g6 FRESH-WATER BIOLOGY 



constantly changing, and varies from time to time not only in the 

 number, but in the nature of the individuals composing it (Fig. 31). 

 Little work has yet been done upon the changes in the kinds of 

 bacteria in river or lake water due to the shifting seasons and other 



factors, but there is no doubt 

 that important differences do 

 exist. Many varieties of bac- 

 teria have been isolated from 

 water. During the course of 

 a study of the bacteria in the 

 water of the IlHnois River 

 the writer found that out of 

 543 cultures, 17 well-defined 

 groups and 41 subgroups were 

 represented. These groups 

 include a number of pigment- 

 , . . producing or chromogenic 



Fig. 31. — Photograph of "plate culture," showing ^ ° r i • i 



different kinds of bacterial colonies. (Original.) formS, SOmC of whlCh arC 



among the most common inhabitants of water, and also a number 

 of bacterid closely related to organisms associated with the 

 production of disease in the higher animals. Among the bacteria 

 commonly found in natural waters may be mentioned B. fluo- 

 rescens vars. liqiiejaciens and non-liqiiefaciens (the green water 

 bacillus), B. suUilis (the hay bacillus), B. mesentericus (the potato 

 bacillus), B. proteus and B. cloacae (commonly associated with the 

 decomposition of vegetable and animal matter), B. liquefaciens, 

 B. hyalinus, B. violaceiis, and many chromogenic and non-chromo- 

 genic micrococci; in polluted waters, B. coli is usually found in 

 large numbers and organisms of the B. proteus t^^De and strepto- 

 cocci are more abundant than in normal waters. 



It is well known that the germs of several of the principal infec- 

 tious diseases of man are commonly conveyed in drinking water. 

 Typhoid fever and Asiatic cholera are familiar examples. Both 

 the typhoid bacillus and the cholera spirillum have been found in 

 water, although, partly because the technical difficulties of investi- 

 gation are considerable, partly because the longevity of these 

 organisms in water is Umited, positive findings have not been very 



