460 APPLICATION OF METHODS OF BACTERIOLOGY 



none, a noticeable difference in morphology can usually 

 be made out: while the forms on the glycerin-agar-agar 

 cultures are throughout small, and pretty regular in size, 

 shape, and stainiiig, those on the plain agar-agar are larger, 

 stain less uniformly, vary more in shape, and when stained 

 by Loffler's blue are not so regularly marked by pale trans- 

 verse lines that give to them the appearance of being made 

 up of numerous short segments. 



Though the outline of this organism is more regular 

 under some circumstances than others, it is nevertheless 

 always conspicuous for its manifold variations in shape. 



Growth on Serum Mixture.- — The medium upon which 

 bacillus diphtheriee grows most rapidly and luxuriantly and 

 which is best adapted for determining its presence in diph- 

 theritic exudates, is, as has been stated, the blood-serum 

 mixture of LofEer. (See chapter on Media.) On the blood- 

 serum mixture the colonies of bacillus diphtherise grow so 

 much more rapidly than the other organisms usually present 

 in secretions and exudations in the throat that at the end of 

 twenty-four hours they are often the only colonies that 

 attract attention; and if others of similar size are present, 

 they are generally of quite a different aspect. Its colonies 

 are large, round, elevated, grayish-white or yellowish, with 

 a centre more opaque than the slightly irregular periphery. 

 The surface of the colony is at first moist, but after a day or 

 two becomes rather dry in appearance. 



A blood-serum tube studded with coalescent or scattered 

 colonies of this organism is so characteristic that one familiar 

 with the appearance can anticipate with tolerable certainty 

 the results of microscopic examination. 



Glycerin-agar-agar. — Upon nutrient glycerin-agar-agar the 

 colonies likewise present an appearance that readily may 



