THE DECOMPOSITION OF MILK 141 



How many of the largest protozoa present, placed end to 

 end, would make an inch? 



10. How do the protozoa and bacteria in the drop 

 compare in numbers? Do these organisms have any rela- 

 tion to each other? If so, explain. 



11. Of what importance are protozoa? Name several 

 well-known protozoa. 



12. State your results in detail and point out any con- 

 clusions that may be drawn and any practical applications 

 that may be made. 



REFERENCE 



MARSHALL: Microbiology, pp. 10-11, 68-80, 82-84. 



EXERCISE 48. TO STUDY THE NATURAL DECOMPO- 

 SITION OF MILK 



Apparatus. 500 c.c. sterile Erlenmeyer flask; two 

 5 c.c. sterile pipettes; ten 1 c.c. sterile pipettes; four 

 10 c.c. sterile pipettes; six 200 c.c. sterile Erlenmeyer 

 flasks; fifteen sterile Petri dishes; physiological salt solu- 

 tion. 



Cultures. Fresh skim milk. 



Method. 1. Prepare " dilution flasks " as given in 

 Exercise 13, p. 52, making two 90 c.c. and four 99 c.c. 

 flasks. Sterilize by heating one hour in flowing steam or 

 five minutes in the autoclav at 120 C. (15 Ibs. pressure). 

 Dilution flasks and all glassware must be sterile before the 

 experiment proper can be started. 



2. Place 200 c.c. of the fresh skim milk in the sterile 

 500 c.c. flask and use this sample for the entire experi- 

 ment. 



3. Plate the milk immediately on nutrient agar, using 

 dilutions according to the age of the milk, as follows. 

 (See Exercise 13, p. 52, for method of using dilution 

 flasks.) 



