Pathogenic Bacteria 331 



are grown on blood agar media, three different reactions are pro- 

 duced by various species with respect to hemolysis : 



Alpha hemolysis— A greenish discoloration of the medium ad- 

 jacent to the colony, called a viridans type reaction. 



Beta hemolysis— A punched-out appearance on blood plates due 

 to complete lysis of red blood cells around the colony. 



Gamma reaction— No visible change noted in blood agar. 



Streptococci are killed by a few minutes exposure to boiling 

 temperature, and pasteurization destroys them. Their resistance 

 to drying, however, aids in their spread from infected persons to 

 susceptible individuals, since the organisms can remain viable in 

 dust for long periods of time. Selected antibiotics, penicillin in 

 particular, are effective in combatting streptococcal infections. 



In addition to their reactions on blood-containing media, 

 streptococci are classified on the basis of serological reactions, 

 particularly the precipitin test. Lancefield classified these organ- 

 isms according to their antigenic components into the following 

 groups: 



The usual reactions accompanying infections with streptococci 

 can be traced to the products of metabolism of these organisms: 



HEMOLYSIN, FIBRINOLYSINS, SPREADING FACTOR, SKIN NECROTIZING 



TOXIN, etc. Antitoxins can be produced to combat exotoxins, but 

 injections of such antibodies will act only against the true toxins, 

 not against the bacteria themselves. Antibiotics and sulfa drugs 

 must be employed to combat the bacteria, but the passive pro- 

 tection afforded by antitoxins is necessary to neutralize the cir- 

 culating poisons. 



