102 THE ACTION OF ENZYMES 



oxidizing agents. Probably in this way are produced 

 the formic, acetic, propionic, butyric, valerianic, caproic, 

 and other similar acids so frequently found in a free 

 state or as ammonium salts in putrefying substances. 

 Cinnamic and oxalic acids are sometimes present. 



Two aromatic bodies, p-cresol and phenol, are often 

 obtained also, and are doubtless derived from tyrosine. 



The most characteristic substances produced in the 

 putrefaction of proteins by bacteria are indole and skatole, 

 two compounds to which the unpleasant odour of faeces 

 is due. These are derived in some instances directly 

 from the proteins, but they are also produced from 

 tryptophane. The amount obtained is very variable, 

 and is dependent upon the species of bacterium causing 

 the decomposition, the nature of the medium in which 

 it is grown and certain factors which are not understood. 

 Some bacteria, such as the cholera organism and Bact. 

 vulgare, manufacture considerable amounts of indole when 

 grown in peptone solutions, while others give rise to little 

 or none. The presence of sugar generally interferes with 

 its formation. Bact. colt is able to form indole in broth 

 or peptone solutions so long as the liquid is neutral, but 

 none is obtained when the medium is acid, or when 

 much sugar is present. In view of the fact that one 

 species may produce it, and another nearly allied form 

 be devoid of that power, the presence or absence of indole 

 in a culture medium may be used as a valuable diagnostic 

 character in the identification of bacteria. Methods of 

 testing for it are given in Ex. 63. 



From among the products of putrefaction a number 

 of weak bases have been isolated, some of which are 

 crystalline compounds, resembling the vegetable alkaloids : 

 they are known as ptomaines. Those of common occur- 



