COCCACE^E AND THEIR PARASITIC RELATIONSHIPS 261 



is no specific distinction between the streptococci found in ery- 

 sipelas and those found in other lesions. The difference in the 

 pathological process depends rather upon the portal of entry of 

 the infection, the virulence of the microbe and the resistance of 

 the host. 



Streptococcus pyogenes lives naturally upon the mucous 

 membranes, especially in the pharynx, nose and mouth, the 

 intestine and on the vaginal mucosa. Such streptococci found 

 in normal individuals are relatively non-virulent. Virulent 

 streptococci occur in erysipelatous lesions of the skin, in infected 

 wounds, on the inflamed pharyngeal mucosa, and in the lochia, 

 uterine wall and in the circulating blood in puerperal fever. 

 Streptococci are frequently found in pyemic abscesses, bacteremia, 

 meningitis and pneumonia. It seems probable that these virulent 

 races originate from the ordinary relatively harmless parasitic 

 forms in some instances, when an opportunity is presented for 

 successful invasion of tissues by a lowered resistance of the host, 

 and that by successive transfer from one susceptible individual to 

 another the virulence is still further enhanced. 



The individual cells of a chain vary in size from 0.6 to 1.5^ 

 and in form from flattened disks to long ovals. The chains are 

 variable in length and in general the more virulent types form 

 longer chains in broth cultures. In old cultures the cells are very 

 irregular in size, and it was once supposed that the larger spheres 

 were special resistant forms, "arthrospores." They are now 

 regarded as involution or disintegrating forms. The streptococcus 

 stains readily and is Gram-positive. 



Cultures on ordinary media are relatively poorly developed 

 and of short life. Broth or glucose broth serves very well, and 

 a few cultures in series may be obtained on glycerin agar or glu- 

 cose agar. Loffler's blood serum is better than these. Serum 

 agar, ascitic-fluid agar and blood agar are the best solid media 

 and ascitic-fluid broth is an excellent fluid medium for cultiva- 

 tion of streptococci. Blood agar is especially valuable in plating 

 pus or exudates because of the rather characteristic appearance 



