Il8 LABORATORY BACTERIOLOGY 



EXERCISE LXI 



THE QUANTITATIVE EXAMINATION OF WATER {Continued) 



174. Work for this Exercise Examine the cultures and 



count the colonies on the plates, and estimate from them the 

 number of bacteria in a cubic centimetre of the water; that 

 is, if there are 40 colonies on the plate containing ,1 c.c. of 

 water, there are 400 bacteria in i c.c. of it. 



From the cultures in the grape-sugar media estimate the 

 number of gas-producing bacteria present. 



Describe the appearance of the different colonies and indi- 

 cate the approximate number of each kind. 



Keep the plate cultures until the following exercise and re- 

 examine and count the colonies. 



Determine the obviously different genera of bacteria by 

 making a microscopic examination of the different colonies. 



175. Estimating the Number of Gas-producing Bacteria in 

 Water. — If there is gas in all of the 10 fermentation tubes in- 

 oculated with o.i c.c. each, it would show that there were 10 or 

 more of these bacteria in each cubic centimetre. If 3 of the 

 5 tubes inoculated with ^ c.c. each contained gas, it would 

 indicate that there were at least 3 gas-producing bacteria in i c.c. 

 The preliminary results must be verified by repeated examina- 

 tions. 



