Normal and Not Normal Bacteria of Water 63 



which, though frequently found in drinking-water, 

 must be regarded as strangers there. They are 

 not normal inhabitants of water. This class of bacteria 

 may again be divided into two groups. The first includes 

 all the harmless kinds that could produce no illness, even 

 if swallowed in very large numbers. The second is com- 

 posed of disease bacteria proper, such as cholera, typhoid, 

 and dysentery. Since many of the bacteria are not nor- 

 mal inhabitants of water, and since they tend to die out 

 when introduced there, it follows that their presence in 

 large numbers is a certain sign of recent pollution. 



The number of bacteria in water. The number of 

 bacteria in water is affected by its composition, by the 

 amount and frequency of rainfall and the resulting 

 drainage, by the contamination with sewage, by the 

 season and climate, the amount and intensity of sun- 

 shine, the depth of the water, the velocity of the cur- 

 rent, and still other factors. Because of the natural 

 variations thus introduced, the number of bacteria in 

 fairly pure river-water may range from a few hundreds, 

 or even less, per cubic centimeter to several thousands. 



The differences in the bacterial content of water 

 between the winter and summer months are largely 

 attributed to the greater amount of surface drainage 

 in the fall and winter, whereby enormous numbers of 

 bacteria are carried to the streams from the forests, 

 fields, and city streets. In the summer months, on the 

 other hand, much of the lighter rainfall is taken up 

 by the growing vegetation, and a larger portion of it 

 is evaporated from the^oil, thus decreasing the amount 

 of surface drainage. The streams must then depend 



