434 AGEICULTUEAL AND INDUSTRIAL BACTEEIOLOGY 



water containing large numbers of bacteria, therefore, is 

 suspicious and should not be used until investigation has 

 shown that the bacteria present are not derived from any 

 source which might prove harmful or injurious. 



Many attempts have been made to formulate a standard 

 for potability of water, using the number of bacteria pres- 

 ent per cubic centimeter as an index. These have in general 

 proved impracticable. However, it may be said that water 

 containing less than a hundred bacteria per cubic centi- 

 meter is usually, although not always, potable, and when 

 bacteria are present in numbers greater than a thousand 

 per cubic centimeter it usually indicates a water which is 

 not suitable for drinking, certainly a water which should 

 be investigated carefully before it is used. 



Inasmuch as the bacteria which transmit disease through 

 water are organisms present in the alimentary tract, it is 

 evident that if organisms can be identified which are char- 

 acteristic of sewage, that is, of human and animal feces, 

 their presence in water will prove the presence of sewage in 

 the water and the unsuitability of the water for drinking 

 purposes, providing, of course, it can be shown that these 

 same organisms do not exist in nature. A large amount 

 of work has been done in recent years on this problem. 

 Obviously, if water upon examination is shown to contain 

 a particular disease microorganism such as the typhoid, it 

 is dangerous. It is evident, however, that any water con- 

 stantly contaminated by sewage will sooner or later contain 

 typhoid bacilli or that it at least is open to pollution from 

 the excretions of typhoid patients. While not impossible, it 

 is very difficult to isolate typhoid and similar bacteria from 

 water in which they are present because they are generally 

 found in numbers much smaller than other intestinal forms. 

 At the present day attempts are therefore rarely made to 

 isolate such organisms from infected water supplies. Atten- 



