BACILLUS DIPHTHERIA 517 



nies are irregularly round or oval, showing a dark, heaped-up, nucleus- 

 like center, fringed about by a loose, coarsely granular disk. The edges 

 have a peculiarly irregular, torn appearance which distinguishes them 

 readily from the sharply defined streptococcus colonies. For these 

 reasons agar is the medium most commonly used for purposes of iso- 

 lation. 



The addition of dextrose 1 per cent, nutrose 2 per cent, or glycerin 

 6 per cent, renders agar more favorable for rapid growth, but unfits it 

 for the preservation of cultures, the organism dying out more rapidly, 

 probably because of acid formation. 



Meat-Infusion Broth. Upon beef or veal broth the diphtheria bacil- 

 lus grows rapidly, almost invariably forming a pellicle upon the surface, 

 another expression of its desire for oxygen. The broth remains clear. 

 Broth tubes with such growth, therefore, have a characteristic appear- 

 ance. 



Meat-infusion gelatin is a favorable medium for the Klebs-Loeffler 

 bacillus, but growth takes place slowly because of the low temperature 

 at which this medium must be kept. Gelatin is not fluidified. 



Milk is an excellent medium, and for this reason may even occa- 

 sionally be a vehicle of transmission. There is no coagulation of the 

 milk. 



Upon potato, B. diphtherias will grow only after neutralization of the 

 acid. It is, at best, however, a poor nutrient medium. 



Upon the various pepton solutions the bacillus of diphtheria produces 

 no indol. 



Many special media have been recommended for the cultivation of 

 this organism. The most important of these are the modification of 

 Loeffler's serum devised by Beck, 1 the horse-blood-fibrin cake used by 

 Escherich, and Wassermann's ascitic-fluid-nutrose-agar, called by him 

 "Nasgar." None of these has sufficient advantages over the simpler 

 media, however, to make its substitution desirable. 



Isolation. Cultures are taken from throats upon Loeffler's blood 

 serum. These are permitted to grow at 37.5 C. for from eighteen to 

 twenty-four hours. At the end of this time about 5 c.c. of bouillon are 

 poured into the tubes and the growth is gently emulsified in the broth 

 with a platinum loop. Two or three loopfuls of this emulsion are then 

 streaked over the surface of glucose agar, serum agar, or nutrose agar. 

 After twenty-four hours' incubation these plates show characteristic 



. 1 M, Beck, Kolle und Wassermann; Brit. Mod. Jour. 



