36 A MANUAL OF BACTERIOLOGY 



addition. Milk which has been just boiled usually under- 

 goes the butyric rather than the lactic fermentation, the 

 spores of the butyric organisms surviving. Lactic acid is 

 first formed, and this is then converted into butyric acid : 



2C 3 H 6 3 - C 4 H 8 2 + 2C0 2 + 2H 



The acetic acid fermentation. The conversion of alcohol 

 into acetic acid is also due to bacteria, familiar examples 

 of which are the souring of beer and wine. 



Bacterial enzymes. Many changes brought about by 

 bacteria and other micro-organisms are due to enzymes, 

 which may be not only intra-cellular but may escape from 

 the cells into the medium in which they are. The most 

 familiar example is the peptonising enzyme produced by 

 bacteria which liquefy gelatin and digest coagulated 

 protein, fibrin, etc. The enzymes differ : an organism 

 which liquefies gelatin does not necessarily digest blood- 

 serum. The proteolytic enzyme is tryptic in nature and 

 escapes from the cells into the surrounding medium, so 

 that some of the liquefied gelatin free from cells or in 

 which their action is inhibited by an antiseptic, liquefies 

 other gelatin if added to it. Amylolytic enzymes are 

 also produced, such as amylase (digesting starch), maltase, 

 lactase, inulase, and invertase. Lipases and rennet-like 

 enzymes also occur. " Fermentation " of urea takes 

 place by means of an enzyme secreted by the Micrococcus 

 ureoe, etc., with the formation of ammonium carbonate. 

 These enzymes do not seem to possess any poisonous 

 action. 



Formation of pigment. Numerous organisms, especially 

 those of air and water, during their growth produce various 

 coloured pigments. They are termed " chromogenic 

 bacteria," examples of which are the Sarcina lutea and 

 Micrococcus cereus, var. flavus, which form citron-yellow 

 pigments ; the Bacillus prodigiosus and Spirillum rubrum t 



