76 THE TYPES OF MILK BACTERIA. 



a slight alkaline reaction in the milk while some do not 

 change the reaction at all. Their distinctive characteristic, 

 however, is the production of enzymes. Two quite distinct 

 types of action on milk are recognized, a curdling action and 

 a digestive action. Not only do we see two types of action, 

 but two distinct enzymes or chemical ferments have been sepa- 

 rated from the bacteria cultures by chemical means, and we 

 have, therefore, demonstration that the action of these organ- 

 isms is through the production of certain chemical ferments. 

 The enzymes produced by these bacteria are as follows : 



Rennet. Most of the organisms in this group, perhaps all 

 of them, produce a certain quantity of an enzyme that has 

 the essential characteristic of rennet (62). Like the ordi- 

 nary rennet, it is capable of curdling milk rapidly and pro- 

 ducing a soft curd, which is commonly either alkaline in 

 reaction or amphoteric. The curd thus produced is sweet 

 rather than sour, and the action of these bacteria is, there- 

 fore, the common cause of a phenomenon appearing in the 

 dairy kno.wn as the " sweet curdling " of milk. The bac- 

 teria commonly require two days' growth in the milk before 

 they curdle it, although some of them act more rapidly and 

 some more slowly. The curd may usually be distinguished 

 from the acid curd by being soft, while the latter is hard. 



Tryptic Ferment. The second enzyme produced by these 

 bacteria is akin to the tryptic ferment, called casease by Du- 

 claux (3). It has characteristics very similar to the trypsin 

 produced by the digestive glands, though it has not been 

 studied in sufficient detail to determine whether it is strictly 

 identical with trypsin. Its most noticeable characteristic is 

 its power of digesting proteid material, which it converts 

 more or less completely into soluble forms, like peptone, etc. 



The action of these bacteria upon milk is striking. The 

 casein in the milk is in the form of a solid or semi-solid 

 material held in practical solution. The bacteria in question, 



