FERMENTATIVE CHANGES. 121 



(b) Amido-acids and other amido-compounds, i.e., compounds 

 containing (NH 2 ) are formed by the putrefaction of 

 albumen. Tyrosine, OH.C (i H 4 .CH 2 .CH(NH 2 ).COOH, 

 para-oxyphenyl a-amido-propionic acid, leucine 

 CH,.(CH. 2 ) H .CH(NH,).COOH, a-amido-caproic acid, 

 asparagine COOH.CH,.CH(NH 2 ).CONH. 2 , amido-suc- 

 cinamic acid, and glycocol, CH. i .(NH 2 ).COOH, amido- 

 acetic acid are among such products. 



Tyrosine is converted by fermentation, if air be 



excluded, into indol, C, ; H 4 <f /CH, carbon dioxide, 



and hydrogen ; in the presence of air other substances, 

 like phenol, C (i H 5 OH, are formed and the nitrogen is 

 converted into ammonia. Leucine by its fermentation 

 forms valerianic acid, C 4 H .COOH, ammonia, carbon 

 dioxide, and hydrogen. 



(c) Putrefactive fermentation is the rapid decomposition of 

 albuminoid substances attended by the evolution of evil- 

 smelling gases and produced by the agency of various 

 species of bacteria. Generally the first step is the con- 

 version of the insoluble or colloidal albuminoids into 

 soluble and diffusible peptones ; these next split up, 

 yielding amido-acids, e.g., leucine, &c. These in turn 

 are decomposed into fatty acids and ammonia and the 

 fatty acids then ferment as described under (a). 



However, the kind of change produced is determined 

 to a great extent by the admission or exclusion of air 

 from the fermenting substances. Indeed, the presence 

 or absence of air from the decomposing mass deter- 

 mines the species of bacteria which can flourish in it. 

 Bacteria are sometimes classed into two great groups : 



Aerobic bacteria, which require the presence of 

 oxygen, and 



Anaerobic bacteria, which only perform their functions 

 in the absence of oxygen. 



The distinction is not altogether satisfactory, as under 

 conditions of air exclusion certain aerobic organisms 

 can carry on their work if nitrates be present. In the 



