ISOLATION OF INDICATOR ORGANISMS 21 



invert. Incubate for 20 24 hours at 37 C. before the plates 

 are examined. 



At least three characteristic colonies should be subcultivated 

 even if they all appear alike. Only one if typical need be 

 worked out, but the others should be subcultivated, so that if the 

 one selected is atypical in any of its characters, these can in 

 turn be fully examined. 



The third stage is the application of determining tests. The 

 value of the different tests has already been considered and 

 those advocated for employment are mentioned under the 

 respective substances to be examined. 



Streptococci. 



In the determination and enumeration of streptococci many 

 of the principles involved are identical with those concerned in 

 the B. coli enumeration and explained above so that a shorter 

 consideration is sufficient. As with other indicator organisms a 

 numerical estimation is required, not merely their detection. 



There are no known chemical agencies which will enable, 

 by macroscopic appearances alone, tubes of liquid media contain- 

 ing streptococci and other organisms to be differentiated from 

 those which do not contain streptococci. For their preliminary 

 determination in liquid media reliance has therefore to be placed 

 upon other procedures. 



When suitable liquid media are employed the presence or 

 absence of streptococci can be judged with considerable accuracy 

 by a careful microscopic examination of the liquid, particularly 

 of any sediment. 



Using this basis of enumeration various quantities- (O'l, ro, 

 10 c.c., etc.) of the water, milk, or other substance under exami- 

 nation are added to liquid media (such as glucose neutral red 

 broth) which allow the streptococci to grow well and the tubes 

 are carefully microscopically examined after incubation at 37 C. 

 for the presence of streptococci. 



It is obvious that such a method is only capable of enabling 

 streptococci as a class, and not the prevalence of any particular 

 kinds, to be enumerated. This method has both the drawbacks 

 of widely spaced results and the errors from unequal distribution 



