346 SPECIFIC MICRO-ORGANISMS 



sensitive to acids. Colonies appear on gelatin at 22 C. in about 

 24 hours as circular disks with somewhat irregular border and 

 granular interior. A few hours later the gelatin begins to liquefy. 

 In pep ton-salt solution both indol and nitrite are formed, so that 

 the addition of sulphuric acid gives rise to the red color due to 

 nitroso-indol. This has been called the cholera-red reaction, but 

 it is of course not a specific test for this organism. In milk there 

 occurs abundant growth without apparent change in the medium. 

 In broth, growth is extremely rapid and a pellicle forms in 24 



FIG. 137. Cholera vibrios, short forms. (From Kolle and Schurmann after Zettnow.') 



hours. The rapid growth in pepton solution (pepton i per cent, 

 salt 0.5 per cent) and the tendency for the organisms to collect 

 at the surface are utilized in practical enrichment for purposes 

 of diagnosis. The spirillum is an obligate aerobe. It is very 

 easily killed. If dried on a cover-glass at 37 C., the organisms 

 are all dead in two hours. It seems impossible, therefore, for the 

 infection to be distributed in dry dust. Moist heat at 56 C. 

 kills the cholera spirilla in 30 minutes. They are also easily 

 killed by chemical germicides. Milk of lime is recommended for 

 the disinfection of excreta. The organism lives for several weeks 



