204 BIOLOGY OF PNEUMOCOCCUS 



when the reaction of the medium was adjusted to pH 8 or 9, viru- 

 lence decreased ; the more alkaline the reaction the more rapid the 

 decrease. 



When cultivated in broth by the same method, 425 the Type I cul- 

 ture lost virulence as the reaction of the medium by adjustment 

 became more acid. Virulence fluctuated with the frequency of trans- 

 fer of the organism to fresh broth. In the case of transplants made 

 every hour, virulence immediately decreased. When the interval 

 was two, four, or eight hours, there was first a rise and then a fall 

 of virulence, the rise being greatest in the case of eight-hour and 

 least with the two-hour transfers. With the hydrogen ion concen- 

 tration at set points the amount of meat infusion influenced the 

 virulence of the culture, the unfavorable action varying in inverse 

 proportion to the concentration of meat in the substrate. The ad- 

 dition of glucose neutralized the action of the meat. Peptone in 

 the broth also affected the virulence of the strain studied, Felton 

 and Dougherty reporting that the nutrient in a concentration of 

 2 per cent maintained and even increased virulence of the strains 

 studied. 



In a later communication (1932), Felton 411 " 2 described other 

 nutritional factors that affect the virulence of pneumococci. Media 

 made from calf lung or heart or from the skeletal muscles of the 

 horse maintained for a long period of time the virulence of the cul- 

 tures used ; conversely, media prepared from calf spleen led to a 

 decrease in pathogenicity. Normal horse serum, or specific immune 

 serum freed from protective antibody, preserved virulence. Media 

 made from rabbit muscle were less suitable for the purpose than 

 media prepared in the same way from the meat of guinea pigs. 

 When grown in the automatic transfer device in a medium which 

 aerobically maintained the virulence of a Type I culture, the addi- 

 tion of pure oxygen or pure carbon dioxide lowered virulence, but 

 no change in infectivity was noted when the organisms were culti- 

 vated in the presence of nitrogen. Gradual increase in the tem- 

 perature of incubation from 36.5° to 42° over a period of ten 



