These are Gram-negative, non-sporing non-acid fast rods, which may be pleomorphic. They are generally motile with one polar or subpolar flagellum or two to six peritrichous flagellae. They chemoorganotrophs and able to utilise many carbohydrates. The tricarboxylic acid cycle and and the Entner-Doudoroff pathway are the main metabolic pathways, while the Embden-Meyerhof pathway plays only a smaller rôle. The optimum temperature for growth of most isolates is around 20°C and some can grow at temperatures as low as 12°C or as high as 44°C. Many are able to fix nitrogen, which is done in symbiotic association with with leguminous plants. The family is basically subdivided into two groups, comprising fast and slow growing strains respectively. The fast growing genus is Rhizobium and the slow growing genus is Bradyrhizobium. Two further genera have recently been described: Azorhizobium and Sinorhizobium.

Genera

Rhizobium: There are currently four species recognized within this genus. They are fast growing with a generation time <6h. They can use pentoses, hexoses as well as mono-, di- and trisaccharides. They have peritrichous flagellae. The symbiotic gene is on a plasmid and the chromosome. The nitrogen-fixing genes H,D and K are on the same operon. They have a high intrinsic antibiotic resistance.

Bradyrhizobium: There is currently only one species recognized within this genus. It is slow growing with a generation time >6h. It can use pentoses and hexoses only, not di- and trisaccharides. It has subpolar flagellae. The symbiotic gene is only on the chromosome. The nitrogen-fixing genes H,D and K are on the separate operons. They have a low intrinsic antibiotic resistance.

Azorhizobium:

Sinorhizobium:

Species Profiles

R. leguminosarum

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