STREPTOCOCCUS PYOGENES 



185 



Fig. 67. — Streptococcus pyogenes ; young 

 culture on agar, showing chains of cocci. 

 Stained with weak carbol-fuchsin . x looo. 



COCCUS brevis and streptococcus longus {vide infra). As division 



may take place in many of the cocci in a chain at the same time, 



the appearance of a chain of 



diplococci is often met with, the 



line of division lying always at 



right angles to the long axis of 



the chain. In young cultures 



the cocci are fairly uniform in 



size, but after a time their size 



presents considerable varia- 

 tions, many swelling up to twice 



their normal diameter. These 



are to be regarded as involution 



forms. In its staining reactions 



the streptococcus resembles the 



staphylococci 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 dies out more readily, being in every 

 respect a more delicate organism. 



In peptone gelatin sl 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 punc- 

 ture. 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. On 

 the agar media growth takes place along 

 the stroke as a collection of small cir- 

 cular discs of semi-translucent appearance, which show a great 



tendency to remain separate (Fig. 68). The separate colonies 



Fig. 68. — Culture of the 

 streptococcus pyogenes on 

 an agar plate, showing 

 numerous colonies — three 

 successive strokes. Twenty- 

 four hours' growth. Natural 

 size. 



