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Enrichment of Azotobacter, a N2 Fixing Bacterium

Background & Introduction

Azotobacter is one of the soil organisms capable of fixing nitrogren. Nitrogen fixers are important in the nitrogen cycle and help convert atmospheric nitrogen into an organic form that is then usable by living creatures like animals and humans. 

diagram of the nitrogen cycle

In this lab, we will attempt to isolate Azotobacter from a soil sample by using an enrichment medium that lacks a nitrogen source. The absence of nitrogen will favor the growth of organisms that can fix atmospheric nitrogen. Mannitol is included in the medium as an energy source and an ion of molybdenum that is essential for nitrogen fixation. This metallic ion appears to serve as an essential ion cofactor for the enzyme nitrogenase, which catalyzes the reduction of nitrogen to ammonia.



N2 fixation 

  1. Collect a bottle of soil culture (nitrogen-free medium).

  2. Using an inoculating loop collect a sample of the culture and streak for isolation onto a nitrogen-free agar plate.


azotobacter colonies on a nitrogen free mediumazotobacter colonies on a nitrogen free medium

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