EXAMINATION OF WATER 175 



and on plain agar and gelatin. The Dunham's solution is tested for indol and 

 the morphology, motility and reaction with Gram's staining observed. If 

 necessary further differential cultures may be made. 



The above technique should be employed when examining a new or unknown 

 sample of water. Samples of water obtained from the same source at frequent 

 intervals, as in routine work, number of examinations showing that the bacteria 

 contained does not vary to any great extent, do not need to be planted in so 

 many different quantities. With water which is usually potable, it is sufficient 

 to plant just three different quantities in fermentation tubes 3 cc., 5 cc. and 

 10 cc. and three different quantities in plates i cc., i^ cc., and 2 cc. 



FIG. 35. WOLFHUGEL'S COLONY COUNTER. 



Water which is grossly contaminated with sewage is frequently as rich in 

 bacteria as dirty milk and hence must be diluted with sterile distilled water 

 before plating or making fermentation tubes. 



For reasons discussed on page 181 it is essential in making a numerical count 

 to obtain plates that show between 20 and 200 colonies. When making the 

 count all colonies that are visible when examined through a reading lens that 

 magnifies three diameters are to be counted. 



When water is suspected to contain the cholera spirillum, to 100 cc. of the 

 suspected water add i Gm. peptone (Witte's) and % Gm. sodium chloride, 

 shake to mix and incubate at 37C. in a tall, narrow container. After 6 hours 

 carefully remove several loopsf ul from the top and transplant as when examining 

 feces (pages 116 and 117). 



