272 SPECIFIC MICRO-ORGANISMS 



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' asci tic-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 

 of the small colony surrounded by a very clear zone of hemolysis 

 which the streptococcus produces on this medium. In making 

 cultures from the blood in bacteremia, plain agar previously 

 melted and cooled to 45" C. is mixed with freshly drawn blood 

 of the patient and allowed to solidy in a Petri dish. In other 

 cases naturally sterile defibrinated rabbit's blood niay be used, 

 the technic of plating being analogous to that described for the 

 gonococcus. The streptococcus grows very slowly below 20° C. 

 and poorly, in ordinary gelatin, which it does not Uquefy. Oil 

 solid media, agar or serum-agar, at 37° C, small round elevated 

 colonies develop, 0.5 to i.o mm. in diameter, and they tend to 

 remain discrete. In broth only a slight cloud develops, but 

 considerable granular deposit made up of streptococci is found at 



