202 



INFLAMMATION AND SUPPURATION 



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 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 lique- 

 faction of the medium occurs. The 

 colonies in gelatin plates have a cor- 

 responding appearance, being minute 

 spherical points of whitish colour. 

 A somewhat warm temperature is 

 necessary for growth ; even at 20° C. 

 some varieties do not grow. On the 

 agar media, growth takes place along 

 the stroke as a collection of small 

 circular discs of semi-translucent 

 appearance, which show a great 

 tendency to remain separate (Fig. 

 47). The separate colonies remain 

 small, rarely exceeding 1 mm. in 

 diameter. Under a low power of the 

 microscope they have a slightly woolly 

 margin. Cultures on agar kept at 

 the body temperature may often be 

 found to be dead after ten days. On 

 potato, as a rule, no visible growth 

 takes place. In milk it produces a 

 strongly acid reaction but no clotting of the medium. It 

 ferments lactose, saccharose, and salicin (Andrewes and Border) ; 

 it produces no fermentation of inulin in Hiss's serum-water- 

 medium, in this respect differing from the pneumococcus. It 

 has usually a strong hsemolytic action, as can be demonstrated 

 by growing it in blood-agar plates (p. 45). In bouillon, growth 

 forms numerous minute granules which afterwards fall to the 

 bottom, the deposit, which is usually not very abundant, having 

 a sandy appearance. The appearance in broth, however, -presents 

 variations which have been used as an aid to distinguish different 

 species of streptococci. It has been found that those which form 



Fig. 4/.— Culture of the 

 streptococcus pyogenes on 

 an agar plate, showing 

 numerous colonies — three 

 successive strokes. Twenty- 

 four hours' growth. Natu- 

 ral size. 



