STREPTOCOCCUS PYOGENES 177 



in a chain at the same time, the appearance of a chain of 

 diplococci is often met with. In young cultures the cocci are 

 fairly uniform in size, but after a time they present con- 

 siderable variations, many swelling up to twice their normal 

 diameter. These are to be regarded as involution forms. In 

 its staining reactions the streptococcus resembles the staphylo- 

 cocci described, being readily coloured by Gram's method. 



Cultivation. In cultures outside the body the streptococcus 

 pyogenes grows much more slowly than the staphylococci, and also 



. . 



-VV ' 



V 



~ 



-f -,"* 



I ** 



FIG. 55. Culture of the 

 streptococcus pyogenes on 



an agar plate, showing FIG. 56. Bacillus pyocyaneus ; young 



numerous colonies three culture on agar. 



successive strokes. Twenty- Stained with weak carbol-fuchsin. x 1000. 



four hours' growth. Natu- 

 ral size. 



dies out more readily, being in every respect a more delicate 

 organism. 



In peptone gelatin a stab culture shows, about the second day, 

 a thin line, which in its subsequent growth is seen to be formed of 

 a row of minute rounded colonies of whitish colour, which may be 

 separate at the lower part of the puncture. They do not usually 

 exceed the size of a small pin's head, this size being reached about 

 the fifth or sixth day. The growth does not spread on the surface, 

 and no liquefaction of the medium occurs. The colonies in gelatin 

 plates have a corresponding appearance, being minute spherical 

 points of whitish colour. A somewhat warm temperature is 

 12 



