198 PATHOGENIC BACTERIA. 



The superficial colonies upon gelatin plates form small, 

 irregular, ill-defined collections, which produce a fluores- 

 cence of the neighboring 

 gelatin. The gelatin soft- 

 ens gradually, and about 

 five days elapse before ;'_; 

 liquefaction is complete. 



The microscope shows 

 the colonies to be round, \ 

 coarsely-granulated masses 





FIG. 57. Bacillus pyocyaneus: colonies upon gelntin (Abbott). 



with notched or filamentous borders. They have a yel- 

 low-green color. Upon the surface they form a delicate 

 clump with a smooth surface, finely granular, distinctly 

 green in the middle and pale at the edges. The colonies 

 sink into the gelatin as the liquefaction progresses. 



In gelatin puncture-cultures most of the development 

 occurs at the upper part of the tube, where a deep saucer 

 of liquefaction forms. The growth slowly descends into 

 the medium, and is the point of origin of a beautiful 

 fluorescence. The bacterial growth sinks to the bottom 

 as it ages. At times a delicate mycoderma forms on the 

 surface. 



Upon agar-agar the growth is at first bright green, 

 developing all along the line of inoculation. The green 

 pigment (fluorescin) is soluble, and soon saturates the cul- 

 ture-medium and makes it very characteristic. As the 

 culture ages, or if the medium upon which it grows 

 contains much peptone, a second pigment (pyocyanim is 

 developed, and the bright green fades to a deep blue- 

 green, dark-blue, or in some few cases to a deep reddish- 

 brown. 



A well-known feature of the growth upon fresh agar- 

 agar, upon which much stress has recently been laid by 



