MANUAL OF BACTERIOLOGY. 



bator, the addition of sulphuric acid develops a red color, 

 owing to the formation of indol and nitrites, the so-called 

 " cholera red " reaction. 



The spirillum of cholera is said to be very sensitive to 

 drying, and, provided the drying be complete, is usually 

 killed within twenty-four hours. It is killed in five minutes 

 at a temperature of 65 C. and in one hour at 55 C. It 



FIG. 92. 



1 





a b c 



Spirillum of cholera, colonies on gelatin plates, X 100 to 150. (a) Twenty- 

 four hours old. (b) Thirty hours old. (c) Forty-eight hours old. 

 (Frankel and Pfeiffer.) 



may retain its vitality in water for a long time; observa- 

 tions vary widely in respect to determining how long. In 

 the ordinary food-substances it may survive long enough to 

 allow them to act as carriers of the infection if eaten raw. 

 The important fact is that the cholera spirillum is not a 

 strict parasite, but under favorable conditions it may main- 

 tain its vitality for some time outside of the human body. 



