32 



BIOLOGY OF BACTERIA. 



pigments are the result of bacterial action on the albumins 

 and peptones. Some pigments are soluble in water and 

 others are not. Therefore we may see either the entire 

 medium colored or only a portion of it ; the bacteria them- 

 selves are free from pigment. It has already been men- 

 tioned that some germs produce pigment only in the presence 

 of light and others only in its absence. Oxygen appears to 

 be necessary for color-production. Some bacteria produce 

 more than one pigment. The Bacillus pyocyaneus produces 

 a blue and a green pigment ; one is soluble and the other is 

 insoluble. High temperatures check color-production. The 

 reaction of the media modifies the shade of the pigment. 



FIG. 9. 



Fermentation-tube on left side, ordinary tube on right side. 



Phosphorescence : Some bacteria exhibit strong phospho- 

 rescence in culture. They are designated as photogenic bac- 

 teria. Most of these varieties are found in sea-water. Suf- 

 ficient illumination may be caused by a single culture to 

 enable one to tell the time by the watch in a dark room. 

 Sea-water gelatin is the best culture-medium. The Bacillus 

 phosphorescens is the type of this group. 



Gases: Certain bacteria produce gases as the result of 

 decomposition and fermentation. These gases are not detected 

 unless searched for by special methods. The most common 

 gases are CO 2 , CH 4 , H 2 S, NH 4 . Gas-production is deter- 

 mined by the use of the fermentation-tube (Fig. 9). The tube 



