STREPTOCOCCUS PYOGENES 361 



Under the ordinary conditions of artificial cultivation 

 fully virulent varieties of Streptococcus pyogenes usually 

 lose their virulence after a short time. This property may 

 sometimes be preserved by cultivation upon nutrient gela- 

 tin for two days at 22 C., keeping the cultures after this 

 time in the refrigerator, and transplanting upon fresh gela- 

 tin every five or six days; or by growing the organism in a 

 mixture of 2 parts of horse or human blood serum and 1 

 part of nutrient bouillon, or of 1 part of ascites fluid and 

 2 parts of bouillon. < ^ 



Its virulence may sometimes be increased by passage 

 through a series of susceptible animals. 



Variations in Streptococci. The obiquity of strepto- 

 cocci and their frequent relationship to pathological condi- 

 tions of the gravest nature combine to make them of more 

 than passing interest. Our knowledge of the group is as 

 yet far from satisfactory, yet it has been enhanced in many 

 important particulars during the past three or four years. 



When streptococci are encountered in the various diseased 

 conditions of the body we cannot longer content ourselves 

 with the fact that they conform to the commonly accepted 

 morphological specifications and possess the customary 

 gross cultural peculiarities as outlined above. We have 

 known for a long time that streptococci vary considerably 

 in a number of particulars, and if we arbitrarily decide to 

 call a given streptococcus a typical example of the species 

 we shall find in cultures from different sources very many 

 deviations from such a standard. 



According to the nature of these deviations or variations 

 numerous efforts have been made to arrange the streptococci 

 in more or less fixed groups. Such efforts have not been 

 entirely satisfactory in their results, though they have con- 



