STAPHYLOCOCCUS PYOGENES AUREUS 



387 



the medium, and giving rise, after forty-eight or more hours, to the 

 formation of a thin surface pellicle. As growth increases, the bacteria 

 sink to the bottom, forming a heavy, mucoid sediment. The odor of 

 old cultures is often peculiarly acrid, not unlike weak butyric acid. 



FIG. 44. STAPHYLOCOCCUS COLONIES. 



In milk, staphylococcus causes coagulation usually within three or 

 four days, with the formation of lactic and butyric acids. 



On potato, growth is abundant, rather dry and usually deeply 

 pigmented. 



Upon coagulated animal sera, rapid growth takes place and even- 

 tually slight liquefaction of the medium occurs. 



In nitrate solutions, reduction of the nitrates to nitrites is caused. 



In Dunham's broth, indol is not formed. Bayne- Jones and Zin- 

 ninger have studied 115 strains of various staphylococci in all kinds 

 of media suitable for the production of indol, and have not found a 

 single indol producer. 



In media containing the carbohydrates dextrose, lactose, or sac- 

 charose acidification takes place with the formation chiefly of lac- 

 tic, butyric, and formic acids. There is no gas formation, however. 



