Beijerinck was first to isolate and cultivate an aerobic gram negative rod-shaped microorganism from the nodules of legume. He named it Bacillus radicicola, which was subsequently placed under the genus Rhizobium. Bacteria belonging to the genus Rhizobium live freely in soil and in the root region of both leguminous and non-leguminous plants. However they can enter into symbiosis only with leguminous plants by infecting their roots and forming nodules on them. Rhizobium present in these root nodules fixes atmospheric nitrogen i.e. gaseous nitrogen from air to organic nitrogen compounds, which is absorbed by plants. Thus role of Rhizobium is noteworthy for their major contributions to soil fertility. 

Yeast Mannitol Broth is used for the cultivation of the symbiotic nitrogen fixing organism’s viz. Rhizobium species . Yeast extract serves as a good source of readily available amino acids; contain vitamin B complex and accessory growth factors for Rhizobia. It also poises oxidation - reduction potential of medium in the range favorable for Rhizobia and serves as hydrogen donor in respiratory process. Mannitol is the fermentable sugar alcohol source. Calcium and magnesium provide cations essential for the growth of Rhizobia.