3l8 MANUAL OF BACTERIOLOGY. 



at a temperature of from 20° to 22° C. In stab-cultures in 

 gelatin a white growth forms around the stab, and at the end of 

 about thirty-six to forty-eight hours a funnel-shaped depression 

 occurs at the surface, owing to the Hquefaction of the gelatin. 

 This depression increases in size, and the surface of the hquefied 

 gelatin seems to be surmounted by an air-bubble, which appears 

 to have taken the place of the part of the fluid gelatin which 

 has evaporated. In the deeper portion of the stab liquefaction 

 is less noticeable. The growths on agar are not characteristic. 



Fig. g6. — Involution Forms of the Spirillum op Cholera. — {Van 

 Ermengem.) 



In bouillon a pellicle forms on the surface. On potato in the 

 incubator the growth is whitish or brownish, not conspicuously 

 elevated. After growing it in Dunham's peptone solution in 

 the incubator the addition of sulphuric acid develops a red 

 color, owing to the presence of indol and nitrites, — the so-called 

 "cholera red" reaction. Considerable doubt has recently been 

 cast upon the formation of nitrites by the cholera spirillum.* 

 The cholera-red reaction is not confined to this organism, and 

 is said to differ from the nitroso-indol reaction. 



The spirillum of cholera is said to be very sensitive to dry- 



* Wherry. Journal oj Injections Diseases. Vol. II. No. 3. June 24, 1905. 



