BACTERIA IN DIPHTHERIA. 361 



It grows readily in nutrient gelatin having a slightly alkaline reac- 

 tion, in nutrient agar, glycerin-agar, or in alkaline bouillon, but the 

 most favorable medium appears to be that first recommended by 

 Loffler viz., a mixture of three 

 parts of blood serum with one part 

 of bouillon, containing one per cent 

 of peptone, one per cent of grape 

 sugar, and 0. 5 per cent of sodium 

 chloride. This mixture is steril- 

 ized and solidified at a low tem- 

 perature, as is usual with blood 

 serum. Upon this the develop- 

 ment is SO rapid in the incubating FIG. 115. Colonies of Bacillus diphtherias 

 ,, ,\_ i . in nutrient agar, end of twenty-four hours. 



oven that, at the end of twenty- x :0 (Fran kei knd pfeiffer.) 

 four hours, the large, round, ele- 

 vated colonies, of a grayish-white color and moist appearance, may 

 be easily recognized, while other associated bacteria will, as a rule, 

 not yet have developed colonies large enough to interfere with the 

 recognition of these. 



Upon nutrient agar plates the deep-lying colonies, when magni- 

 fied about eighty diameters, appear as round or oval, coarsely granu- 

 lar discs, with rather ill-defined margins, or, when several colonies 

 are in juxtaposition, as figures of irregular form. The superficial col- 

 onies are grayish-yellow in color, have an irregular, not well-defined 

 outline and a rough, almost reticulated surface. The growth upon 

 glycerin-agar is very similar. The first inoculations in a plain nu- 

 trient agar tube often give a comparatively feeble growth, which be- 

 comes more abundant in subsequent inoculations in the same medium. 

 In stick cultures in glycerin or plain agar, growth occurs to the 

 bottom of the line of inoculation, and also upon the surface, but is 

 not at all characteristic. The same may be said with reference to 

 cultures in nutrient gelatin. Plate cultures in this medium contain- 

 ing fifteen per cent of gelatin, at 24 C., give rather small colonies, 

 which are white by reflected light and under the microscope are seen 

 as yellowish-brown, opaque discs, having a more or less irregular 

 outline and a granular structure. In alkaline bouillon the growth is 

 sometimes in the form of small, whitish masses along the sides and 

 bottom of the tube, but at others a diffusely clouded growth occurs 

 in this medium ; after standing for some time in the incubating oven 

 a thin, white pellicle may form upon the surface of the bouillon. 

 The reaction of the bouillon becomes at first acid, but later it has an 

 alkaline reaction (Welch). With reference to the growth on potato, 

 authors have differed, probably because the growth is scarcely vis- 

 ible ; upon this point we quote from Welch and Abbott : 

 28 



