THE COLON BACILLUS GROUP 235 



GELATIN. In gelatin plates colonies are developed in from eighteen 

 to thirty-six hours. They resemble greatly the colonies of the typhoid 

 bacillus, except that many of them are somewhat larger and more 

 opaque. When located in the depths of the gelatin and examined by 

 a low-power lens they are at first seen to be finely granular, almost 

 homogeneous, and of a pale yellowish to brownish color; later they 

 become larger, denser, darker, and more coarsely granular. In shape 

 they may be round, oval, or whetstone-like. The superficial colonies 

 appear as small, dry, irregular, flat, blue-white points, that are com- 

 monly somewhat dentated at the margins. When the gelatin is not 

 firm the margins of many colonies are broken by outgrowths, which 

 are rather characteristic of colon bacilli. Under the same conditions 

 the colonies of typhoid bacilli usually show thread-like outgrowths. 



FIG. 85 



Colon bacilli. Twenty-four-hour agar culture. X 1100 diameters. 



In stab cultures on gelatin the growth usually takes the form of a 

 nail with a flattened head, the surface extension generally reaching 

 out rapidly to the sides of the tube. 



NUTRIENT AGAR. In plate cultures : Surface colonies mostly circular, 

 finely granular, and rather opaque. The deep colonies are apt to have 

 protuberances. There are no marked differences between colonies of 

 colon and typhoid bacilli. In streak cultures an abundant, soft, white 

 layer is quickly developed, but the growth is not characteristic. 



'BOUILLON. In bouillon the bacillus coli produces diffuse clouding 

 with sedimentation; in some cultures a tendency to pellicle formation 

 on the surface is occasionally seen. 



POTATO. On potato the growth is rapid and abundant, appearing 

 after twenty-four to thirty-six hours in the incubator as a yellowish- 

 brown to dark cream-colored deposit covering the greater part of the 

 surface. But there are considerable variations from the typical growth 

 on potato; there may be no visible growth at all, or it may be scanty and 

 of a white color. These variations are due at times to the bacillus, 

 but more often to variations in the potato. 



