462 APPLICATION OF METHODS OF BACTERIOLOGY 



When magnified slightly the centre is seen to be more 

 dense than the surrounding zone or zones, for they are 

 sometimes marked by a concentric arrangement of zones. 

 The periphery is irregularly notched. Like the colonies 

 seen on agar-agar, they are granular, but are much more 

 granular when seen in the depths of the gelatin than when 

 on its surface. On gelatin the colonies rarely become very 

 large; usually they do not exceed 1.5 mm. in diameter. 



Bouillon. — ^In bouillon it usually 'grows in fine clumps, 

 which fall to the bottom of the tube, or become deposited 

 on its sides without causing diffuse clouding of the bouillon. 

 Sometimes there are exceptions to this naked-eye appear- 

 ance; the bouillon may be diffusely clouded; but if one 

 inspect it very closely, particularly if he examine it micro- 

 scopically as a hanging drop, the arrangement in clumps will 

 always be detected, but the clumps are so small as not to 

 be discernible by the unaided eye. 



In bouillon kept at a temperature of 35°-37° C. a soft, 

 whitish pellicle often forms upon the surface. 



The reaction of the bouillon frequently becomes at first 

 acid, and subsequently again alkaline, changes which can 

 be observed in cultivations in bouillon to which a little 

 rosolic acid has been added. This play of reactions has 

 been attributed to the primary fermentation of the muscle- 

 sugar often present in the bouillon. It does not occur 

 when the medium is free from carbohydrates. 



Potato. — On potato at a temperature of 35°-37° C. its 

 growth after several days is invisible, only a thin, dry glaze 

 appearing at the point at which the potato was inoculated. 

 Microscopic examination of scrapings from the potato, 

 after twenty- four hours at 35°-37° C, reveals a decided 

 increase in the number of individual organisms planted. 



