CLASSES OF BACTERIA 345 



numbers in sterilized water from a polluted well containing con- 

 siderable organic matter and kept in the dark at 20 degrees, while 

 in purer water or in the light they died out in from two to six 

 weeks. In unsterilized water the results may be just the opposite, 

 for in the presence of an abundant supply the saprophytes may 

 multiply at the expense of the pathogens. 



Whipple and Mayer find that the presence of oxygen is essential 

 to the existence of typhoid and colon bacilli in water, and even 

 small quantities of acid and alkali are fatal. It is for this reason 

 that we find few organisms in acid and alkali water of various 

 regions. The factors, therefore, which are at work on the puri- 

 fication of water are numerous, and "although it is hard to estimate 

 the exact importance of each factor, the general phenomena of the 

 self-purification of streams are easy to comprehend. A small 

 brook, immediately after the entrance of polluting material from 

 the surface of the ground, contains many bacteria from a diversity 

 of sources. 



"Gradually those organisms adapted to life in the earth or in the 

 bodies of plants and animals die out, and the forms for which water 

 furnishes ideal conditions survive and multiply. It is no single 

 agent which brings this about, but that complexity of little-under- 

 stood conditions which we call the environment." 



Classes of Bacteria. The bacteria found in water may be roughly 

 classed as: (1) Natural-water bacteria, (2) soil bacteria and (3) 

 sewage or intestinal bacteria. There is no hard and fast line 

 between these classes, for organisms belonging to the water flora 

 are found in the soil and water draining from manured soil will 

 contain intestinal organisms. The classification, however, is valu- 

 able; for the first two groups usually contain the saprophytes, 

 whereas the third contains the pathogens. 



A number of attempts have been made to classify water bacteria. 

 Ward, in his study of the bacterial flora of the Thames River, 

 arranged them into twenty-one groups. But the work is beset 

 with certain difficulties which were recognized by Ward, for he 

 made the following statement: "My work goes to show that 

 species cannot be made out, but that the limits of the species are, 

 in most cases, far wider than is assumed in descriptions in- other 

 words, that many so-called species in books are merely variation 

 forms, whose characters, as given, are not constant but depend on 

 treatment. How far this is true for any given case will have to 

 be tested on the particular form in question." 



Fuller and Johnson, from a study of the bacteria in the rivers 

 of America, suggested a classification containing thirteen groups. 

 Their system was based mainly on morphological data, and hence 

 they experienced considerable difficulty in differentiating short 

 bacilli from cocci. 



