DIPHTHERIA BACILLUS. 341 



medium employed to get by plating methods a pure 

 culture from the original serum tube. The agar should 

 be freshly melted and poured in the Petri dish for this 

 purpose. After it has hardened the layers in a number 

 of plates are streaked across with bacteria from colonies 

 on the serum culture, which appear in size and color 

 like the diphtheria bacilli. Other plates are made from 

 a general mixture of all the bacteria, selected, as a rule, 

 from the drier portion of the serum. The plates are left 

 in the incubator for twelve hours at 37 C. In the 

 examination of the plates one should first seek for 

 typical colonies and then later for any that look nearest 

 the characteristic picture. Diphtheria colonies are very 

 apt to be found at the edges of the streaks of bacterial 

 growth. 



Growth in Bouillon. The diphtheria bacillus usu- 

 ally grows readily in broth slightly alkaline to litmus. 

 The characteristic growth in neutral bouillon is one 

 showing fine grains. These deposit along the slles and 

 bottom of the tube, leaving the broth nearly clear. A 

 few cultures in neutral bouillon and many in alkaline 

 bouillon produce for twenty-four or forty-eight hours 

 a more or less diffuse cloudiness, and frequently a film 

 forms over the surface of the broth. On shaking the 

 tube this film breaks up and slowly sinks to the bottom. 

 This film is more apt to develop during the growth of 

 cultures which have long been cultivated in bouillon, and 

 indeed after a time the entire development may appear 

 on the surface in the form of a friable pedicle. The 

 diphtheria bacillus in its growth causes a fermentation 

 of the meat sugars and the glucose, and thus changes 

 the reaction of the bouillon, rendering it distinctly less 

 alkaline within forty-eight hours, and then, after a vari- 



