EXAMINATION OF WATER 1 25 



Keep the plate cultures until the following exercise and 

 reexamine and count the colonies. 



Determine the obviously different genera of bacteria by 

 making a microscopic examination of the different colonies. 



180. Estimating the number of gas-producing bacteria in 

 water. If there is gas in all of the 10 fermentation tubes 

 inoculated with o.i cc. each, it would show that there were 10 

 or more of these bacteria in each cubic centimeter. If 3 of 

 the 5 tubes inoculated with 0.2 cc. each contained gas, it would 

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

 each cubic centimeter. The preliminary results must be veri- 

 fied by repeated examinations. 



