THE PYOGENIC COCCI 159 



in long chains. A differentiation, however, cannot be based on this 

 feature, since cultivation on media relatively unsuitable causes it to 

 disappear. All streptococci are stained by the usual anilin dyes ; 

 the pyogenic group are for the most part Gram positive. 



Cultivation. The most favorable temperature for their growth 

 is from 30 C. to 37 C. ; below 15 C. and above 43 C. growth 

 rarely takes place. They are faultative anaerobes growing both 

 in the presence and absence of oxygen. Upon ordinary nutrient 

 agar their growth is rather scanty. Much more satisfactory results 

 are obtained by cultivating them on media having meat infusion 

 as a basis or on media to which blood, serum, or ascitic fluid has 

 been added. One to two per cent of glucose added to the medium 

 favors development also. In a gelatin stab growth is seen about 

 the second day as a thin line, later it appears to be formed of a 

 row of minute rounded whitish colonies; the growth does not 

 spread on the surface and no liquefaction occurs. On agar or 

 gelatin surface colonies appear as fine grayish, opalescent points, 

 smooth and round, or, as seen under the low-power lens, with a 

 lacelike edge composed of chains of streptococci arranged in loops. 

 Acid is produced in milk and usually coagulation of the casein. 

 In slightly alkaline broth after twenty-four to forty-eight hours 

 the growth frequently appears as a deposit of tiny flakes. The 

 addition of blood or serum to the broth seems to have a marked 

 effect on their chain formation ; the same strain will often remain 

 attached in long chains in the latter case, whereas in ordinary 

 broth they may separate in twos or threes. On Loeffler's blood 

 serum medium growth is rapid and luxuriant. 



The pyogenic streptococci have been divided into two groups 

 on the basis of their ability to cause hemolysis. If blood agar 

 plates are prepared by adding 1 c.c. of fresh or defibrinated blood 

 to 6 c.c. of agar at 43 C. and then inoculated with hemolytic 

 streptococci and incubated for twenty-four hours, each of the 

 colonies will appear to be surrounded with a clear zone due to the 

 destruction of the red blood cells. Related streptococci, on the 

 other hand, produce a zone of a greenish color. The latter are not 

 so virulent and cause rather a chronic form of inflammation. 



