532 BIOLOGY OF PNEUMOCOCCUS 



malin remained in combination with the sedimented organisms. 

 This fact implies that when formalin is added in a concentration 

 greater than that necessary for devitalizing pneumococci, the ex- 

 cess may remain uncombined, and participate in some of the dis- 

 turbing reactions in animals injected with formalinized vaccines. 



Results of investigations by Barnes and Wight* (1935) suggest 

 that formalinized suspensions of pneumococci, particularly those 

 of Type II, undergo rapid autolysis during storage in the re- 

 frigerator; vaccines killed by heat at 56° for one hour are some- 

 what more stable, while those heated at 100° for fifteen minutes re- 

 tain their original turbidity for periods of at least four months. 

 The antigenic potency, in rabbits and horses, of pneumococcal 

 vaccines devitalized by heat at 100° appears to be as high, if not 

 higher, than that of the more readily autolyzable preparations. 



In the authors' experience, pneumococcal vaccines prepared by 

 heating the cultures at 100° for fifteen minutes have yielded satis- 

 factory results in the immunization of rabbits and horses and have 

 been adopted in the routine immunization of the latter animals for 

 the production of antipneumococcic serum. f 



Dried antigens have also been employed, largely on an experi- 

 mental basis, but the preparations have not been used routinely to 

 a sufficient extent to warrant detailed description. 



Standardization. Regardless of the method used in preparing 

 pneumococcal antigens for the routine immunization of animals, it 

 is desirable to standardize the vaccine. In order to obviate labori- 

 ous and time-consuming bacterial counts on each preparation, 

 many of the standardization methods have depended upon a com- 

 parison of the density of the bacterial suspensions with that of 

 suspensions of inert materials such as silicates, fuller's earth, 

 barium sulfate, and so forth, in which a given amount of the mate- 

 rial chosen is made to correspond to a given bacterial count. 



Gates 504 contrived a simple device for determining opacity by 

 measuring the depth of disappearance of a wire loop when im- 



* Unpublished. f See Dubos, p. 332, Chapter X. 



