134 



A TEXT-BOOK OF BOTANY 



The most important relations of bacteria to man may be 

 grouped under the following three heads: (1) those that 

 induce fermentation; (2) those that induce disease; (3) and 

 those that fix nitrogen. 



(1) Bacteria that induce fermentation. In general, fer- 

 mentation is the decomposition of carbohydrates and 

 proteids by the action of living forms directly or by the 

 enzymes ( 47) which they produce, and conspicuous among 

 these forms are bacteria. When proteids (meat, etc.) con- 

 taining nitrogen and sulphur are decomposed in this way, 

 offensive gases are liberated, such decomposition being 

 often called putrefaction. When the word fermentation is 

 ordinarily used it refers to the decomposition of sugars in 



A B C 



FIG. 122. Certain bacteria of fermentation and disease: bacteria of souring milk 

 (A), of vinegar (B), of diphtheria (C), of tetanus or lockjaw (Z>); C and D show 

 the formation of the so-called " spore." After FISCHER. 



solution, as in various fruit juices, which breaks them up 

 into alcohol and carbon dioxide, the latter rising as bubbles 

 through the solution, which is then said to be working. 

 Such fermentations are produced chiefly by yeasts, which 

 are considered in the next section; but bacteria are con- 

 cerned in the souring of milk and of fruit juices and in 

 the manufacture of vinegar (Fig. 122). These saprophytic 

 bacteria that induce fermentation and putrefaction are of 

 much service as scavengers, being the chief agents in the 

 destruction of dead bodies. The various processes for 



