Question

What conditions would limit other bacteria and enrich for the growth of the genus Rhizobium? Ammonia is produced during nitrogen fixation. If the bacteria fix nitrogen, why would you want to add nitrogen in the form of yeast extract? Please answer in a way that is easily understandable. Thank you!

Answer 1

As you know Rhizobium, a gram negative rod shaped bacteria. It fixed atmospheric nitrogen only when it form symbiosis relationships with leguminous plant. But when we want to grow in our laboratory it cannot fixed atmospheric nitrogen. Generally, Yeast Extract Mannitol Agar (YEMA) used for cultivation of Rhizobium. YEMA contain Mannitol as source of carbohydrates and Yeat extract as a source of nitrogen. Without source of Yeast extract or any other nitrogen source it is not possible to grow in lab. Rhizobium loss it's atmospheric nitrogen fixings ability due to absence of it's partners and presence of oxygen in media.Nitrogenease enzymes perform only microaerophilic conditions.

After incubation Rhizobium appears as colorless colony on YEMA plate. Hope you understand.