132 



GENERAL BACTERIOLOGY 



(using about 1 cc. of water) also one from fermentation tube and 

 kept at 38 C. Examine 24 hours later for acid colonies. 

 c . Pathogenic Bacteria. See Chapter X. 



REFERENCES. A. 579 ; H. 457 ; McF. 234 ; M. & R. 133 ; P. 245 ; 

 Prescott & Winslow, Elements of Water Bacteriology. For the 

 determination of the various species present see Frankland's Micro- 

 organisms of Water ; Fuller : Keport Am. Public Health Assoc., 1899, 

 580; Chester. 



SPECIAL DIRECTIONS. Analyze a surface water (lake or river) 

 and a deep well or a spring water. 



EXERCISE 69. ESTIMATION OF NUMBER OF BACTERIA IN SOIL. 



a. With a sterile knife collect a sample of soil in. a 

 sterile test-tube or Petri dish. Samples at various 

 depths can be secured by means of an earth borer. 

 (Fig. 28.) 



&. Weigh out 1 gram and dilute 1000 times with 

 sterile water. 



c. Make three gelatin plate cultures using 1 cc., 

 % cc. and T V cc. of this suspension. Incubate. 



d. Count the colonies as they develop and estimate 

 the number of bacteria per gram of soil. 



e. Many of the bacteria of the soil are anaerobic 

 and can only be grown in the absence of free oxygen. 

 See Part II. Chap VII. for methods of cultivation. 



REFERENCES. A. 609 ; H. 481 ; M. & R. 128 ; McF. FIG. 28. 

 240 ; Cn. 3. e1 ' 8 Soii 



EXERCISE 70. QUANTITATIVE ANALYSIS OF MILK. 



a. Obtain a sample of milk in a sterile vessel. 

 6. Dilute milk 1000 times with sterile water. 



c. Make plates as under soil (69). 



d. Count colonies and estimate number of bacteria per cc. 



EXERCISE 71. EFFICIENCY OF PASTEURIZATION. 



a. Place same milk as used in previous experiment in the bot- 

 tles of a pasteurizing apparatus, such as Freeman's, and pasteurize 

 as per printed directions. Or, place the milk in ordinary milk bot- 



