RELATION OF AZOTOBACTER TO OXYGEN 65 



Exercise 30 



Effect of Variation in Culture-media on the Growth of 

 Azotobacter (Optional) 



1. Prepare eight tubes of agar slopes. 



2. Arrange the culture-media as follows: 



(a) i and 2, mannit agar. 



(b) 3 and 4, agar without mannit. 



(c) 5 and 6, agar with lactose in place of mannit. 



(d) 7 and 8, mannit agar without phosphate. 



3. Inoculate all cultures from a suspension of Azoto- 

 bacter. 



4. Incubate at 28 C. Examine every two days for 

 twelve days or longer. 



5. Record the growth of Azotobacter on the various 

 culture-media. 



Exercise 31 



Relation of Azotobacter to Oxygen 



1. Prepare six mannit agar slope cultures of Azotobacter 

 chroococcum. 



2. Treat as follows: 



(a) i and 2, untreated. 



(6) 3 and 4, place in an atmosphere free of oxygen (see page 133). 

 (c) 5 and 6, seal the tubes by melting the glass and drawing out the 

 ends. 



3. Incubate at room temperature. 



4. Examine weekly for growth and pigment formation. 

 If the sealed tubes fail to show a brown to black pigment 

 after two weeks, open and note the change in color after 

 three or four days. 



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