506 PATHOGENIC BACTERIA. 



media. Upon gelatin plates the colonies develop in 

 twenty-four hours. Those situated below the surface 

 appear round, yellow-brown, and homogeneous. As they 

 grow older they increase in size and become opaque. The 

 superficial colonies are larger and spread out upon the 

 surface. Their edges are dentate and resemble grape- 

 leaves, often showing radiating ridges suggestive of the 

 veins of a leaf. They may have a slightly concentric 

 appearance. The colonies rapidly increase in size and 

 become more and more opaque. The gelatin is not 

 liquefied. 



In gelatin punctures the culture, developing rapidly 

 upon the surface, and also in the needle's track, causes 

 the formation of a nail-like growth. The head of the 

 nail may reach the walls of the test-tube. Not infre- 

 quently gas is formed in ordinary gelatin, and when i 

 per cent, of glucose is dissolved in the medium the gas- 

 production is often so copious and rapid as to form large 

 bubbles, which by their distention subsequently break it 

 up into irregular pieces. Sometimes the gelatin becomes 

 slightly clouded as the bacilli grow. 



Upon agar-agar along the line of the inoculation a 

 grayish-white, translucent, smeary growth takes place. 

 It is devoid of any characteristics. The entire t surface 

 of the culture-medium is never covered, the. growth re- 

 maining confined to the inoculation-line, except where 

 the moisture of the condensation-fluid allows it to spread 

 out at the bottom. Kruse says that in old cultures crys- 

 tals may form. 



Bouillon is soon evenly clouded by the development 

 of the bacteria. Sometimes a delicate pellicle forms upon 

 the surface. There is rarely much sediment in the cul- 

 ture. 



Wiirtz found that the bacillus produced ammonia in 

 culture-media free from sugar, and thus caused an intense 

 alkaline reaction in the culture-media. The cultures 

 usually give off an odor that varies somewhat, but is, as 

 a rule, unpleasant. 



