566 



PATHOGENIC MICROORGANISMS 



For these reasons agar is the medium most commonly used for purposes 

 of isolation. 



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

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

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



probably because of acid forma- 

 tion. 



Meat-infusion Broth . Upon 

 beef or veal broth the diphtheria 

 bacillus grows rapidly, almost in- 

 variably 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 appearance. 



Meat-infusion gelatin is a favor- 

 able medium for the Klebs-Loeffler 

 bacillus, but growth takes place 

 slowly because of the low temper- 

 FIG. 60 COLONIES OF DIPHTHERIA ature at which this medium must 

 ON GLYCERINE AGAR. be kept. Gelatin is not fluidified. 



Milk is an excellent medium, 



and for this reason may even occasionally be a vehicle of transmis- 

 sion. There is no coagulation of the milk. 



Upon potato, B. diphtherise 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, 11 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 



11 M. Beclc, Kolle uiul Wasscrmann; Brit. Mettl. Jour. 



