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Cultural Characteristics of Rhizobium
Rhizobium can live on relatively simple synthetic medium but it is incapable of fixing nitrogen. It can fix nitrogen only in the root nodules of leguminous plants (Fig. 43.2). Fast growing Rhizobium like R. trifolii, R. leguminosarum can utilise all sources of carbohydrates but slow growing Rhizobium are specific in nutrients and they need simple sugars like xylose, mannitol, arabinose for their growth. Rhizobium cells are small to medium sized gram negative bacilli. They are non-spore formers. They are motile when young. Cells contain characteristic granules of B-hydroxybutyrate granules which stain with sudan black and appear as highly refractile bodies. Most strains produce gum (extracellular polysaccharide slime).