BACILLUS DIPHTHERI^E 517 



growth may occasionally be slow and extremely delicate. Colonies 

 on agar appear within twenty-four to thirty-six hours as small, 

 rather translucent, grayish specks. The appearance of these colonies 

 is quite characteristic and easily recognized by the practiced observer. 

 Surface colonies are irregularly round or oval, showing a dark, 

 heaped-up, nucleus-like center, fringed about by a loose, coarsely 

 granular disk. The edges of these colonies have a pecuharly irregular, 

 torn appearance which distinguishes them readily from the sharply 

 defined chain-fringed streptococcus colonies. For these I'easons agar 

 is the medium most commonly used for purposes of isolation. The 

 deep colonies in this medium are dense and sharply outlined. 



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. 



Meai^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 character- 

 istic appearance. 



Meatr-infiision 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. diphtheriae 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,' the horse-blood-fibrin cake used by 

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

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

 media, however, to make its substitution desirable. 



Isolation. — ^Because of the comparative ease with which B. diph- 

 theria is isolated from mixed cultures, it is not necessary to give in de- 



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



