292 Pneumonia 



cause of the metabolic formation of formic acid. The addi- 

 tion of alkali to the culture medium favors the growth of 

 the pneumococcus by neutralizing this acid. 



Colonies. The colonies which develop at 24 C. upon 

 gelatin plates (15 per cent, of gelatin should be used to 

 prevent melting at the temperature required) are described 

 as small, round, circumscribed, finely granular white points 

 which grow slowly, never attain any considerable size, 

 and do not liquefy the gelatin. 



If agar-agar be used instead of gelatin, and the plates 

 kept at the temperature of the body, the colonies appear 

 transparent, delicate, and. dewdrop-like, scarcely visible to 

 the naked eye, but under the microscope appearing dis- 

 tinctly granular, the dark center being surrounded by a paler 

 marginal zone. 



Upon the medium recommended for isolating the pneu- 

 mococcus by Buerger, the colonies appear in from eighteen 

 to twenty-four hours, the surface colonies appearing circular 

 and disc-like. When viewed from above, the surface appears 

 glassy with a depressed center. When viewed from the side 

 or by transmitted light, they appear as distinct milky rings 

 with a transparent center. This "ring type" is regarded as 

 characteristic and enables the organism to be separated 

 without difficulty from the streptococcus. 



Gelatin Punctures. In gelatin puncture cultures, made 

 with 15 instead of the usual 10 per cent, of gelatin, the 

 growth takes place along the entire puncture in the 

 form of minute whitish granules distinctly separated from 

 one another. The growth in gelatin is always very meager. 



Agar-agar and Blood-serum. Upon agar-agar and 

 blood-serum the growth consists of minute, transparent, 

 semi-confluent, colorless, dew-drop-like colonies, which die 

 before attaining a size which permits of their being seen 

 without careful inspection. Upon glycerin agar-agar the 

 growth is more luxuriant. The addition of a very small 

 percentage of blood-serum greatly facilitates the growth. 



Bouillon. In bouillon the organisms grow well, slightly 

 clouding the medium. 



Milk. Milk is an appropriate culture medium, its casein 

 being coagulated. Alkaline litmus milk is slowly acidified. 



Potato. The pneumococcus does not grow upon potato.* 



* Ortmann asserts that the pneumococcus can be grown on potato at 

 37 C., but this is not generally confirmed. The usual acid reaction of 

 the potato would indicate that it was a very unsuitable culture medium. 



