REACTION OF B. COLI ON ENDO MEDIUM 549 



some light on the original problem, of the cause of the red colo- 

 nies on the Endo plate. 



When strong aldehyd is added to decolorized fuchsin, a strong 

 purple color is produced. Weak aldehyd produces a red color; 

 weak aldehyd in an acid solution, a strong cherry red color. 

 If a proper combination of aldehyd and acid be added to the 

 decolorized fuchsin in a test tube, a metallic film will appear on 

 the surface in a few minutes. This film, however, is not per- 

 manent, disappearing after a few hours. Acetone and alcohol 

 in faintly acid solutions will also bring back the color to decolor- 

 ized fuchsin. This, it might be added, is a matter of text-book 

 knowledge. 



It was further observed that, if glucose be substituted for 

 lactose in Endo medium, not only are typically metallic colonies 

 produced by the colon group, but by the typhoid-dysentery 

 group as well. The same was also found to be true when the 

 triatomic alcohol, glycerol, and the hexatomic alcohol, mannitol, 

 were substituted for lactose in Endo medium. 



Finally, the following experiments were performed. Two 

 1000 cc. Erlenmeyer flasks containing 300 cc. of 1 per cent lac- 

 tose broth, and 1 per cent glycerol broth respectively, were 

 inoculated with a fresh agar slant growth of B. coli-communior 

 obtained from feces. These flasks were employed in order to 

 permit large surface exposure. After twenty-four hours' incu- 

 bation the contents of the flasks were rendered alkaline with 

 sodium carbonate to hold back the organic acids, and distilled. 



When the distillates were added to decolorized fuchsin, no 

 change could be observed. When, however, the distillates were 

 rendered- faintly acid with acetic acid and added to decolorized 

 fuchsin, the appearance of the color was far more marked than 

 when the same concentration of acetic acid alone was employed. 

 This would indicate that another substance, or possibly other 

 substances, besides acid, are formed, when B. coli is gro^vn 

 under partial anaerobic conditions in lactose or glycerol medium, 

 which help to bring back the color to decolorized fuchsin. 



In this connection, it might be well to review briefly the 

 chemical nature of some of the substances under discussion. 



