Cultivation and Identification of Bacteria 131 



medium and some of it is caught in the gas trap. Displacement of 

 the Hquid in the inner inverted tube is evidence of gas formation. 

 Five visible changes may take place in tubes of sugar broth under- 

 going microbial action. If the bacteria do not possess the enzymes 

 required to attack specific sugars, growth in the broth will occur, 

 as evidenced by clouding of the medium, but no color change will 

 take place in the medium and no gas will be liberated to be trap- 

 ped in the inverted vial. Acid production without gas formation 

 is a second possibility, and acid together with gas evolution rep- 

 resents the third visible change in the sugar broth. Alkali pro- 

 duction is a fourth possible change, and alkali production coupled 

 with <jas formation is the fifth reaction. 



Brom Cresol Purple Milk 



If brom cresol purple indicator is added to milk, physiological 

 changes taking place in the inoculated milk can be detected and 

 interpreted. Lactose, the natural sugar found in milk, may be at- 

 tacked by some organisms, and if the amount of acid produced is 

 great enough to push the pH down to the curdling point, the milk 

 will exhibit a hard acid curd. Coagulation of milk protein 

 (casein) may also be brought about in the absence of acids, how- 

 ever, by an enzyme called rennet. This enzyme may be extracted 

 from the stomach of calves, and it has been purified and sold on the 

 market as a constituent of custard-like desserts. Some bacteria 

 also produce rennet, and their physiological action may cause 

 sweet-curdling of milk. The curd is called "sweet" because it 

 normally occurs at or near neutrality. Such curds may be easily 

 distinguished from the acid curds which are harder and form only 

 at a low pH. 



Another reaction that may take place in milk is the digestion, 

 or proteolysis, of the milk protein. The breakdown of casein re- 

 sults in the appearance of a clear liquid lacking the opaqueness of 

 ordinary milk. The liquid formed in such proteolysis occurs at or 

 near neutrality and should not be confused with the whey that 

 separates from an acid curd at a low pH. 



