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 
tnduce fermentation; (2) those that ¢aduce discase; (3) and 
those that fix nitrogen. 
(1) Bacterta 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, ete.) 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 
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A B 
Fia. 122.—Certain bacteria of fermentation and disease: bacteria of souring milk 
(A), of vinegar (B), of diphtheria (('), of tetanus or lockjaw (D); C and D show 
the formation of the so-called ‘‘ spore.””— After Fiscurr. 
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 
