212 BIOLOGY OF SPECIALIZED GROUPS 



succinic acid, carbon dioxide, and hydrogen. Acetic, lactic, 

 butyric, proprionic, malic, citric, succinic, and other acids on 

 fermentation are each converted into varying amounts of the 

 others and eventually into carbon dioxide and water. The 

 M. pneumonias also ferments mannite. 



Fischer refers to the various dextro- and levo-rotatory 

 acids which may be produced in fermentations. For example, 

 the enzymes of B. coli change dextrose into lactic acids which 

 differ in the direction and the extent to which polarized light 

 is rotated. The source of the nitrogen supply seems to be a 

 determining factor in this case. If ammonium phosphate is 

 supplied, levo-rotatory lactic acid results ; while if peptone 

 is used as a source of nitrogen, dextro-rotatory acid is pro- 

 duced. Lactic acid produced by other fermentations is 

 optically inactive (levo- and dextro-rotatory). 



Zymo genie Bacteria in Milk and its Products. The 

 lactose or milk sugar which is normally present in milk under- 

 goes decomposition by the action of the enzymes of various 

 species of bacteria after the milk has been standing for some 

 time at a warm temperature. The acidity is evident to the 

 sense of taste when it reaches the equivalent of 0.3 per cent 

 lactic acid. As the acid is produced, the caseinogen of the 

 milk is acted upon by bacterial enzymes similar to rennet, 

 and, as a result, the casein formed is precipitated, thus pro- 

 ducing a clot or curd. This takes place when the acid reaches 

 a concentration of 0.35 per cent to 0.4 per cent. The result- 

 ing coagulum or curd becomes more solid as the acid increases. 



