THE PNEUMOCOCCUS Ai^D PNEUMONIA 445 



Upon mixtures of whole rabbit's blood and agar, the pneumococcus 

 grows well, and forms, after four or five days, thick, black surface 

 colonies, not unlike sun blisters on red paint. These colonies are easily 

 distinguished from those of streptococci, and are of considerable 

 differential value. 22 



Pneumococcus colonies on blood plates may cause a slight halo of 

 hemolysis and methemoglobin formation with a zone of greenish color 

 about the colony after 48 hours or longer in the incubator. 



The hemolysin formation by pneumococcus is slight, but quite 

 definite. It occurs late, rarely sooner than 48 hours, but is mentioned 

 because errors of diagnosis through ignorance of this might occur. 

 Cole 23 has particularly studied the hemotoxins of this organism. 



Guarnieri 24 has recommended a medium with a pepton-beef- 

 infusion basis rendered semisolid by mixtures of agar and gelatin. 

 A modification of this medium has been described by Welch 25 and has 

 been much employed. Cultivation within eggs and upon egg media 26 

 has been used. Wadsworth 27 has recommended a medium composed 

 of ascitic fluid to which agar has been added sufficient to give a soft, 

 jelly-like consistency. He observed prolonged viability and the preser- 

 vation of the virulence on this medium. 



For the purpose of differentiating pneumococci from streptococci, 

 Hiss 28 devised a medium of beef serum one part, and distilled water 

 two parts, to which is added one per cent of inulin (c. p.), and enough 

 litmus to render the medium a clear, transparent blue. By fermenta- 

 tion of the inulin the pneumococcus acidifies this mixture, causing 

 coagulation of the serum. Streptococci do not ferment inulin. 



Since inulin fermentation is a very important differential character- 

 istic of the pneumococcus, it is necessary to say a few words about 

 the irregularity with which it occurs. Occasionally, failure to ferment 

 inulin is due to the fact that the particular strain of pneumococcus 

 does not grow well in the inulin medium made up by the older method 

 of Hiss. The addition of one per cent of pepton to this medium has 

 been suggested by Burger and is a distinct improvement. It may 



22 Hiss, loc. cit. 



23 Cole, Jour. Exp. Mc<L, 1914, 20, 346 and 363. 

 s * Giumticri, Alt. dell' Aead. di Roma, 1883. 

 -Wr1ch, Johns Hopk. Hosp. Bull., iii, 1892. 

 2 SSV/ro, Kiv. d'lgiene, 1894. 



27 Wadswortli, Proc. N. Y. Path. Soc., 1903. 



28 Hiss, Jour. Exp. Med., vi, 1905. 



