CHAPTER IV. 



DETERMINATION OF THE NUMBER OF ORGANISMS 

 DEVELOPING AT THE BODY TEMPERATURE. 



THE count of colonies upon the gelatin plate measures 

 as we have pointed out, the number of those micro-organ- 

 isms associated with the decomposition of organic matter 

 wherever it may occur. In this great class, however, 

 there are a few species, like B. subtilis and B. ramosus, 

 which will grow under a great variety of conditions and 

 which are likely to be present with more or less constancy 

 in water, and others which through a semi-parasitic 

 mode of life have become specially adapted to the pecu- 

 liar conditions characteristic of the animal body. The 

 latter in particular possess the property of developing 

 most actively at the temperature of the human organism, 

 37 C., which altogether checks the growth of the majority 

 of normal earth and water forms. The determination of 

 the number of organisms growing at the body temperature 

 may throw light, then, on the presence of direct sewage 

 pollution, since the bacteria from the alimentary canal 

 flourish under such conditions, while most of those derived 

 from other sources do not. The count at 37 helps to 

 distinguish contamination by wash of the soil of a virgin 

 woodland from pollution by excreta, since in the latter 



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