BIOLOGY OF PNEUMOCOCCUS 45 



initiate growth in the medium. With potato broth the beginning 

 period of growth was prolonged and cell death delayed. Moreover, 

 in plant-tissue medium, the zone of hydrogen ion concentration 

 within which growth could be initiated was considerably extended 

 beyond the optimal range in ordinary bouillon. Fresh plant tissue, 

 that is tissue which has not been subjected to heat or oxidation, 

 was necessary for this effect, and this fact explains the action of 

 the tissues in promoting the growth of certain anaerobic bacteria 

 in the presence of oxygen. In the case of Pneumococcus, the action 

 of the oxidizing-reducing system of the V and the X factors* leads 

 to the destruction of toxic peroxides. 



Yeast, in the form of autolysate, in combination with clotted 

 horse blood, peptone, agar, glucose, and maltose was found by 

 Hitchens 654 to be a favorable medium for promoting the growth of 

 pneumococci. 



APPEARANCE OF GROWTH 



In a suitable broth, the first evidence of growth is a faint, uni- 

 form clouding of the medium. There is a slow deposition of a light, 

 flocculent sediment at the bottom of the container. On further 

 incubation there is a gradual clearing of the medium with the ac- 

 cumulation of a heavier deposit, while some of the falling particles 

 may adhere to the side of the vessel. There is no pellicle formation. 

 Growth is more rapid and luxuriant when glucose, the accessory 

 substances already mentioned, or some sterile body fluid are added. 



In milk, Pneumococcus thrives and develops capsules, and if 

 transfers are made at six or eight-hour intervals, the vigor and 

 virulence of the organism are maintained. Prolonged cultivation in 

 this medium without frequent transplants, however, is detrimental 

 to the cocci owing to the acid produced. Milk is coagulated by 



fresh blood in beef-heart broth for the cultivation of pneumococci, while Larson 

 and Thompson"^ employ broth containing sterile, fresh, unheated potato ex- 

 tract for growing pneumococci for type determination and bile-solubility tests. 

 The latter medium has the advantage over serum or blood broth of being free 

 from extraneous substances which might interfere in these tests. 

 * See p. 43. 



