THE DIPUTUEHIA BACILLUS 425 
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, 
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 centers, and may attain 
a diameter of 1 to 1.5 mm. Diphtheria bacilli grow somewhat more 
Fig. 60. — Bacillus diphtherise, methylene blue stain. X 1000. 
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 
than that characteristic of the streptococcus. Pseudodiphtheria bacilli 
do not produce hemolysis on this medium.^ The growth of diphtheria 
bacilli in gelatin is slow, and the organisms do not produce liquefaction 
of the medium. In plain broth the organism grows rather slowly; 
repeated transfers are usually followed by the development of a pel- 
licle which floats on the surface.'' This pellicle may sink, but a new 
one usually takes its place. The growth in glucose broth is more 
luxuriant than is plain broth, but no pellicle forms. There is, however, 
1 Hill: Jour. Med. Res., 1902, 7, 202. 
2 Mandelbaum and Heinemann: Centralbl. f.. BakterioL, orig., 1910, 53, 356. 
Rankin: Jour. Hyg., 1911, 11, 271. 
3 It is essential for the production of toxin that the organisms be cultivated until 
they produce a pellicle, leaving the underlying medium perfectly clear and free from 
bacilli. 
