32 BIOLOGY OF PNEUMOCOCCUS 



dia, stained by an aniline dye, phenol, and iodine, with a mixture 

 of one part 40 per cent aqueous solution of urea and nine parts 

 absolute alcohol. Organisms which retained the dye were called 

 positive; those from which the dye was extracted were negative; a 

 third class exhibited fluctuations in their ability to retain the ani- 

 line color; while the fourth group displayed periodic changes from 

 positive to negative and vice versa. He reported that all strains of 

 streptococci tested were without exception negative, while Strepto- 

 coccus mucosus and thirty-nine strains of pneumococci showed a 

 change from positive to negative or vice versa. Some of the latter 

 gave predominantly positive results while some were usually nega- 

 tive with only an occasional positive reaction. These latter he con- 

 sidered as transition forms between the predominantly positive 

 pneumococci and the uniformly negative strain of Streptococcus. 

 For demonstrating pneumococci in tissues the method described 

 by Wadsworth* may be used to advantage, while the Gram stain 

 is to be preferred to the Gram-Weigert technique.f 1506 



THE CAPSULE 



Modern chemical study of the capsular material has given a ra- 

 tional basis for the serological classification of all pneumococci 

 into definite and specific types. It is this peculiar and complex com- 

 ponent of the pneumococcal cell that determines its specific anti- 

 genic stimulus and its immunological behavior in the presence of 

 antibodies. This knowledge enables us to utilize in a far more in- 

 telligent way Pneumococcus or its components in the induction of 

 active immunity and consequently in the production of immune 

 serums, and furnishes us with a delicate reagent for measuring the 

 body's response to various immunizing procedures, and for deter- 

 mining the potency of antipneumococcic serums. 



The capsule interested and perplexed many of the earlier investi- 

 gators. Described first by Pasteur 1066 as an aureole, and by Fried- 



* Zinsser and Bayne-Jones.1579 



f Neufeld prefers the Gram technique; Zinsser the Gram-Weigert technique. 



