THE EXAMINATION OF WATER 651 



]\Ielt the gelatin in three tubes in a water bath or incubator at 

 blood heat. 



Inoculate three sterile Petri dishes with 0*5, o'3, 0*2 c.c. of water 

 well shaken. 



Pour the contents of a gelatin tube into each Petri dish. 



]\Iove quietly to mix the water and gelatin. 



Allow to solidify on a flat surface. 



Count the colonies at the end of seventy-two hours, but inspect 

 daily. 



1,000 organisms can be counted on a 10 cm. dish, but 200 colonies 

 should be the maximum. 



In pure water less than 10 per cent, will liquefy gelatin. 



In the purest upland streams i c.c. contains often under 100 

 bacteria, while in town sewage the same quantity contains millions. 



In deep well water over 100 organisms per c.c. is conclusive 

 evidence of contamination. 500 per c.c. and above is suspicious water, 

 1,000 or more means condemnation of the water for drinking purposes. 



(b) To enumerate bacteria grown at blood heat. 



Inoculate two sterile Petri dishes with i and o'l c.c. of the sample. 

 Melt two tubes of nutrient agar and allow to cool to 45° or 40° C. 

 and pour into the dishes. Mix quickly and place to cool. 

 Count them at the end of forty-eight hours. 



(c) To enumerate Bacillus coli present. The most important datum 

 of bacterial examination. 



The sample should be 50 c.c. of shallow well and surface water. 



The sample should be 100 c.c. of deep well water. 



Add a medium that contains a substance fermented by the Bacillus 

 coli, and also a substance to inhibit the growth of other water 

 organisms as Pake's method, glucose-formate broth. 



Savage's method, neutral red glucose broth. 



MacConkey's method, bile-salt broth, generally used. 



In this is lactose, the Bacillus coli ferments giving acid gas. 



Sodium taurocholate... ... ... ... o"5 per cent. 



Wittes' peptone ... ... ... ... 2*0 „ 



Lactose ... ... ... ... ... ro „ 



Agar 2-0 „ 



Made with tap water and i per cent, solution of neutral red added. 

 A double strength is also made. 



Bacillus coli gives bright red colonies, and Bacillus typhosus white 

 colonies without fermentation. 



Inoculate four tubes containing 5, 10, 15, 20 c.c. of this double 

 strength medium with 5, 10, 15, 20 c.c. of the water sample. 



Incubate at 42° C. for twenty-four to forty-eight hours. 



