Notwithstanding the prodigiously mischievous effects that are justly attributed to the action of bacteria, it must be admitted that they are essential in the economy of life. It is only through the action of bacteria that the great cycle of vital activities that constitute the organic world is made possible. For example, the soil from which plants grow owes its origin to the action of bacteria. It is through bacteria growing in the soil that the nitrogen of the air is fixed and prepared for the use of plants. The extent of this activity, which has not been understood until recent times, is so great as to be almost in-credible. No better illustration of the prodigious activity of micro-organisms could be offered than the observations of Muntz and Lainé who found that a peat bed six and a half feet thick and covering an area of twenty-five acres, if inoculated with nitrifying bacteria, might be made to produce the enormous quantity of fifteen hundred tons of nitrate daily. The “frost” that appears in cellars is saltpetre or nitrate of lime formed by the bacteria of the soil (Burnet).
It is believed by some eminent authorities that the great coal beds, and even the extensive de-posits of petroleum found buried in the earth, are the result of bacterial action. It is certainly known that it is only through the action of bacteria that the dead bodies of plants and animals are returned to the soil through the processes of fermentation and putrefaction. Except for this beneficent action of the bacteria the soil would sooner or later become exhausted and the earth would be encumbered with the desiccated forms of plants and animals which had finished their life history. It is thus only through the action of bacteria that the cycle of organic activity is maintained.
Another and notable example of the utility of micro-organisms was brought to light through the remarkable discovery of Tissier, made known to the world by Metchnikoff, that the acid-forming bacteria which are active in fermentation, may be successfully used to combat the poison-forming bacteria that are active in the processes of putrefaction. Fermentation and putrefaction are antagonistic processes. Fermentation produces acid products which are for the most part harmless to human beings but inimical to putrefactive bacteria.
This wise provision is of greatest importance in the economy of nature. Vegetable foods contain sugars, starches and dextrines, substances which ferment, and so when undergoing decay do not in general give rise to the obnoxious and poisonous gases and other substances which ac-company the decay of animal tissues.
Milk likewise ferments because of the large amount of sugar which it contains. Eggs and meat do not ferment but undergo putrefaction giving rise to highly offensive and poisonous products. This is because of the absence of sugar. Eggs or meat placed in a strong solution of sugar will not decay. Sugar is thus a preservative.
Another useful employment of fermentation is in. the production of silage. The silo is a de-vice for fermenting green stuffs and so preserving them from destruction and providing green food for cattle during the winter months.