Determination of Bacteria in Water 239 



Ice always contains bacteria if the water contained them before 

 it was frozen. In Hudson River ice Prudden found an average of 

 398 colonies in a cubic centimeter. 



A sample of water when collected for examination should be 

 placed in a clean sterile bottle or in a hermetically sealed pre-ster- 

 ilized glass bulb, and must be examined as soon as possible, as the 

 bacteria multiply rapidly in water which is allowed to stand for a 

 short time. If the water to be examined must be transported any 

 considerable distance before the manipulations are performed, it 

 should be packed in ice. The greatest care must always be exercised 

 that the unnatural conditions arising from the bottling of the water, 

 the changes of temperature, and the altered relationship to light and 

 the atmosphere, do not modify the number of contained bacteria. 



1OO 



Fig. 85. Frost's plate counter, for counting colonies of bacteria on Petri 

 dish or plate cultures. The cross-lines divide the figure into square centimeters. 

 The numbers at the top of the figure indicate the area in centimeters of the various 

 discs. The area of each sector (a and b) is one-tenth of the whole area. 



The detection of such important bacteria as the colon, typhoid 

 and dysentery bacilli, and the cholera spirillum, will be considered 

 in the chapters treating of those respective organisms. 



Drinking-water, especially that furnished to large cities, is not 

 infrequently contaminated with sewage, and contains intestinal 

 bacteria Bacillus coli. For the ready determination of this organ- 

 ism, which is an important indication that the water is polluted, 

 Smith* has made use of the fermentation-tube in addition to the 

 * "Amer. Jour. Med. Sci.," 1895, ex, p. 301. 



