302 



APPLIED BACTERIOLOGY 



found by a direct gelatine plate culture, which is made on 

 the water immediately upon the receipt of the sample. If 

 a sample of water is polluted with sewage, a great increase 

 in the number of the organisms will be found to have taken 

 place as the result of the incubation. All the organisms 

 normally present in faeces grow and multiply vigorously 

 at blood-heat, whereas this temperature is fatal to the 

 majority of the common water bacteria ; therefore a cor- 

 responding decrease in the number of the organisms will 

 be found to have taken place in a pure water. 



A more convenient plan is to prepare an agar-agar plate 

 culture with a fraction of a c.c. of the water. The result- 

 ing plate is incubated at blood-heat for thirty-six hours. 

 This method is the most satisfactory, as it has the advan- 

 tage that the actual number of the micro-organisms that 

 will grow at blood-heat is ascertained. 



The following examples show the value of these two 

 methods : 



