Specificity 



453 



blood. An interesting paper upon this subject has been written by 

 E. C. Rosenow.* 



The conditions under which it enters the lung to produce pneu- 

 monia are not known. It is probable that some systemic depravity 

 is necessary to establish susceptibility, and in support of this view 

 we may point out that pneumonia is very frequent, and exceptionally 

 severe and fatal, among drunkards, and that it is the most frequent 

 cause of death among the aged. Whether, however, any particular 

 form of vital depression is necessary to predispose to the disease, 

 further study will be required to tell. 



Virulence. Pneumococci vary greatly in virulence, and rapidly 

 lose this quality in artificial culture. When it is desired to maintain 

 or increase the virulence, a culture must be frequently passed through 

 animals. Washbourn found, however, that a pneumococcus isolated 



Fig. 171. Diplococcus pneumoniae. Colony twenty-four hours old upon 

 gelatin. X 100 (Frankel and Pfeiffer). 



from pneumonic sputum and passed through one mouse and nine 

 rabbits developed a permanent virulence when kept on agar-agar 

 so made that it was not heated beyond iooC., and alkalinized 4 cc. 

 of normal caustic soda solution to each liter beyond the neutral point 

 determined with rosolic acid. The agar-agar is first streaked with 

 sterile rabbit's blood, then inoculated. The cultures are kept at 

 37-5C. Ordinarily pneumococci seem unable to accommodate 

 themselves to a purely saprophytic life, and unless continually trans- 

 planted to new media die in a week or two, sometimes sooner. 

 Lambert found, however, that in Marmorek's mixture (bouillon 2 

 parts and ascitic or pleuritic fluid i part) the organisms would some- 

 times remain alive as long as eight months, preserving their virulence 

 during the entire time. 



* "Jour. Infectious Diseases," 1904, I, p. 280. 



