GENUS BACTERIUM 295 



tensely than the rest of the organism. Sometimes these highly 

 stained bodies are thicker than the rest of the organism, and 

 again they are small and surrounded by a more lightly stained 

 portion. The bacteria stain in this peculiar manner at a cer- 

 tain period of their growth, so that only a portion of the 

 organisms taken from a culture at any one time will show the 

 characteristic staining. The granules which take the methy- 

 lene blue more intensely than the remainder of the organism 

 are brought out more distinctly by the Neisser stain. 



Cultivation. Bacteria of diphtheria will grow on all the 

 ordinary media made from meat infusion. Its most charac- 

 teristic growth, however, is on Loeffler's blood serum. 



Agar. On slightly alkaline, nutrient or glycerin-agar the 

 growth is less certain than upon blood serum ; but the appear- 

 ance of the colonies when examined under a low-power lens, 

 though very variable, is often far more characteristic. For 

 this reason, nutrient agar in Petri dishes is used to obtain 

 pure cultures. The diphtheria organisms taken from cultures 

 which have been under cultivation for some time grow fairly 

 well on nutrient agar. 



Gelatin. The growth on this medium is much slower,, 

 more scanty, and less characteristic than that on the other 

 media. 



Serum. The growth of diphtheria bacteria in pure cul- 

 ture on blood serum from eight to twelve hours will show 

 small colonies which appear as pearl-gray, whitish-gray, or 

 more rarely, yellowish-gray, slightly raised points. The 

 colonies when separated from each other may increase in 

 forty-eight hours to a diameter of 4 mm. The borders are 

 usually somewhat uneven. The colonies lying together be- 

 come confluent when the serum is moist. During the first 

 twelve hours the colonies are about equal in size to those of 

 other pathogenic bacteria which are often present in the 

 throat; but after this time the diphtheria colonies become 

 larger than those of the streptococci and smaller than those of 

 the staphylococci. Serum is not liquefied. 



Bouillon. Diphtheria bacteria obtained from about one- 



