556 APPLICATION OF METHODS OF BACTERIOLOGY 



whitish clump, the colony itself. As growth continues the 

 . depression increases in extent and ultimately assumes an 

 appearance that consists in the apparent sinking of the 

 liquefied portion in such a way as to leave a perceptible 

 air-space between the top of the liquid and the surface of 

 the solid gelatin. The growth now appears to be capped by 

 a small air-bubble. The impression given by it at this 

 stage is not only that there has been a liquefaction, but 

 also a coincident evaporation of the fluid from the liquefied 

 area and a constriction of the superficial opening of the 

 funnel. (See a, b, c, and d, Fig. 91.) Liquefaction is not 

 especially active along the deeper portions of the track 

 made by the needle, though in stab-cultures in gelatin the 

 liquefaction is much more extensive than that usually seen 

 around colonies on plates. It spreads laterally at the upper 

 portion, and after about a week a large part of the gelatin 

 in the tube may have become fluid, and the growth will 

 have lost its characteristic appearance. 



Stab- and smear-cultures on agar-agar present nothing 

 characteristic. 



Its growth in bouillon is luxuriant, causing a diffuse 

 clouding and the ultimate production of a delicate film 

 upon the surface. 



In sterilized milk of a neutral or amphoteric reaction at 

 a temperature of 36 to 38 C. it develops actively, and 

 gradually produces an acid reaction, with coagulation of 

 the casein. It retains its vitality under these conditions 

 for about three weeks or more. The blue color of milk to 

 which neutral litmus tincture has been added is changed to 

 pink after thirty-six or forty-eight hours at body-temperature. 



Its growth in peptone solution, either that of Dunham 

 (see Special Media) or the one preferred by Koch, viz., 



