234 Microbes and You 



common diseases we are trying to avoid, why don't we look for 

 these specific organisms when we analyze water bacteriologically? 

 The answer is that these pathogenic bacteria are relatively difficult 

 to isolate from water, even from badlv polluted water. The pro- 

 cedure available for the cultivation and identification of these 

 species are too involved and time-consuming to be very practical. 

 If a quart of water harbored one t)^phoid organism, it would be 

 extremely difficult to isolate that one cell. Yet, that apparently 

 low concentration of organisms in a water supply might well be 

 the cause of a serious typhoid outbreak. 



However, by using the index organism, Escherichia coli, we can 

 determine with a relatively high degree of accuracv whether a 

 water supply is polluted. Escherichia coli is found in extremely 

 high numbers in the intestinal wastes (feces) of warm-blooded 

 animals, and apparently in few other places. If we can detect this 

 organism in water, feces can be said to have found their way into the 

 supply, and such water is potentially dangerous. Since a small per- 

 centage of humans who have had typhoid fever remain for months 

 or years as healthy carriers of these organisms which they excrete 

 intermittently, there is always the potential possibility that the 

 Escherichia coli found in the water came from the intestines of 

 such an infected individual. Not all polluted water contains 

 dangerous pathogens, but in order to be ultra-safe, sanitarians 

 prefer to condemn all polluted water to protect the public from 

 possible infection from sewage originating from carriers. It is 

 a much wiser policy to condemn more water than necessary than 

 to allow one sample to be called safe when it may not be free 

 from pathogenic bacteria. 



Persons interested in the finer details of bacteriological water 

 testing are referred to Standard Methods for the Examination of 

 Water and Sewage prepared through the joint efforts of the 

 American Public Health Association and the American Water 

 Works Association. It is not the intent of this chapter to repeat 

 all of the laboratory procedures, but general comments on the 

 underlving principles will be discussed. 



The physical appearance of water is not a reliable index of the 



