ENDO MEDIUM FOR ISOLATION OF B. DYSENTERIAE 439 



was required for every 100 cc. of agar. It is necessary, however, 

 to determine the exact amount for each fresh lot of agar. 



Double sugar medium. Having developed the colonies on the 

 Endo plate, it is necessary to carry the differentiation further 

 so as to confirm the nature of the selected colonies. Russell 

 (1911) makes use of a double s'ugar agar containing 0.1 per cent 

 glucose and 1 per cent lactose. While this medium is effective, 

 it does not permit the distinction of B. typhosus and the two 

 classevS — alkaline or Shiga and acid or Flexner, etc. — of dysentery 

 bacilli from each other. We have substituted 0.5 per cent man- 

 nite for the lactose, with the effect of accomplishing the purpose. 



The selected colonies are stabbed in the butt of the slanted 

 tube and also streaked on the surface. Andrade's indicator is 

 used. The differentiation is made after twenty-four hours' 

 incubation at 37°C. The Shiga bacillus yields a slight redden- 

 ing of the butt but no change of the surface. The acid class of 

 dysentery bacilli and B. typhosus color the entire medium red; 

 no gas is produced. B. alcaligenes produces no changes; while 

 the paratyphoid bacilli cause both reddening and gas formation. 

 B. proteus, on the other hand, produces gas but no reddening. If 

 decolorized fuchsin is employed as indicator, then the paratyphoid 

 bacilli A and B can be distinguished from each other, as the 

 latter brings about final decolorization of the medium. How- 

 ever, the main advantage of this medium is that it permits of a. 

 more rapid separation of the two classes of dysenteric bacilli — - 

 the Shiga and the Flexner. 



, SUMMARY 



A study of the Endo medium, as applied to B. dysenteriae, 

 indicated that the most important single condition that must be 

 carefully controlled, particularly if the medium is intended for 

 the isolation of dysenteric bacilli, is the end reaction. The 

 ingredients are of singificance in so far as they furnish the ele- 

 ments essential for the growth of the organisms. Given an 

 otherwise favorable Endo medium, however, the Shiga bacillus 

 will or will not grow, depending on whether the end reaction is. 

 Ph 7.6-7.8 or P^ 8.4-8.8, the reaction ordinarily obtained. 



