin of name

The prefix “Azo-” comes from the French word “azote”, which means nitrogen.  This prefix is used to denote the ability of the bacteria to Nitrogen fixation, fix atmospheric nitrogen.  The ending “-spirillum” refers to the shape of the bacteria which is similar to spiral-shaped bacteria in the genus ''Spirillum.''


Ecological and agricultural significance

''Azospirillum'' are found in freshwater and soil habitats, especially in close relationships with plant roots. Associations with plants are thought to be largely beneficial. Over 113 species of plants in 35 different plant families have been documented to have benefited from association with a species of ''Azospirillum''. In addition to Vascular plant, vascular plants, the growth of the algae Chlorella vulgaris was positively affected by the presence of ''Azospirillum''. Since the 1970s, ''Azospirillum'' strains have been researched for their effects in improving agricultural yields and improving growth of wild plants. In 2009, the first commercial inoculants containing ''Azospirillum'' came on the market, and by 2018, over 3 million doses were applied annually to crops by farmers, mainly in South America.


Plant growth promotion

''Azospirillum'' promote plant growth through a variety of mechanisms. Many ''Azospirillum'' excrete Plant hormone, plant hormones that alter how the roots of plants grow. Affected roots frequently grow more branches and fine root hairs, which may help the plants acquire water and nutrients more efficiently. In addition to these changes, ''Azospirillum'' can also alter the forms of Plant nutrients in soil, plant nutrients such as nitrogen and phosphorus to make them more available to plants. However, how much nitrogen Azospirillum contribute to crop plants via biological fixation is debated. ''Azospirillum'' also make Antioxidant, antioxidants that protect the plant roots from stresses due to drought and flooding. Plant growth can also be promoted indirectly by ''Azospirillum'' reducing plant disease. ''Azospirillum'' competes with pathogens on the roots for space and for trace nutrients such as iron. The plants' immune systems can also be primed by ''Azospirillum'' to Plant disease resistance, resist attack by pathogens, a process known as induced systemic resistance.


Known species


''References''


Further reading

* * * * * {{Taxonbar, from=Q794056 Rhodospirillales Bacteria genera