BACTERIA IN WATER 



1021 



"Procedure. Shake at least twenty-five times the bottle which contains 

 the sample. Withdraw 1 c.c. of the sample with a sterilized pipette and 

 deliver it into a sterilized Petri dish, 10 cm. in diameter. If there be reason 

 to suspect that the number of bacteria is more than 200 per c.c., mix 1 c.c. 

 of the sample with 9 c.c. of sterilized tap or distilled water. Shake twenty-five 

 times and measure 1 c.c. of the diluted sample into a Petri dish. If a higher 

 dilution be required proceed in the same manner, e.g., 1 c.c. of the sample to 

 99 c.c. of sterilized water, or 1 c.c. of the once diluted sample to 9 c.c. of 

 sterilized water, and so on. In the case of an unknown water or a sewage 

 it is advisable to use several dilutions for the same sample. To the liquid 

 in the Petri dish add 10 c.c. of standard agar at a temperature of about 

 40 C. Mix the medium and water thoroughly by tipping the dish back and 

 forth, and spread the contents uniformly over the bottom of the plate. Allow 

 the agar to cool rapidly on a horizontal surface and transfer to the 37 C. 

 incubator as soon as it is hard. Incubate the culture for twenty-four hours 

 at a temperature of 37 C. in a dark, well-ventilated incubator where the 

 atmosphere is practically saturated with moisture. 8 After the period of 

 incubation place the Petri dish on a glass plate suitably ruled and count 

 the colonies with the aid of a lens which magnifies at least five diameters. 

 So far as practicable the number of colonies upon the plate shall not be 

 allowed to exceed 200. The whole number of colonies upon the plate shall 

 be counted, the practice of counting a fractional part being resorted to only 

 in case of necessity. 



"It will be found advantageous to use Petri dishes with porous earthen- 

 ware covers in order to avoid the spreading of colonies by the water of 

 condensation. 9 



"Expression of Results. In order to avoid fictitious accuracy and yet to 

 express the numerical results by a method consistent with the precision of the 

 work the rules given below shall be followed : 



''Numbers of Bacteria per c.c. 



From 



1 to 



50 Recorded as found 



51 " 100 



101 " 250 



251 " 500 



501 " 1,000 



1,001 " 10,000 



10,001 " 50,000 



50,001 " 100,000 



100,001 " 500,000 



500,001 " 1,000,000 



1,000,001 " 10,000,000 



to the nearest 



5 

 10 

 25 



50 



100 



500 



1,000 



10,000 



50,000 



100,000" 



Qualitative Bacterial Analyses of Water. Of far greater im- 

 portance than quantitative analysis is the isolation of bacteria either 



8 Whipple, Tech. Quar., 1899, 12, p. 276. 



9 Hill, Jour. Med. Res., 1904, N.-S., 8, p. 93. 



