300 



SPECIFIC MICRO-ORGANISMS 



Loffler's blood serum is the medium of choice. The colonies 

 develop at 37° C. in 8 to 12 hours as grayish, slightly elevated 

 points and become 2 to 3 mm. in diameter in the course of 48 



A ' B 



Pig. 121. — Forms of B. diphtheria in cultures onjLoffler's serum. A, Charac- 

 teristic cliibbed and irregular shapes with irregular st twining of the cell contents. 

 Xiioo. B, Irregtilar shapes with even staining. Xiooo. (^After Park and 

 Williams.) 



hours. Contiguous colonies become confluent. On glycerin agar 



after 25 hours at 37" C, the colony is coarsely granular with 



somewhat jagged outline. Many variations from this typical 



A B 



Pig. 122. — Forms of B. diphtheria in cultures on agar. A, Bacilli small and 

 uniform. Xiooo. B, Spherical forms in culture 24 hours old. On Loffler's serum 

 this same organism produced granular forms. X1410. (After Park and Williams.) 



appearance occur. Growth in gelatin is slow and ceases below 

 20° C. The medium is not liquefied. The bacillus grows in 

 milk without producing coagulation. In broth the growth 



