280 BACTERIOLOGICAL EXAMINATIONS 



To Count Bacteria in Water 



The sample must be collected in a sterile bottle, and the plates 

 poured immediately, since bacteria multiply enormously after a 

 few hours. 



Take ^fo c.c. or J^j c.c. or i c.c. of the water in sterile pipettes 

 and mix with a tube of melted gelatine or agar, pour quickly into 

 cool sterile petri dishes and place in a cool dry place. The Ameri- 

 can Public Health Association also recommends the use of + i 

 percent agar plates grown both at room and body temperature. 

 The counts for the two are averaged. After forty-eight hours 

 count the colonies and the result (after multiplication where 3^0 

 or J^ c.c. of water was used) will be the number of bacteria per 

 cubic centimeter. It may be necessary to dilute the water five 

 or ten times before pouring plates. A glass plate ruled into 

 squares, known as a WolrThiigel plate, should be used for counting. 

 The number of bacteria in potable waters varies in many ways, 

 according to the amount of pollution, or albuminous matter in 

 the water, while depth, and the swiftness with which it flows are 

 conditions that modify bacterial contents. The water in a 

 reservoir becomes almost free from bacteria during the first ten 

 days. The number of bacteria diminishes- 10 percent per day 

 for the first five or eight days, due no doubt to gravitation of 

 the bacteria to the bottom, also in part to the action of light, 

 which plays an important role in the destruction of the bacteria 

 of water supplies. 



In general, water containing less than 100 bacteria per cubic 

 centimeter is considered to be from a deep source, and uncon- 

 taminated by drainage. Deep artesian wells often contain but 

 from 5 to 15 bacteria per cubic centimeter, water from rivers 

 often contain 12,000 or 20,000 depending somewhat upon the 

 season of the year. Rains cause an augmentation of the bac- 

 terial content. Summer causes a diminution. 



In identifying a certain water supply as the cause of an epi- 



