24 THE NATURE OF THE ACTIVITIES OF MICROORGANISMS. 
processes of both animals and plants. Among the chemical changes 
which are brought about by bacteria, some are of this synthetic 
character. 
Analytical Processes. Decomposition: Katabolism. The 
most noticeable action of bacteria is that of decomposition. The 
great majority of them, just like animals, live upon complex chemical 
foods, and these compounds are broken to pieces by their action and 
reduced to simpler molecules. Acting in this way, the fungi are 
the most important agents in nature for reducing to a simpler condi- 
tion the great quantity of organic matter which would otherwise 
accumulate upon and within the soil, or in bodies of water. The 
chemistry of the decomposition of organic substances is still in its 
infancy, and as yet only the general nature of the changes is under- 
stood. The decomposition of these compounds in general brings 
the elements back to simpler conditions and nearer to the form in 
which they can serve as food for ordinary plants. 
Both synthetical and analytical processes are carried on, to a 
certain extent, by all bacteria. If they grow and multiply they 
must be manufacturing proteid and protoplasm out of the food prod- 
ucts, for each new bacterium is made of protoplasm. This build- 
ing of protoplasm is a synthetic process, and is, of course, character- 
istic of all growing bacteria. On the other hand, all bacteria like- 
wise produce a certain amount of decomposition of the materials 
which serve them as food, giving rise to simpler products as excre- 
tions. But while all bacteria thus perform both types of chemical 
change, the decomposition activity is, in general, much greater than 
that of synthesis; they are destructive rather than constructive agents. 
In still another respect the chemical changes produced by bacteria 
are two-fold. In some cases the new products which arise are of 
the nature of excretions. By this is meant that certain substances 
are taken into the bacteria and then subjected to a series of changes 
within their bodies. These changes are classed together under the 
name of metabolism. As a result of the metabolic changes there 
arise new chemical products which may be eventually eliminated 
from the body of the bacteria as excretions. Some of the new 
products arising in a mass of organic material undergoing decompo- 
