444 PATHOGENIC MICROORGANISMS 



adapted to the production of mass cultures for purposes of immuniza- 

 tion or agglutination. 21 The addition of ascitic fluid or blood serum 

 to broth, in the proportion of one to three, makes an extremely favor- 

 able medium in which growth is rapid and profuse. 



Upon agar plates, pneumococcus growth is not unlike that of strep- 

 tococcus. The colonies are small, round, and slightly more transparent 

 than those of the streptococci. They appear more moist than strepto- 

 coccus colonies and often are more flat. Microscopically examined, the 

 colonies are finely granular, with dark centers and slightly corrugated 

 lighter-colored peripheral areas. Under high magnification no such 

 intertwining convolutions can be seen as those noticed under similar 

 magnification in streptococcus cultures. The addition of animal 

 albumin to agar results in the more rapid development, larger size, 

 and deeper opacity of the colonies. 



Agar stab cultures show growth within twenty-four to thirty-six 

 hours, which takes place with equal thickness along the entire course 

 of the stab. There is nothing distinctive in these cultures to differen- 

 tiate them from similar streptococcus cultures. 



In gelatin plate and stab cultures at 22 C., growth, as a rule, does 

 not take place. This, however, is not true of all races of penumococci. 

 Occasionally strains are met with which will grow fairly abundantly in 

 gelatin at a temperature of 22 C. When the gelatin is rendered 

 sufficiently firm to bear 25 to 26 C. without melting, growth appears 

 slowly and sparsely as minute, grayish-white, transparent colonies. 

 The gelatin is not liquefied. 



Growth upon milk is profuse, resulting in the production of acid 

 and coagulation of the medium. Kaces are encountered in which this 

 is suppressed and coagulation in milk is absent or long delayed. 



Upon potato, thin, gray, moist growth occurs, hardly visible and 

 indistinguishable from an increased moisture on the surface of the 

 medium. This is not a favorable medium. 



Upon Loeffler's coagulated ~blood'serum, the pneumococcus develops 

 into moist, watery, discrete colonies which tend to disappear by a 

 drying out of the colonies after some days, differing in this from 

 streptococcus colonies, which, though also discrete, are usually more 

 opaque and whiter in appearance than those of the pneumococcus 

 and remain unchanged for a longer time. This medium, as will be 

 seen, is useful in differentiating pneumococci from the so-called 

 Streptococcus mucosus. 



21 Hiss, Jour. Exp. Med., vii, 1905. 



