88 PRINCIPLES OF BACTERIOLOGY 



In the presence of oxygen some of the decomposition products that are 

 formed by the attack of the anaerobic bacteria are further decomposed 

 and oxidized by the aerobes; they are thereby rendered, as a rule, inert 

 and consequently harmless. Some bacteria have adapted themselves 

 to the exclusive use of compound oxygen, using those compounds from 

 which oxygen can be obtained, and others the obligatory anaerobes 

 are able to live only in the presence of free oxygen. The facts of 

 anaerobiosis are of great importance to technical biology and to path- 

 ology. Many parasitic bacteria are found to produce far more poison 

 in the absence of air than in its presence. The following three types 

 of chemical activity can be separated: 1. The bacteria develop their 

 tissues. 2. The bacteria produce and liberate ferments or enzymes 

 which tend to make the foodstuff in their neighborhood more assimi- 

 lable. 3. The bacteria assimilate substance and liberate it changed to 

 other material. These changes may be due to ferments retained in 

 the cells. 



Fermentation. The term fermentation is differently used by different 

 authors. Some call every kind of decomposition due to bacteria or 

 their products a fermentation, speaking thus of the putrefactive fermen- 

 tation of albuminous 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 decomposition of an 

 organic compound, induced by the life processes of living organisms 

 (organized 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 life processes necessary for the growth of the 

 organisms producing the ferment, as in the formation of acetic acid 

 from alcohol by the action of the vinegar plant, and in the second the 

 enzyme, either within or outside of the organism and having no 

 direct connection with the growth of the organism, causes a structural 

 change without losing its identity, as in digestion. E. Buchner 

 (Berichte d. Deutsch. chem. Gesellsch., xxx. 117-124 and 1110-1113) 

 has shown that, even in those cases of fermentation in which formerly 

 it was believed the organized cell itself was necessarily concerned, the 

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

 changes as the organized cells. This brings fermentation by unorgan- 

 ized and organized ferments very closely together, the one being a sub- 

 stance 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. These enzymes, even when present in the most 

 minute quantities, have the power of splitting up or decomposing 

 complex organic compounds into simpler, more easily soluble and 

 diffusible molecules. The changes thus made may greatly aid in 

 rendering the foodstuff suitable for bacterial growth. We can only 

 speak of chemical ferments when it can be demonstrated that the 

 fermentation continues in the absence of all living bacteria. This 



