160 BACTERIOLOGY FOR NURSES 



The inulin serum medium of Hiss is frequently used to differen- 

 tiate the streptococcus pyogenes from pneumococci. The latter 

 produce acid and coagulation of the serum, while the former are 

 unable to ferment inulin. 



Resistance. On culture media, unless transplanted, strepto- 

 cocci do not live more than two to fourteen days ; in body dis- 

 charge they may live for several weeks. They are killed by 

 exposure to a temperature of 54 C. for twenty minutes; low 

 temperatures have less effect upon them. Exposure to sunlight 

 kills them in a few hours. Mercuric chloride 1 to 1000 destroys 

 them in from five to ten minutes, carbolic acid 1 to 100 in from 

 five to forty-five minutes. 



Pathogenesis. Attempts have been made to classify the 

 streptococci according to their pathogenicity or their growth on 

 artificial culture media; such attempts have not been very suc- 

 cessful because the differences observed are not constant. What 

 may have been thought to be a definite characteristic may become 

 totally changed under other conditions. The animals ordinarily 

 used for experimentation are not so susceptible to streptococci 

 infection as man, and different animals show different degrees of 

 susceptibility to different cultures. A virulent strain when in- 

 jected into a mouse will cause septicemia; those of a little less 

 virulence will produce the same result if the quantity is increased ; 

 others still less virulent will produce septicemia if injected into a 

 vein, but if introduced into subcutaneous tissue will produce an 

 abscess or erysipelas. Others even less virulent when injected in 

 large amounts will only produce a slight inflammation or no re- 

 action at all. 



Experiments have shown that streptococci originally virulent 

 may become non-virulent after long cultivation on artificial 

 culture media, but that after passage through an animal they 

 regain their lost power. 



In man streptococci are responsible for a greater variety of 

 lesions than any other microbes, and in addition to the number 

 of diseases they themselves cause they are present in " secondary " 

 or " mixed " infection more often than any other organisms. 



