A NOTE ON THE NATURE OF THE REACTION OF B. 

 COLI ON ENDO MEDIUM 



REUBEN L. KAHN 



From the Department Laboratory, United States Army, Southeastern Department, 



Atlanta, Georgia 



Received for publication June 17, 1918 



There appears to exist much difference of opinion as to the 

 factor or factors which enter into the production of typical 

 colonies of B. coli on the Endo plate. Endo (1904), who devel- 

 oped this medium for the differentiation of B. typhosus from 

 B. coli, states that the red colonies are due to acid produced by 

 the B. coli. *" 



Harding and Ostenberg (1912) claim that the red colonies of 

 these organisms on the Endo plate are due to aldehyd formation, 

 and that the gradual disappearance of the color after the first 

 twenty-four to forty-eight hours incubation, is due to oxidation 

 of aldehyd to acid. According to these workers, acid decolor- 

 izes Endo medium. De Bord (1917), on the other hand, claims 

 that acid is essential for the production of red colonies; that 

 aldehyds will not bring out the red color in the Endo medium, 

 but acid and aldehyd will. This worker, like Levine, Weldin, 

 and Johnson (1917) speaks of the Endo reaction as the ''fuchsin- 

 aldehyd reaction," insisting however that acid is essential for 

 this reaction. 



According to Robinson and Rettger (1916), organic acid, espe- 

 cially lactic acid, is the cause of the reddening of colon colonies 

 on Endo medium, and they maintain that the later decoloriza- 

 tion of the colonies, is due to alkali formation by the bacteria. 

 These investigators find that a drop of lactic acid added to an 

 Endo plate, produces a shade of red somewhat similar to that 

 produced by B. coli. On the other hand, a drop of neutral 



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THE JOURNAL OP BACTERIOLOGY, VOL. Ill, NO. 6 



