64 LABORATORY GUIDE IN BACTERIOLOGY 



and which animals ? What diseases are 

 caused by the organism in man or animal ? 



Note. — ^It may be well here to call attention to the terms 

 "proteolysis," " enzym-production," and "coagulation." 



"Proteolysis" is the breaking up of complex nitrogenous 

 compounds (proteids), rendering them soluble. This process is 

 also expressed by the terms "peptonization" and "liquefaction." 

 The liquefaction of gelatin is one kind of proteolysis. Gelatin 

 is composed of nitrogenous matter (albuminoid or gelatinoid), 

 and it is for this reason mainly that gelatin stab-cultures are 

 made. If the gelatin is liquefied, we conclude that the organism 

 is capable of producing a "proteolytic" enzym. In milk the 

 process is more complex, and this medium, on account of its 

 composition (fat, milk-sugar, casein, lactalbumin), offers excel- 

 lent opportunities for the organism to develop different charac- 

 teristics. Milk is one of the most important media. The casein- 

 ogen, contained in milk in colloid solution, may be precipitated 

 by an enzym or by an acid. This precipitate forms the coagu- 

 lum. At least 0.4 per cent, of acid, which is largely lactic acid 

 produced by splitting of milk-sugar (lactose), is required for 

 precipitation, and this amount of acid will turn the blue litmus to 

 a decided pink. A coagulum may also be produced by the presence 

 of a "coagulative"' or "rennet"-like enzym, which is the result 

 of the metabolic activity of the organism. Such coagulation may 

 take place in milk of an amphoteric or alkaline reaction, as well 

 as in milk of a slightly acid reaction. The coagulum formed by 

 any of the mentioned agents may gradually contract, and a straw- 

 yellow, opalescent liquid is squeezed out, called "whey." If, 

 now, the organism also produces a proteolytic enzym, this will 

 attack the coagulum and gradually dissolve it (proteolysis, pep- 

 tonization). At first the coagulum shows a broken-up surface; 

 lumps separate and settle to the bottom, and finally the coagulum 

 may completely disappear. Theoretically, coagulation is always 

 necessary before proteolysis sets in, but in the case of some organ- 

 isms the proteolytic enzym is so powerful as to produce immediate 

 dissolution of the casein. 



Another phenomenon frequently observed in litmus milk is 



