620 



THE PYOGENIC STAPHYLOCOCCI 



precipitate : the funnel of liquefaction subsequently enlarges and reaches 

 the sides of the tube and little by little becomes cylindrical, but it rarely 

 extends to the bottom of the tube. Some strains of the 

 Staphylococcus aureus liquefy gelatin much more slowly 

 than others. One of these strains which in every other 

 respect resembled the Staphylococcus aureus did not com- 

 mence to liquefy gelatin until it had been incubated at 

 20 C. for a fortnight, and even then the liquefaction was 

 always minimal in amount (Besson). 



Single colonies. After incubating for 2-4 days at 20 C. 

 small greyish circular colonies with a yellow centre appear : 

 a little later an annular liquefaction occurs around these 

 colonies, which extends more or less rapidly. Yellow flakes 

 can be seen swimming in the cloudy liquid. 



Agar. Coagulated serum.- At 37 C. numerous white 

 rounded colonies appear in 24 hours along the line of in- 

 oculation ; these rapidly coalesce to form a more or less 

 shiny moist broad band, and the growth soon acquires a 

 colour varying between a dirty yellow and a bright orange 

 yellow. Sometimes the colour does not appear until about 

 the eighth or tenth day. 



Potato. It is on this medium that the Staphylococcus 

 aureus produces the most intense colour. Towards the second 

 or fourth day at 37 C. the growth forms 

 a more or less bright yellow thick layer. 

 Milk. Growth rapidly leads to coagu- 

 g3n! tab Culture in lation of the medium. 



Staphylococcus pyogenes albus. 



This organism has the same characteristics as the Staphylo- 

 coccus aureus except that the growths are always white. On 

 agar the colour is dull white like porcelain. It often 

 liquefies gelatin more slowly than the Staphylococcus aureus 

 [and Gordon has shown that some strains fail to liquefy 

 gelatin. The larger number of the strains examined by Gordon did not clot 

 milk]. 



Staphylococcus pyogenes citreus. 



Here again, the characteristics are the same as those of the Staphylococcus 

 aureus save that the colour of the cultures is a citron yellow. [Two out of 

 the three strains examined by Gordon did not clot milk.] 



coccus pyogenes aureus. 



FIG. 291 . Staphylo- 

 coccus pyogenes aureus. 

 Surface colony on a 

 gelatin plate (5 days). 

 xlO. 



SECTION III. BIOLOGICAL PROPERTIES. 

 1. Viability. Virulence. 



Viability. The staphylococci do not form spores : but retain their vitality 

 in culture for a very long time. In broth they will still be found living after 

 the lapse of a year and on gelatin after an even longer time. 



Cultures of staphylococci are easily killed by heat : e.g. exposure to a 

 temperature of 55 C. for 24 hours, or of 80 C. for 15 minutes is sufficient to 

 sterilize them. In dried pus or other albuminous material staphylococci 

 will resist the action of steam at 100 C. for several minutes. 



Staphylococci in culture are very susceptible to the action of antiseptics 

 but when mixed with albuminoid substances are much more resistant. 



