Rhizobia

(nodule bacteria), bacteria of the genus Rhizobium, which form small nodules on the roots of certain legumes, symbiotically fix molecular nitrogen, and produce in the process a number of physiologically active substances with beneficial effects on the legumes.

Rhizobia play an important part in enriching soils with nitrogen. The young bacteria, which measure 0.5–0.9 X 1.2–3 microns, are nonsporiferous, motile, gram-negative, and aerobic. Entering a root hair, they bring about the active division of the root cells. This leads to the appearance of a small nodule. The rhizobia grow in the nodule, turning into thickened and branched forms called bacteroids. These actively fix molecular nitrogen. Rhizobia also assimilate ammonium salts, nitrates, and amino acids. Their sources of carbon can be monosaccharides, disaccharides, certain polysaccharides, organic acids, and alcohols. On smooth culture media, rhizobia form round, colorless, transparent, mucoid colonies, which grow well at 25°C. The nodules formed by active rhizobia contain the pigment leghemoglobin, which gives them a rosy color.

Different rhizobia cause the dvelopment of nodules in specificlegumes; Rhizobium meliloti in alfalfa and melilot; Rh. leguminosarum in vetch, peas, and fodder beans; Rh. trifolii inclover, and Rh. japonicum in soy. When the nodules are destroyed the rhizobia survive in the soil as saprophytes. The seeds of legumes are infected with rhizobia in order to promote nodule formation; this is an aspect of what is known as bacterial fertilization.