410 APPLICATION OF METHODS OF BACTERIOLOGY 



Varieties. The foregoing general description of pneumo- 

 coccus suffices for the recognition of the organism as it is 

 frequently found in the normal upper air passages and in 

 cases of pneumonia; that is, it is a lancet-shaped, Gram- 

 positive, encapsulated diplococcus, having the property of 

 fermenting inulin, of dissolving in bile or bile salts and of 

 usually causing septicemia when introduced into the bodies 

 of mice and rabbits. But this description by no means 

 includes certain other important aspects of the subject 

 that have been revealed by special researches. 



It has been shown that the variations in virulence upon 

 animals of those pneumococci isolated from the mouth 

 of normal human beings is of but small importance to 

 an interpretation of the role of the organism in the causa- 

 tion of pneumonia in man; and intimate study of pneu- 

 monias in man, together with the organisms associated with 

 them, have revealed a state of affairs not only not suspected 

 a few years ago, but of the utmost importance to an under- 

 standing of the variations in the disease; of the greater 

 fatality of one expression of the disease over another and 

 the likelihood of the transmission -of the disease from the 

 sick to the well. 



These studies have brought out the fact that in about 80 

 per cent, of all cases of pneumonia pneumococci are present 

 that are markedly different in their specific immunologic 

 reactions from those often found in the normal mouth, and 

 that are distinguished only by such reactions. Such varie- 

 ties of pneumococcus are found only in cases of pneumonia, 

 and if more than one case of pneumonia occur in succession 

 among persons domiciled together the same type of pneu- 

 mococcus will frequently be found in all of them; suggesting 

 the transmission of this particular variety of the disease 

 from one person to another. 



