CLASSIFICATION

The classification is based on growth rate, which distiguishes the genus. It belongs to one of the three distinct genetic groups into which the family has been subdivided. It also includes the species R. meliloti and S. fredii . Based on DNA-DNA homology studies slightly different relationships are evident. R. leguminosarum was split into two biovars viciae and trifolii . Further studies showed that there was greater diversity among R. leguminosarum strains than any other bacterial species. The full taxonomic relationships have not yet been established. The GC content of the DNA is 57-65 mol%.

BIOCHEMISTRY

R. leguminosarum is not pigmented. Many strains grow at 12°C and are able to grow in the presence of 0.5% NaCl or 8% KNO3 and at pH8.0. Acid production from glucose is common but some produce alkali. Many strains can denitrify. All strains can grow on many mono and disaccharides. Many strains can grow on simple carbon compounds such as propionate, valerate, malonate, succinate, 2-ketoglutarate, fumarate, glutarate, DL-malate, DL-lactate, citrate, pyruvate, aconitate, D-glucuronate, a-D-galacturonate, cis -aconitate, erythritol, mannitol, sorbitol ribitol, many amino acids, methylamine, D-glucosamine, betaine, sarcosine and hippurate.

On the basis of antisera raised in rabbits different somatic 'O', flagellar 'H' and capsular 'K' antigens have been recognized. Both bacterial preparations from root-nodules as well as pure cultures can be used as antigens. Both agglutination and fluorescent antibody studies have been successfully used. When only small amounts of antigen are available such as from small nodules ELISA techniques have been used. There is a great deal of antigenic heterogeneity among strains of Rhizobium.

NORMAL FLORA

Found in soil.

Non-pathogenic.

LABORATORY ID

Following thorough cleaning of the root nodules, from soil particles, they can either be cultured directly or following drying (for storage or for transport to the laboratory). Crushed or cut (rehydrated if dried) nodules are cultured on yeast extract mannitol agar and incubated at 28°C and examined daily for typical rhizobial colonies.

Serological methods can also be used directly on nodules or to identify cultures.

Normally found in soil.

INDUSTRIAL USES

These organisms are important in agriculture as a means of providing nitrogen into the soil.

None developed.

SPECIAL FEATURES

Symbiotic Association:

The appropriate strain proliferates in the region of the root hairs of the host legume plant. Following a chemotactic response the bacterial cells are attracted to the root hair surface, whose growth they affect such as to deform them. The root hairs are then infected, an infection thread is formed and the vegetative cells are transformed to enlarged pleomorphic bacteroids which fix nitrogen.

Rhizobiaceae

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