DIPHTHERIA. 



333 



elements. They differ greatly in size and shape, often in the same 

 cultures, and still more in cultures obtained from different sources. 

 Spore formation is unknown. In unstained preparations there are 

 highly refractive elements which correspond with the deeply stained 

 parts of the bacillus. They stain readily with the ordinary aniline 

 dyes. At certain stages of their growth they stain irregularly, the 

 protoplasm of the rod being broken up into irregular segments. 

 The bacillus is non-motile, and does not liquefy gelatine ; it grows 

 at 20 C., but much more readily at higher temperatures. Colonies 

 in gelatine plate -cultivations are 

 yellowish-brown, and opaque, 

 granular, and circular, but with 

 more or less irregular margin. 



In plate -cultivations on agar 

 and on glycerine agar the same 

 description applies. 



On the surface of gelatine 

 the appearances depend greatly 

 on the method of inoculation. 

 The growth may occur in the 

 form of a whitish film, but if a 

 sub-culture has been prepared 

 from broth the growth is often 

 composed of a number of iso- 

 lated white colonies (Fig. 128, a). 



On blood serum, after twelve 

 hours the colonies appear in the 

 form of little elevated greyish - 

 white or pearl-grey dots, which 

 coalesce, forming a film if the 

 serum is moist. On the surface 

 of 1 per cent, alkaline glycerine 

 agar, the appearances are found 

 to vary, and this medium is not so suitable for the cultivation of the 

 bacillus. In slightly alkaline broth, with or without the addition of 

 1 per cent, grape-sugar, the culture is cloudy, or a fine granular 

 deposit occurs along the sides and bottom of the tube, while the 

 broth remains clear. 



On potato the growth is almost invisible, in the form of a dry, 

 thin glaze. Irregular forms are very numerous on microscopical 

 examination, whilst the rods are thicker than usual (Welch and 

 Abbott). In milk the organisms grow readily. 



FlG. 128. PUKK-CULTURES OF BACILLUS 



DIPHTHERIA ON GELATINE : a, isolated 

 colonies ; b, filmy growth. 



