454 BACTERIA IN DIPHTHERIA. 
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 
Léffler—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- 
Fic. 113,—Colonies of Bacillus diphtherz ment is so rapid in the incubating 
ty nutrient agar ond of emirfour POE. oven that, at the end of twenty- 
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 bouzllon 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 : 
