390 



THE DIPHTHERIA BACILLUS GROUP 



Loffler's methylene blue. With methylene blue the granules above 

 mentioned are brought out very sharply, and it is observed that these 

 granules exhibit the phenomenon known as metachromatism, that is, 

 they stain mahogany red while the rest of the organism stains blue. 

 Diphtheria bacilli are Gram-positive, but prolonged washing with 

 alcohol removes the Gram-positive stain. Cultures prepared directly 

 from diphtheritic membranes stain more uniformly than organisms 

 obtained from cultures on Loffler's blood serum. 



Diphtheria bacilli are non-motile, possess no capsules and form no 

 spores. Very frequently the organisms are arranged in a definite 

 and characteristic manner, occurring very commonly in pairs, each 

 pair of organisms forming a configuration very similar to a capital 

 "L," and a series of these angulated pairs are arranged in parallel, 



FIG. 55. Bacillus diphtherias, methylene-blue stain. ( X 1000.) 



very much like chevrons. This angular arrangement of the organisms 

 is due to their method of reproduction. 



Isolation and Culture. The diphtheria bacillus grows best on Loffler's 

 alkaline blood serum and this medium is almost specific for the 

 organism, which during the first nine to eighteen hours' incubation 

 outgrows all other organisms with which it is usually associated in 

 characteristic lesions, except staphylococci. Colonies of diphtheria 

 bacilli on this medium after eighteen hours' incubation at 37 C. are 

 gray-white, round, rather dull, with darker centres, and may attain 

 a diameter of 1 to 1.5 mm. Diphtheria bacilli grow somewhat more 

 slowly on plain agar, forming small, non-characteristic colonies. The 

 organisms produce a well-marked zone of hemolysis around the 

 individual colonies on blood agar, but the hemolytic area is smaller 



