THE DIPHTHERIA BACILLUS 323 



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. 349) deep inky-coloured dots, appearing 

 somewhat larger in diameter than the rod, occur at the 

 poles of the organism and occasionally at the centre. 



Cultural reactions. The diphtheria bacillus is an 

 aerobic and 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 

 skatole-carboxylic acid (see below, p. 344). 



The diphtheria bacillus attacks glucose and lactose 

 with the formation of acid only, no gas (see Table, p. 348). 

 As regards the production of acid, Neisser found that 

 during the first nine hours there is little or none ; at the 

 end of twenty-four hours a considerable quantity has 

 been formed, and the amount increases until the end of 

 the second day, after which the production ceases. 



The B. diphtherice is distinguished from all other diph- 

 theroids by giving acid in glucose and dextrin but not in 

 saccharose (Hine). 



The B. diphtherice is agglutinated by the serum of 

 patients and by a diphtheria serum, but the test is diffi- 



