BACTERIOLOGICAL TECHNIC 63 



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 red. 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 amphoteric or alkaline 

 reaction, as well as in milk of slightly acid reaction. The coagu- 

 lum formed by any of the mentioned agents may gradually 

 contract, and a straw-yellow, opalescent liquid will be squeezed 

 out, called "whey." If the organism also produces a proteolytic 

 enzym, this will attack the coagulum and gradually dissolve it 

 (proteolysis, peptonization). At first the coagulum shows a 

 broken-up edge; lumps separate and settle to the bottom, and 

 finally the coagulum may disappear completely. Theoretically, 

 coagulation is always necessary before proteolysis sets in, but in 

 the case of some organisms the proteolytic enzym is so powerful 

 as to produce immediate solution of the casein. (See Figs. 21, 

 22, 23, 24.) 



Another phenomenon frequently observed in litmus milk 

 is the decolorization of litmus, whether this be pink or blue. 

 This is due to the fact that the organism takes up the oxygen 

 necessary to maintain the coloration. It may be frequently 

 observed that at the surface, where atmospheric oxygen has 

 access, the color remains or is restored. The color may also be 

 restored by shaking the milk vigorously, thus bringing it into 

 intimate contact with the oxygen of the air. 



The production of an amylolytic enzym (diastase, amylase) is 

 demonstrated by gas production on potatoes. This medium con- 

 tains starch in large amounts. The starch is converted into 

 maltose by diastase (amylase). Maltose may then be split by 

 the action of an inverting enzym into dextrose, which is then 

 fermented with gas production. 



Second page — 



1. Note the reaction of the medium. 



2. The incubation temperature may be (a) 37° C. 

 (thermostat), (6) room temperature (20° C.),' (c) ice 

 chest. 



' Gelatin cultures are to be incubated at room temperature. 



