166 BACTERIOLOGY FOR NURSES 



characteristics which aid in differentiating them from the strepto- 

 cocci. 



The pneumococci may be isolated from mixed cultures or sputum 

 by smearing the material over blood agar plates. After twenty-four 

 hours the fine colonies are easily distinguished from all others 

 except the streptococci; from the latter they differ only in that 

 they are transparent, a little more delicate in appearance, and 

 have a smoother edge. 



By another method a small mass of sputum is washed free of 

 extraneous organisms by gently rinsing it in salt solution and 

 then injecting it into the peritoneal cavity of a white mouse. If 

 virulent pneumococci are present, death will occur in from twenty- 

 four to forty-eight hours and the organisms will be found in pure 

 culture in the heart's blood and in the peritoneal exudate. 



Resistance. The pneumococcus is decidedly frail. Apart from 

 the body or body discharges growth soon ceases. On artificial 

 culture media it must be transplanted every three or four days 

 in order to keep it alive, and even then occasional passage through 

 a mouse or rabbit is sometimes necessary to maintain its virulence. 

 In dried sputum it may live for several months and retain its 

 pathogenic power. Low temperatures slightly above zero are 

 quite favorable to the preservation of its vitality. In direct 

 sunlight it dies within an hour. It is quickly killed by a moderate 

 degree of heat; ten minutes' exposure to 52 C. is sufficient. 

 Mercuric chloride 1 to 1000 will destroy it in five minutes and car- 

 bolic acid 1 to 100 in from five to ten minutes. 



Pathogenesis. The pneumococcus is frequently found in the 

 saliva of healthy individuals. The New York Commission reported 

 its presence in 45 per cent of a number of persons examined. Thus 

 many apparently normal individuals are pneumococcus " carriers." 

 It has been shown, however, that the majority of such cases are 

 only " temporary carriers." The organism leaves the body mainly 

 in the discharges from the mouth and nose and enters the system 

 through the same channels. 



Most strains are pathogenic for a number of animals. A small 

 amount of sputum containing virulent pneumococci will cause 



