170 SUPPURATION PUS COCCI. 



slightly greenish color. They are very granular, with an 

 irregular border, and exhibit some radiation like a strepto- 

 coccus colony. The gelatin is rapidly liquefied, the colony 

 sinking into the medium" as liquefactiorT'progresses. The 

 colony is darker at its centre than at the periphery. 



In a gelatin stab culture the growth develops rapidly at the 

 surface and rather slowly along the line of the needle inocu- 

 lation. The medium is liquefied, the culture settling gradu- 

 ally to the bottom. The liquefaction is not characteristic. 



FIG. 71. 



Bacillus pyocyaneus, from an agar-agar culture. X 1000. (Itzerott and Niemann.) 



The liquefied medium may be colored green or the solid por- 

 tion blue, or the blue and green color may both be present in 

 the same culture. 



On agar-agar the growth develops very rapidly along the 

 stroke. The growth remains white, while the medium is 

 colored green because of the formation of fluoresein, a soluble 

 pigment. If the medium contains an excess of peptone, the 

 green color is displaced by a beautiful deep blue. Bacillus 

 pyocyaneus forms two pigments, fiuorescin, a green pigment, 

 and pyocyanin^ a blue pigment. Pyocyanin is crystallizable. 



The growth on potato is very luxuriant and of a brownish 



