40 ELEMENTS OF WATER BACTERIOLOGY 



off from 535,000 to 54,500; from 412,000 to 50,500, 

 and from 329,000 to 73,000, respectively. It is, there- 

 fore, important that even iced samples should not be 

 kept too long; and it is desirable to adhere strictly 

 to the recommendations of the Standard Methods 

 Committee that the interval between sampling and 

 examination should not exceed 12 hours in the case of 

 relatively pure waters, 6 hours in the case of relatively 

 impure waters, and i hour in the case of sewage. 



Plating. The bottle containing the sample of water 

 is first shaken at least twenty-five times in order to 

 get an equal distribution of the bacteria. If the num- 

 ber of bacteria present is probably not greater than 200, 

 i c.c. is then withdrawn with a sterile i c.c. pipette 

 and delivered into a sterile Petri dish of 10 cm. diameter. 

 To this is added 5 c.c. of standard 10 per cent gelatin 

 at a temperature of about 30 C., or standard agar 

 (7 c.c.) at 40-42 C. Should the number of bacteria 

 per c.c. probably exceed 200, dilution is necessary. 

 This is best accomplished by adding i c.c. of the water 

 in question to 9, 99 or 999, etc., c.c. of sterile tap water 

 according to the amount of dilution required. The 

 diluted sample is then shaken thoroughly and i c.c. 

 taken for enumeration. In order to determine the 

 number of bacteria originally present it is only neces- 

 sary to multiply by the factor 10, 100, or 1000, etc. 



When a sample of water from an unknown source 

 is to be examined it is generally desirable to make 

 two check plates at each of the above dilutions, select- 

 ing those which give nearest to 200 colonies on the 

 plates after incubation as the ones on which to rely 



