''Azospirillum'' is a

Gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

microaerophilic A microaerophile is a microorganism that requires environments containing lower levels of dioxygen than that are present in the atmosphere (i.e. < 21% O₂; typically 2–10% O₂) for optimal growth. A more r ...

, non- fermentative and

nitrogen-fixing Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...

bacterial genus from the family of

Rhodospirillaceae The Rhodospirillaceae are a family of Pseudomonadota. The majority are purple bacteria, purple nonsulfur bacteria, producing energy through photosynthesis; originally all purple nonsulfur bacteria were included here.George M. Garrity, Don J. Bre ...

. ''Azospirillum'' bacteria can promote

plant growth Important structures in plant development are buds, Shoot (botany), shoots, roots, leaf, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. ...

Characteristics

The genus ''Azospirillum'' belongs in the

Alphaproteobacteria ''Alphaproteobacteria'' or ''α-proteobacteria'', also called ''α-Purple bacteria'' in earlier literature, is a class of bacteria in the phylum '' Pseudomonadota'' (formerly "Proteobacteria"). The '' Magnetococcales'' and '' Mariprofundales'' ar ...

class of bacteria. ''Azospirillum'' are

gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

, do not form spores, and have a slightly twisted oblong-rod shape. ''Azospirillum'' have at least one

flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

and sometimes multiple flagella, which they use to move rapidly. ''Azospirillum'' are aerobic, but many can also function as microaerobic

diazotroph Diazotrophs are organisms capable of nitrogen fixation, i.e. converting the relatively inert diatomic nitrogen (N2) in Earth's atmosphere into bioavailable compound forms such as ammonia. Diazotrophs are typically microorganisms such as bacteria ...

s, meaning, under low oxygen conditions, they can change inert nitrogen from the air into biologically usable forms. At least three species, ''A. melinis'', ''A. thiophilum'', and ''A. humicireducens'' are 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 led 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 Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

. This prefix is used to denote the ability of the bacteria to 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 (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignin, lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified Ti ...

s, the growth of the algae ''

Chlorella vulgaris ''Chlorella vulgaris'' is a species of green microalga in the division Chlorophyta. This unicellular alga was discovered in 1890 by Martinus Beijerinck, Martinus Willem Beijerinck as the first microalga with a well-defined nucleus. It is the typ ...

'' 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 (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of Organ (anat ...

s 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 such as

and

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

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 are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...

s 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 Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

. The plants' immune systems can also be primed by ''Azospirillum'' to resist attack by pathogens, a process known as induced systemic resistance.

Known species and genetic diversity

Azospirillum genus harbor over than 20 described species. Despite the remarkable plant growth-promotion properties, less than half of ''Azospirillum'' species have the genome sequenced: ''A. brasilense'', ''A. thiophilum'', ''A. lipoferum'', ''A. oryzae'', ''A. palustre'', ''A. doebereinerae'', ''A. halopraeferens'' and several undescribed ''Azospirillum'' sp. strains. When accessing a phylogenetic tree with all ''Azospirillum'' genomes, it is possible to identify two monophyletic groups, one harboring exclusively ''A. brasilense'' strains and another the remaining species. This strongly suggests a higher differentiation of ''A. brasilense'' from the remaining strains. The second clade also has very high diversity and not enough resolution to determine strains species only using genetic data.