REACTION OF B. COLI ON ENDO MEDIUM 553 



given period of incubation, will be more pronounced on the 

 Endo agar slant than on the Endo plate. This is explained as 

 due to the greater opportunity for evaporation from the slant 

 han from the inverted plate. 



There are a large number of red colonies on Endo medium 

 which workers recognize to be of non-colon types, These 

 colonies, as a rule, possess a red shade that is somewhat different 

 from that of B. coli and they never possess a metallic film. It is 

 possible that some organisms cause the production of red colonies 

 by reducing sodium sulfite to hydrogen sulfide, thus liberating 

 the fuchsin color. It is more likely, however, in view of the 

 ease with which bacteria attack carbohydrates, that these red 

 colonies are produced by the conversion of lactose to some non- 

 volatile acid, most probably lactic acid. This acid has a tend- 

 ency to absorb moisture from the air, and such moisture on the 

 surface of the colony would tend to keep the fuchsin in solution, 

 rendering the colony red but not metallic. 



CONCLUSIONS 



1. Chemical and physical factors enter into the formation of 

 typically metallic colonies of B. coli on Endo agar. 



2. During the growth of these organisms on Endo medium, 

 they adsorb the various soluble substances contained in the 

 medium and proceed to break down the lactose first (Kendall). 



3. After ten to fifteen hours incubation, the trace of lactose 

 adsorbed, is probably transformed into lactic and other organic 

 acids, and the colony is colored red. 



4. On further incubation, the organic acids are probably 

 reduced by the bacteria to aldehyd and alcohol, which volatilize 

 from the surface, leaving the non-volatile fuchsin behind, thus 

 producing a metallic sheen. 



5. The carbohydrate being disposed of, the organisms proceed 

 to attack the nitrogenous materials. Ammonia and other sub- 

 stances are produced in which the fuchsin goes back into solu- 

 tion, and ultimate decolorization takes place. 



