PATHOGENIC BACTERIA. 



391 



alkaline media, and is very sensitive to a small amount of acid. 

 It liquefies gelatin. The colonies on gelatin plates have a 

 very characteristic appearance. They are nearly round at 

 first, and granular as seen under the low power of the micro- 

 scope; but at the end of about twenty-four hours the outline 

 is slightly irregular, and the surface looks as though it were 

 covered with finely broken glass. The outline later becomes 

 still more irregular or scalloped. As liquefaction of the 

 gelatin takes place a funnel-shaped depression is formed, into 

 which the colony sinks. Gelatin plates should be kept at a 



Fig. ioi. — Involution forms of the spirillum of cholera. — {Van Ermengem.) 



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 on the surface, owing to the liquefaction of the gelatin. 

 This depression increases in size, and the surface of the lique- 

 fied gelatin seems to be surrounded 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 o the stab 

 liquefaction is less noticeable. The growths on agar are not 

 characteristic. 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 



