VITAL PHENOMENA OF BACTERIA. 63 



visible change, when sugar is added decompose it, with 

 the production of gas. The term fermentation is dif- 

 ferently used by different authors. Some call every 

 kind of decomposition due to bacteria a fermentation, 

 speaking thus of the putrefactive fermentation of albu- 

 minous substances; others limit the term to the process 

 when accompanied by the visible production of gas; 

 others, again, take fermentation to mean only the 

 decomposition of carbohydrates, with or without gas- 

 production. 



Fermentation may be defined as a chemical decom- 

 position of an organic compound, induced by living 

 organisms or substances contained within them (organ- 

 ized ferments), or by chemical substances thrown off 

 from the bacteria (unorganized or chemical ferments 

 or enzymes). In the first the action is due to the 

 growth of the organisms producing the ferment, 1 as 

 in the formation of acetic acid from alcohol by the 

 action of the vinegar-plant, and in the second the 

 enzyme causes a structural change without losing its 

 identity, as in digestion. These enzymes even when 

 present in the most minute quantities have the power 

 of splitting up or decomposing complex organic com- 

 pounds into simpler, more easily soluble and diffu- 

 sible molecules. We can only speak of chemical fer- 

 ments when it can be demonstrated that the fermenta- 

 tion continues in the absence of all living bacteria. 



1 Buchner (Berichte d. deutsch. chem. Gesellsch., xxx. 117-124 and 1110- 

 1113) has shown that even in those cases of fermentation in which, until 

 lately, we have believed the organized cell itself was necessarily concerned 

 that the cell protoplasm squeezed from its capsule is able to cause the same 

 changes as the organized cells. This brings fermentation by unorganized and 

 organized ferments very closely together, the one being a substance thrown 

 off from the cell, the other a substance ordinarily retained in the cell. The 

 increase of both ceases with the death of the bacteria producing them. 



