NUMBER OF BACTERIA ON DIFFERENT CULTURE-MEDIA 21 



10. Reduce all results to number of bacteria in i gram of 

 soil. 



Exercise 4 



Comparison of the Number of Bacteria on Different 

 Culture-media 



1. Weigh out a representative sample of field soil, 

 about 20 grams, and transfer to a sterile 2oo-c.c. water 

 blank. 



2. Carry through dilutions as given in the previous 

 exercise. In the last dilution use 2 c.c. to 18 c.c. of water 

 instead of i c.c. to 9 c.c. 



3. At the same time the count is made weigh a sample 

 of soil and determine the moisture. 



4. Pour triplicate plates of the following media: Heyden- 

 Nahrstoff, sodium asparaginate, soil-extract, casein agars, 

 and soil-extract gelatin. After melting, the gelatin may 

 be cooled to 30 C. before pouring plates. 



5. Gelatin should be incubated at a constant temperature, 

 about 20 C., for one week. At regular intervals of two 

 days each remove the plates and count the number of 

 colonies, differentiating between the liquefiers and non- 

 liquefiers. In order to prevent a rapid liquefaction of the 

 gelatin, the peptonizing organisms may be killed by touch- 

 ing them with the point of a silver nitrate pencil. 



6. Compare the number of peptonizing colonies on casein 

 agar with the number of liquefiers on gelatin. 



Note. After counting the total number of colonies on casein agar, flood 

 the plates with N/2O lactic acid. After the medium turns white the acid 

 may be poured off. The lactic acid precipitates the casein, which produces 

 an opaque white medium except around the peptonizing colonies, where the 

 casein has been digested. These colonies may be distinguished by the 

 clear zone. 



