122 LABORATORY BACTERIOLOGY 



EXERCISE LV 



THE QUANTITATIVE BACTERIOLOGIC EXAMINATION 

 OF WATER 



175. This is to determine the number of bacteria in water. 

 In preparing media for this purpose the directions given in the 

 Journal of the American Public' Health Association for Janu- 

 ary, 1898, p. 60, should be followed. The conditions of tem- 

 perature and of media which favor growth differ for different 

 species. Many bacteria found in water will not grow at the 

 incubator temperature, while others which may be in it grow 

 very slowly at the room temperature. To determine numbers 

 it is better to grow the bacteria in gelatin plates at the tem- 

 perature of the room. (In an actual examination a much 

 larger number of plate cultures than can be managed here 

 should be made.) 



In this exercise students may work in small groups. 



176. Work for this exercise. Make from the properly col- 

 lected water 4 gelatin plates, using a definite quantity of water 

 for each. To begin with, it may be safe to inoculate these 

 tubes with o.i, 0.25, 0.50, and i.oo cc., respectively. 



To determine if there are gas-producing bacteria, and the 

 approximate number of these if any, inoculate 10 fermentation 

 tubes with o.i cc. each and 5 with 0.2 cc. each of the water. 

 In place of the fermentation tubes glucose agar may be used. 

 In this case i fermentation tube of glucose bouillon should be 

 inoculated with 3 cc. of the water to determine the quantity 

 of gas produced, if there is any. If a large fermentation tube 

 is used, add 5 cc. of the water. From the gas produced in 

 these tubes determine approximately the number of the gas- 

 producing bacteria. 



Careful and full notes should be taken on this examina- 

 tion. The preliminary methods for making a bacteriologic 



