76 BACTERIOLOGY FOR NURSES 



means the absence of pathogenic bacteria except in such extremely 

 rare cases as contamination by urine. The presence of the colon 

 bacillus does not necessarily signify danger, but it does mean pollu- 

 tion with fecal discharges. Deep well water should be condemned 

 if any colon bacilli are found in it. On the other hand, surface 

 water may contain one colon bacillus per c.c. without the presence 

 of pathogenic organisms being suspected, particularly if it is known 

 to drain an inhabited area. The fact that the colon bacillus is 

 found in the feces of animals makes it difficult to determine whether 

 pollution is of animal or human origin. A fresh hillside stream 

 may contain colon bacilli brought to it by rain washings from 

 manured fields through which it passes or by a stray horse or cow. 

 Any water, however, containing ten colon bacilli per c.c. is usually 

 considered as decidedly polluted and unsafe for human consump- 

 tion. 



Generally speaking, no single test should be relied on alone, al- 

 though the colon test surpasses all others in delicacy in determin- 

 ing pollution. An inspection of the surroundings, a chemical and 

 bacteriological examination, are all necessary for complete infor- 

 mation. 



Bacteriological Analysis. In the bacteriological examination 

 of water two lines of inquiry are generally followed. First, the 

 approximate number of bacteria per c.c. is estimated, and second, 

 the presence or absence of the colon bacillus is determined, and 

 if present in what number per c.c. it occurs. 



As in the case of soil, so also in a given sample of water, it is 

 impossible to determine the exact number of living organisms pres- 

 ent. This fact, however, does not prevent a numerical estimation 

 from being of value. An approximate idea of the number present 

 can be obtained, and also certain inferences of importance can 

 be drawn by comparing the results obtained by different methods. 

 For example, if a moderate number of bacteria develop at 20 C. 

 and very few at 37 C. the water may, from a sanitary standpoint, 

 be comparatively pure. If, however, the condition is reversed and 

 many colonies appear upon agar plates incubated at 37 C. and 

 few at 20 C., then the majority of the organisms present may be 



