Cultivation 467 



may be made. A small portion of the dilute mixture was poured at a point near 

 the periphery of the prepared plates. By a slight tilting motion. the fluid was 

 carefully distributed over the whole surface of the plates. Care must be taken 

 to avoid an excess of fluid. It was found that plates made in this way gave 

 a sufficiently thick and discrete distribution of surface colonies." 



Cultivation. — The organism grows upon all the culture-media ex- 

 cept potato, but only between the temperature extremes of 24° and 

 42°C., the best development being at about 37°C. The growth is 

 always meager, probably because of the metabolic formation of lac- 

 tic and formic acids. The addition of alkali to the culture-medium 

 favors the growth of the pneumococcus by neutraUzing this acid. 

 Hiss and Zinsser* advise that the culture-media used for the pneu- 

 mococcus be made with 3 to 4 per cent, of peptone. 



Colonies. — The colonies which develop at 24°C. upon gelatin 

 plates (is per cent, of gelatin should be used to prevent melting at 

 the temperature required) are described as small, round, circum- 

 scribed, 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 appear distinctly granular, a dark center being sur- 

 rounded by a paler marginal zone. 



Upon the medium recommended by Buerger for isolating the 

 pneumococcus, the colonies appear in from eighteen to twenty-four 

 hours, the surface colonies being circular and disk-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. 



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 gran- 

 ules distinctly separated from one another. The growth in gelatin 

 is always meager. The medium is not liquefied. 



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

 the growth consists of minute, transparent, semi-confluent, colorless, 

 dewdrop-Uke colonies. The medium is not liquefied. Upon 

 glycerin agar-agar the growth is more luxuriant. The addition of a 

 very small percentage of blood-serum facihtates growth. 



Bouillon.^ — In bouillon the organisms grow well, slightly clouding 

 the medium. With the death of the organisms and their sedimenta- 

 tion, the medium clears again after a few days. 



Milk.^ — Milk is an appropriate culture-medium, its casein being 

 coagulated. AlkaUne litmus milk is slowly acidified. 



Potato. — The pneumococcus does not grow upon potato. f 



Text-book of Bacteriology," 1910, p. 356. 

 tOrtmann asserts that the pneumococcus can be grown on potato at 37°C., 

 put this is not generally admitted. The usual acid reaction of potato makes. 

 It an unsuitable culture-medium. 



