458 



Diphtheria 



vital resistance is such that the bacilli grow scantily without 

 producing disease of the throat. The bacteriologic examin- 

 ation is, therefore, only an adjunct to the clinical diagnosis, 

 and must never be taken as positive in itself. 



Fig. 150. Diphtheria bacilli (from photographs taken by Prof. E. 

 K. Dunham, Carnegie Laboratory, New York): a, Pseudobacillus ; 

 b, true bacillus; c, pseudobacillus. 



Gelatin. Gelatin is not an appropriate medium for the 

 cultivation of the bacillus. Upon the surface of gelatin 

 plates the colonies attain but a small size and appear to 

 the naked eye as whitish points with smooth contents and 

 regular, though sometimes indented, borders. Under the 

 microscope they appear granular and yellowish-brown, with 

 irregular borders (Fig. 151). The growth in gelatin punc- 

 ture is characterized by the occurrence of small spheric 

 colonies along the line of inoculation. The gelatin is not 

 liquefied. 



