268 A MANUAL OF BACTERIOLOGY 



are abundant ; on blood-serum club-shaped involution 

 forms also occur, but sparsely in a young, eighteen to 

 twenty hours' culture, in a forty- eight hours' culture 

 more numerously. 



Staining reactions. The B. diphtheria stains well with 

 the ordinary anilin dyes and is Gram-positive. With 

 Loffler's methylene blue the coloration is usually some- 

 what irregular, more deeply stained portions alternating 

 with paler intervals, the so-called segmentation, and 

 especially marked with agar cultures. The ends of the 

 organisms are also frequently deeply stained, the so-called 

 polar staining, while the phenomenon known as " meta- 

 chromatism " is often marked both at the poles and also 

 in the rod, appearing as granules of a purplish tint and 

 contrasting with the blue of the methylene blue. With 

 Neisser's stain (p. 294) deep inky coloured dots, appearing 

 somewhat larger in diameter than the rods, occur at the 

 poles of the organism and occasionally at the centre. 



Cultural reactions. The diphtheria bacillus is an aerobic 

 and also a facultatively anaerobic organism, and grows 

 well on all the ordinary culture media, forming cream- 

 coloured growths or colonies, the latter on serum tending 

 to be somewhat flattened, with regular margins. It grows 

 slowly on gelatin, forming a raised whitish growth without 

 liquefaction of the medium, and flourishes in milk, with 

 the production of an acid reaction, but without curdling. 

 In broth some strains give a granular growth on the sides 

 and at the bottom of the tube, the broth remaining clear, 

 sometimes with a thin surface pellicle ; other strains may 

 render the broth turbid throughout. On potato the 

 growth is slight and invisible. 



The indole reaction can be obtained in peptone-water 

 cultures either with or without a nitrite, but the writer 

 has shown that this reaction is due, not to indole, but to 

 skatolecarboxylic acid (see below, p. 288). 



