THE DIPHTHERIA BACILLUS 391 



than that characteristic of the streptococcus. Pseudodiphtheria bacilli 

 do not produce hemolysis on this medium 1 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. 2 This pellicle may sink, but a new 

 one usually takes its place. The growth in dextrose broth is more 

 luxuriant than in plain broth, but no pellicle forms. There is, however, 

 a well-marked turbidity. The diphtheria bacillus grows well in milk, 

 producing an initial acid reaction during the first two or three days 



FIG. 56. Bacillus diphtherias, branching. ( X 800.) 



of incubation, followed by the gradual development of an alkaline 

 reaction. 3 No gross changes, however, are produced in the milk, even 

 with prolonged cultivation. The growth on potato is very slight 

 provided the reaction of the potato is alkaline; no growth at all 

 takes place on acid potato. 



The diphtheria bacillus is an aerobic, facultatively anaerobic organ- 

 ism. Its limits of growth are 17 C. as a minimum, 43 C. as a maxi- 

 mum, with the optimum at 37 C. Ten minutes exposure to 60 

 C., five minutes at 70 C., or one minute at 100 C. readily kills diph- 

 theria bacilli. The organisms are occasionally transmissible through 

 milk, and in this connection it should be remembered that the ordinary 

 method of heating milk in an open vessel will not certainly kill diph- 



1 Mandelbaum and Heinemann, Centralbl. f. Bakt., Orig., 1910, liii, 356; Rankin, 

 Jour. Hyg., 1911, xi, 271. 



2 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. 



3 Kendall, Day and Walker, Jour. Am. Chem. Assn., 1914, xxxvi, 195. 



