254 



GENERAL MICROBIOLOGY 



cells of Azotobacter. The former are almost always a type 

 resembling the nodule bacteria, B. radiobacter, and which 

 can also fix nitrogen to a slight extent. Besides these, 

 especially when Azotobacter is less in evidence, many other 

 sporing and non-sporing bacteria participate in the process. 

 Azotobacter, however, is the most vigorous free nitrogen- 

 fixing organism yet discovered. 

 (See reference, E. B. Fred, 

 Exercise 9, Soil Microbiology.) 



5. What characteristic odor 

 is produced in these cultures? 

 Add a drop of concentrated 

 H2SO4 to a small portion of 

 the culture liquid. This in- 

 tensifies the odor. 



6. When a brownish surface 

 film develops, make plates from 

 this culture, using relatively 

 high dilutions. 



7. After a rather long period 

 of incubation (six to seven 

 days) examine the organisms in 

 the various colonies and isolate 

 Azotobacter chroococcum if possi- 

 ble upon a mannit agar slant. 

 On account of the slimy prop- 

 erty of its cell wall, its separa- 

 tion from B. radiobacter is often very difficult. The quickest 

 way is to reinoculate first into the mannit solution. 



8. Save several of the plates having well-isolated colo- 

 nies and note any changes which may occur. 



9. If any brown colonies develop, examine them in 

 stained preparations. Measure the bacteria stained. 



10. Are these pure cultures? If not, plate from several 

 such colonies in mannit agar to isolate the different organ- 

 isms present. 



FIG. 55. Azotobacter. xlOOO; 

 Smear from Six-day Old Cul- 

 ture on Ashby's Agar at 

 25 C. Showing organisms 

 and capsules in various 

 stages of development. (Dan 

 H. Jones.) 



