VARIATIONS IN VIRULENCE OF THE BACILLUS. 369 



in the membrane. Cartwright Wood has also found that 

 when diphtheria cultures in an albumin-containing medium 

 are filtered germ-free and exposed to 65 C. for an hour 

 (the supposed ferments being thus destroyed), there still 

 remain albumoses which produce febrile reaction and are 

 active in developing immunity. The existence of ferments, 

 though a possibility, cannot, however, be considered to be 

 yet completely proved. Nor is it certain whether the 

 proteids obtained by precipitation from cultures and from 

 the tissues are in themselves toxic, or whether the toxic 

 bodies are carried down along with them. 



Immunity. This is described in the general chapter 

 on immunity. It is sufficient to state here that a high 

 degree of immunity, against both the bacilli and their 

 toxines, can be produced in various animals by gradually 

 increasing doses either of the bacilli or of their filtered 

 toxines (vide Chap. XIX.). 



Variations in the Virulence of the Diphtheria Bacillus. 

 In cultures on serum the diphtheria bacilli retain their 

 virulence fairly well, but they lose it much more quickly 

 on less suitable media, such as glycerine agar. Roux and 

 Yersin found that, when the bacilli were grown at an 

 abnormally high temperature, namely 39.5 C., and in a 

 current of air, the virulence so much diminished that they 

 became practically innocuous. When the virulence was 

 much diminished, these observers found that it could be 

 restored if the bacilli were inoculated into animals along 

 with streptococci, inoculation of the bacilli alone not being- 

 successful for this purpose. If, however, the virulence had 

 fallen very low, even the presence of the streptococci was 

 insufficient to restore it. As a rule, the cultures most 

 virulent to guinea-pigs are obtained from the gravest cases of 

 diphtheria, though to this rule there are exceptions. Jt 

 has been abundantly established that after the cure of 

 the disease, the bacilli may persist in the mouth for weeks, 

 though they often quickly disappear. Roux and Yersin 

 found by making cultures at various stages after the 

 termination of the disease, that these bacilli in the mouth 



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