BIOLOGY OF PNEUMOCOCCUS 47 



of colonies after some days, differing in this from streptococcus 

 colonies."* 



DIFFERENTIAL MEDIA 



In addition to the diagnostic features of pneumococcal colonies 

 grown on Loeffler's medium and on the rabbit blood-agar mixture, 

 other media have been recommended as serving to develop charac- 

 ters that distinguish pneumococci from streptococci. Hiss/ 50 for 

 this purpose, devised a medium consisting of one part of beef se- 

 rum and two parts of distilled water, to which was added one per 

 cent of inulin (C.P.) and enough litmus to render the medium a 

 clear, transparent blue. By fermentation of the inulin, Pneumococ- 

 cus acidifies the mixture, causing coagulation of the serum. The 

 method is useful except in the case of those rare strains of pneu- 

 mococci which are not inulin fermenters. A similar, but solid, me- 

 dium for the same purpose was that recommended by Ruediger. 1190 

 To sugar-free broth, containing one per cent of Witte peptone and 

 1.5 per cent of agar, he added approximately 1.5 per cent of pure 

 inulin and a small amount of Merck's highest purity litmus. One 

 cubic centimeter of ascites fluid heated to 65° was added to each 

 tube of melted agar. Pneumococci are distinguished by the forma- 

 tion of red colonies on this medium. 



Buerger 163 claimed that the ring type of colon} 7 on serum, or bet- 

 ter serum-glucose agar, was distinctive of Pneumococcus when 

 compared with colonies of Streptococcus on the same medium. 

 The method required a close inspection of the growth under trans- 

 mitted and reflected artificial light. When viewed from the side or 

 by transmitted light, the pneumococcal colony shows a distinct 

 milky ring enclosing a transparent center, while the streptococcal 

 colony has a prominent periphery and a definite nucleus. These 

 and other physical characters serve to differentiate these two spe- 

 cies of cocci. 



* From Zinsser and Bavne-Jones.i579 



