446 BACTERIOLOGY. 



Growth on Gelatin. In gelatin plates colonies are 

 developed in from twenty-four to forty-eight hours, 

 which vary considerably in their appearance according 

 to their age, and in different cultures in the same 

 medium. They resemble greatly the colonies of the 

 typhoid bacillus, except that they are somewhat larger 

 for the same period of growth. 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, in structure round, and of a pale yellow- 

 ish to brownish color; later they become larger, denser, 

 darker, and more coarsely granular. In shape they 

 may be round, oval, or " whetstone-like. " The super- 

 ficial colonies appear as small, dry, irregular, flat, 

 blue- white points that are commonly somewhat dentated 

 at the margin. 



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. 



On Nutrient Agar and Blood-serum. On nutrient 

 agar and blood-serum an abundant, soft, white layer is 

 quickly developed in the incubator, but the growth 

 is not characteristic. 



In Bouillon. In bouillon the bacillus coli produces 

 diffuse clouding with sedimentation; in some cultures 

 a tendency to pellicle formation on the surface is seen 

 occasionally. In old cultures, in the absence of sugar, 

 the reaction becomes alkaline, and a decided fecal odor 

 may be noticed. 



The colon bacillus produces indol in bouillon and in 

 peptone solutions, this reaction being most pronounced 

 after a week's development in the incubator. It pos- 



