88 



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. 



156. Estimating the number of gas producing bac- 

 teria in water. If there is gas in all of the ten fermenta- 

 tion tubes inoculated with i c. c. each, it would show that 

 there were one or more of these bacteria in each cubic centi- 

 meter. If 3 of the 5 tubes inoculated with ^ c. c. each con- 

 tained gas it would indicate that there were at least 3 gas 

 producing bacteria in one cubic centimeter. The preliminary 

 results must be verified by repeated examinations. 



EXERCISE LVIIL 



THE QUALITATIVE EXAMINATION OF WATER. 



157. Explanatory note. The qualitative examination 

 of water consists in determining the species of bacteria 

 present. From a sanitary standpoint it ' consists in finding, 

 if present, those species which may be the cause of disease 

 among people or animals consuming it. The pathogenic 

 bacteria which may be in the water will depend upon the 

 conditions, but usually in this country water is examined for 

 typhoid and hog cholera bacilli, B. coli communis and B. 

 pyocyaneus. 



In India the spirillum of Asiatic cholera may be found in 

 the water. Occasionally, anthrax may be suspected. It 

 should be stated that B. fluorescens liquefaciens , pseudo ty- 

 phoid and the transitional form of the colon group are to 

 be carefully differentiated from B. pyocyaneus and B. ty- 

 phosus. Owing to imperfect descriptions many of the com- 

 mon soil and water bacteria cannot be readily identified. 

 The genera of these is all that is expected here. 



158. General directions. Make at least four gelatin 

 plate cultures, and such others as may seem necessary to 



