ENZYME-FORMING BACTERIA. 77 



by means of the rennet which they produce, first commonly 

 curdle the milk into a soft curd ; then the curd begins slowly 

 to be digested by the tryptic ferment, and, as it is digested, 

 it is also dissolved. This digestion and solution of the curd 

 may continue until the whole is completely dissolved, form- 

 ing a more or less transparent liquid. With some species, 

 however, the digestion does not continue so far and stops 

 after the curd has been half dissolved, so that the final result 

 is a small mass of soft curd floating in a liquid (Fig. 14, 

 c and d). There are many species of bacteria belonging to 

 this group and they produce a variety of different effects on 

 the milk. Sometimes the milk which has been acted upon 

 by them becomes almost as clear as water ; more commonly 

 it is slightly cloudy, sometimes very cloudy. It may be color- 

 less, or it may become of a brownish, an amber, or a yellow- 

 ish, or even a greenish or chocolate color. The chemical 

 changes that occur are extremely complex, but we know 

 almost nothing in regard to them. In general they are 

 changes of decomposition and frequently those called putre- 

 factive. The resulting digested milk very commonly develops 

 prominent odors, and sometimes very strong odors of putre- 

 faction and decay. This group of bacteria in general must 

 be looked upon as containing the putrefying bacteria, and all 

 of them produce chemical decomposition. 



Both enzymes are commonly developed by each of the 

 species of bacteria in this class. In most cases the rennet at 

 the outset is developed rapidly in sufficient quantities to 

 curdle the milk. Later the milk is dissolved by the action 

 of the tryptic ferment. In some cases, however, the tryptic 

 ferment is developed so' rapidly at first that the digestion and 

 solution of the casein occurs before the rennet ferment has 

 developed sufficiently to curdle the milk. In this case the 

 milk shows no signs of curdling, but slowly becomes trans-- 

 parent or semi-transparent. Experiment has shown, how- 



