Yeast Mannitol Agar w/ Congo Red

Principle And Interpretation

Rhizobium can fix atmospheric nitrogen only in root nodules of legumes and that too when it is in the bacteroid stage of its life cycle. It possesses the entire complement of genes for nitrogen fixation, which are normally latent and become active only under special conditions. Rhizobium makes nitrogen available to the plant and in turn, the bacteria derive nutrients from the tissues of the plants (1). Yeast Mannitol Agar with Congo Red is used for the cultivation of Rhizobium species and for studying root nodulation (2).

Yeast extract serves as a good source of readily available amino acids, vitamin B complex and accessory growth factors for Rhizobia. It also poises the oxidation-reduction potential of medium in the range favourable for Rhizobia and serves as hydrogen donor in respiratory process (3). Mannitol is the fermentable sugar alcohol source. Magnesium provides cations essential for the growth of Rhizobia. Congo red inhibits penicillin-susceptible strains. Colonies of Rhizobia stand out as white, translucent, glistening and elevated, with entire margins (2).