PRINCIPLES IN METHODS OF ISOLATION 107 



species of bacteria. As was said above, and as we shall soon 

 see for ourselves, certain bacteria elaborate in the course of 

 their growth digestive enzymes or ferments that in their 

 action upon proteid matters are strikingly like pepsin in 

 some and trypsin in other instances. When bacteria en- 

 dowed with this physiological property are cultivated upon 

 bone gelatin their growth is accompanied by the progressive 

 digestion (liquefaction) of the gelatin, which liquefied 

 gelatin cannot again be brought to a solid condition. We 

 know of no bacteria capable of producing a similar lique- 

 faction of agar-agar or vegetable gum. 



As a rule, the colony-formations seen upon gelatin are 

 much more characteristic than those which develop on agar- 

 agar, and for this reason gelatin is to be preferred when 

 circumstances will permit. Both gelatin and agar-agar may 

 be used for the isolation of species from mixtures. 



