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Exercise 20. Effect of Variation in Food on the Growth 
of Azotobacter 
MATERIALS: 
2 tubes of nitrogen-free agar 
2 tubes of same agar without sucrose 
2 tubes of the same agar with lactose in place of sucrose 
2 tubes of the same agar without phosphate 
1 9 c. c. sterile water blank 
Pure culture of Azotobacter 
1. Prepare a heavy suspension of organisms from the pure culture by trans¬ 
ferring them to the sterile water. Shake well until they are evenly distributed. 
2. After carefully labeling the agar slopes, inoculate each one by making a 
single streak along the surface of the agar with a loopful of the suspension. 
3. Incubate the cultures for a period of two weeks. 
4. Keep careful watch of the cultures and record the extent and color of the 
growth on the various media. 
(a) What food supply is best for Azotobacter ? 
( b) Why is some soluble carbohydrate so important? 
(c) Which one of the compounds used appears to be the most 
essential for Azotobacter ? 
( d ) What are the essential elements for most bacteria? 
