io8 BACTERIA AND THE CARBON DIOXIDE CYCLE 



not putrescible. They are broken up by ' fermentation,' carbonic acid 

 being again the final shape in which the carbon is set free*. 



In nature, fermentative organisms are more widely distributed even 

 than putrefactive. With these they labour to disintegrate and remove dead 

 animals and plants. But in addition to being indispensable scavengers 

 they are of prime importance to man in numerous industries in the pre- 

 paration of butter-milk, cheese, ' sauerkraut,' and bread, and in the 

 manufacture of alcohol. Some species, on the other hand, are dangerous 

 enemies to the dairyman and the brewer, and in many technical processes 

 bacteria play a part useful or injurious as the case may be (77). 



There is a diversity of opinion as to what shall be understood by the 

 word fermentation. Some would apply it to every kind of chemical change 

 arising from the activity of micro-fungi, and would include putrefaction, 

 nitrification, and the oxidation of SH 2 by the thiobacteria, in short all 

 biochemical processes. From this it is but a short step to regard human 

 life itself as a fermentation. In the author's opinion, a more restricted 

 application is desirable, both in view of the history of the word and as 

 inducing to clearness. The word ' fermentation ' will therefore be used here 

 to designate the decomposition by means of particular living organisms the 

 fermentation organisms of non-nitrogenous organic compounds, particularly 

 carbohydra tes. 



Given fermentable substances, the necessary conditions for fermentation 

 are a supply of food-stuffs (above all a special source of nitrogen) for the 

 micro-organisms and, as in putrefaction, moisture arid a suitable tempera- 

 ture. Even before the nature of fermentation was well understood, we knew 

 that in addition to the fermentable substance a certain something, a ferment, 

 was necessary for the production of the phenomenon, and it was thought by 

 many to be a chemical body. But the classical researches of Pasteur (78) 

 showed that every fermentation was the work of some living organism, a 

 fermentum vivum, and not of a chemical ferment or enzyme. We have there- 

 fore to distinguish between living or organized ferments and unorganized 

 chemical ferments or enzymes. The enzymes (79) are substances secreted 

 by cells (e.g. pancreatin, pepsin, ptyalin), and are capable, even in the 

 absence of the cells that produced them, of causing specific chemical changes 

 in certain organic substances. The organized ferments are living cells. 

 They also are able to set up chemical changes in organic bodies, but only so 

 long as they, the cells, are alive. 



Organized ferments and enzymes have much in common. In each case 



* In qualification of this somewhat broad statement it should be mentioned that very little is 

 known as yet concerning the biochemical decomposition of fats (76). Preliminary experiments seem 

 to have shown that certain bacteria (cholera, typhoid, B. pyocyaneus) are able to split olive-oil or 

 beef-fat into glycerine and fatty acids, thus making them accessible to fermentation. 



