What Bacteria do and Where They May be Found.—A walk through a crowded city street on any warm day makes one fully alive to odors which pervade the atmosphere. Some of these unpleasant odors, if traced, are found to come from garbage pails, from piles of decaying fruit or vegetables, or from some butcher shop in which decayed meat is allowed to stand. This characteristic phenomena of decay is one of the numerous ways in which we can detect the presence of bacteria. These tiny plants, "man's invisible friends and foes," are to be found "anywhere, but not everywhere," in nature. They swarm in stale milk, in impure water, in soil, in the living bodies of plants and animals and in their dead bodies as well. Most "catching" diseases we know to be caused directly by them; the processes of decay, souring of milk, acid fermentation, the manufacture of nitrogen for plants are directly or indirectly due to their presence. It will be the purpose of the next paragraphs to find some of the places where bacteria may be found and how we may know of their presence.
A steam sterilizer.
How we catch Bacteria to Study Them.—To study bacteria it is first necessary to find some material in which they will grow, then kill all living matter in this food material by heating to boiling point (212°) for half an hour or more (this is called sterilization), and finally protect the culture medium, as this food is called, from other living things that might grow upon it.
One material in which bacteria seem to thrive is a mixture of beef extract, digested protein and gelatine or agar-agar, the latter a preparation derived from seaweed. This mixture, after sterilization, is poured into flat dishes with loose-fitting covers. These petri dishes, so called after their inventor, are the traps in which we collect and study bacteria.
Where Bacteria might Grow.—Expose a number of these sterilized dishes, each for the same length of time, to some of the following conditions:
- (a) exposed to the air of the schoolroom.
- (b) exposed in the halls of the school while pupils are passing.
- (c) exposed in the halls of the school when pupils are not moving.
- (d) exposed at the level of a dirty and much-used city street.
- (e) exposed at the level of a well-swept and little-used city street.
- (f) exposed in a city park.
- (g) exposed in a factory building.
- (h) dirt from hands placed in dish.
- (i) rub interior of mouth with finger and touch surface of dish.
- (j) touch surface of dish with decayed vegetable or meat.
- (k) touch surface of dish with dirty coin or bill.
- (l) place in dish two or three hairs from boy's head.
- This list might be prolonged indefinitely.
Colonies of bacteria growing in a petri dish.
Now let us place all of the dishes together in a moderately warm place (a closet in the schoolroom will do) and watch for results. After a day or two little spots, brown, yellow, white, or red, will begin to appear. These spots, which grow larger day by day, are colonies made up of millions of bacteria. But probably each colony arose from a single bacterium which got into the dish when it was exposed to the air.