Conditions favorable to growth of Yeast.--_Experiment._--Label three pint fruit jars A, B, and C. Add one fourth of a compressed yeast cake to two cups of water containing two tablespoonfuls of mola.s.ses or sugar. Stir the mixture well and divide it into three equal parts and pour them into the jars. Place covers on the jars. Put jar A in the ice box on the ice, and jar B over the kitchen stove or near a radiator; pour the contents of jar C into a small pan and boil for a few minutes. Pour back into C, cover and place it next to B. After forty-eight hours, look to see if any bubbles have made their appearance in any of the jars. If the experiment has been successful, only jar B will show bubbles. After bubbles have begun to appear at the surface, the fluid in jar B will be found to have a sour taste and will smell unpleasantly. The gas which rises to the surface, if collected and tested, will be found to be carbon dioxide. The contents of jar B have fermented. Evidently, the growth of yeast will take place only under conditions of moderate warmth and moisture.

Carbohydrates necessary to Fermentation.--Sugar must be present in order for fermentation to take place. The wild yeasts cause fermentation of the apple or grape juice because they live on the skin of the apple or grape.

Various peoples recognize this when they collect the juice of certain fruits and, exposing it to the air, allow it to ferment. Such is the _saki_ or rice wine of the j.a.panese, the _tuba_ or sap of the coconut palm of the Filipinos and the _pulque_ of the Mexicans.

Beer and Wine Making.--Brewers' yeasts are cultivated with the greatest care; for the different flavors of beer seem to depend largely upon the condition of the yeast plants. Beer is made in the following manner.

Sprouted barley, called malt, in which the starch of the grain has been changed to grape sugar by digestion, is killed by drying in a hot kiln. The malt is dissolved in water, and hops are added to give the mixture a bitter taste. Now comes the addition of the yeast plants, which multiply rapidly under the favorable conditions of food and heat. Fermentation results on a large scale from the breaking down of the grape sugar, the alcohol remaining in the fluid, and the carbon dioxide pa.s.sing off into the air. At the right time the beer is stored either in bottles or casks, but fermentation slowly continues, forming carbon dioxide in the bottles. This gives the sparkle to beer when it is poured from the bottle.

In wine making the wild yeasts growing on the skin of the grapes set up a slow fermentation. It takes several weeks before the wine is ready to bottle. In sparkling wines a second fermentation in the bottles gives rise to carbon dioxide in such quant.i.ty as to cause a decided frothing when the bottle is opened.

Commercial Yeast.--Cultivated yeasts are now supplied in the home as compressed or dried yeast cakes. In both cases the yeast plants are mixed with starch and other substances and pressed into a cake. But the compressed yeast cake must be used fresh, as the yeast plants begin to die rapidly after two or three days. The dried yeast cake, while it contains a much smaller number of yeast plants, is nevertheless probably more reliable if the yeast cannot be obtained fresh.

[Ill.u.s.tration: _a_ _b_ _c_]

The cut ill.u.s.trates an experiment that shows how yeast plants depend upon food in order to grow. In each of three fermentation tubes were placed an equal amount of a compressed yeast cake. Then tube _a_ was filled with distilled water, tube _b_ with a solution of glucose and water, and tube _c_ with a nutrient solution containing nitrogenous matter as well as glucose. The quant.i.ty of gas (CO{2}) in each tube is an index of the amount of growth of the yeast cells. In which tube did the greatest growth take place?

Bread Making.--Most of us are familiar with the process of bread making.

The materials used are flour, milk or water or both, salt, a little sugar to hasten the process of fermentation, or "_rising_," as it is called, some b.u.t.ter or lard, and yeast.

After mixing the materials thoroughly by a process called "kneading," the bread is put aside in a warm place (about 75 Fahrenheit) to "rise." If we examine the dough at this time, we find it filled with holes, which give the ma.s.s a spongy appearance. The yeast plants, owing to favorable conditions, have grown rapidly and filled the cavities with carbon dioxide.

Alcohol is present, too, but this is evaporated when the dough is baked.

The baking cooks the starch of the bread, drives off the carbon dioxide and alcohol, and kills the yeast plants, besides forming a protective crust on the loaf.

Sour Bread.--If yeast cakes are not fresh, sour bread may result from their use. In such yeast cakes there are apt to be present other tiny one-celled plants, known as _bacteria_. Certain of these plants form acids after fermentation takes place. The sour taste of the bread is usually due to this cause. The remedy would be to have fresh yeast, to have good and fresh flour, and to have clean vessels with which to work.

Importance of Yeasts.--Yeasts in their relation to man are thus seen to be for the most part useful. They may get into canned substances put up in sugar and cause them to "work," giving them a peculiar flavor. But they can be easily killed by heating to the temperature of boiling. On the other hand, yeast plants are necessary for the existence of all the great industries which depend upon fermentation. And best of all they give us leavened bread, which has become a necessity to most of mankind.

BACTERIA IN THEIR RELATION TO MAN

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.

[Ill.u.s.tration: 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 pa.s.sing.

(_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.

[Ill.u.s.tration: 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.

How we may isolate Bacteria of Certain Kinds from Others.--In order to get a number of bacteria of a given kind to study, it becomes necessary to grow them in what is known as a pure culture. This is done by first growing the bacteria in some medium such as beef broth, gelatin, or on potato.[20] Then as growth follows the colonies of bacteria appear in the culture media or the beef broth becomes cloudy. If now we wish to study one given form, it becomes necessary to isolate them from the others. This is done by the following process: a platinum needle is first pa.s.sed through a flame to _sterilize_ it; that is, to kill all living things that may be on the needle point. Then the needle, which cools very quickly, is dipped in a colony containing the bacteria we wish to study. This ma.s.s of bacteria is quickly transferred to another sterilized plate, and this plate is immediately covered to prevent any other forms of bacteria from entering.

When we have succeeded in isolating a certain kind of bacterium in a given dish, we are said to have a _pure culture_. Having obtained a pure culture of bacteria, they may easily be studied under the compound microscope.

Footnote 20: For directions for making a culture medium, see Hunter, _Laboratory Problems in Civic Biology_. Culture tubes may be obtained, already prepared, from Parke, Davis, and Company or other good chemists.

[Ill.u.s.tration: A pure culture of bacteria. Notice that the bacteria are all the same size and shape.]

Size and Form.--In size, bacteria are the most minute plants known. A bacterium of average size is about 1/10000 of an inch in length, and perhaps 1/50000 of an inch in diameter. Some species are much larger, others smaller. A common spherical form is 1/50000 of an inch in diameter.

They are so small that several million are often found in a single drop of impure water or sour milk. Three well-defined forms of bacteria are recognized: a spherical form called a _coccus_, a rod-shaped bacterium, the _bacillus_, and a spiral form, the _spirillum_. Some bacteria are capable of movement when living in a fluid. Such movement is caused by tiny lashlike threads of protoplasm called _flagella_. The flagella project from the body, and by a rapid movement cause locomotion to take place. Bacteria reproduce with almost incredible rapidity. It is estimated that a single bacterium, by a process of division called _fission_, will give rise to over 16,700,000 others in twenty-four hours. Under unfavorable conditions they stop dividing and form rounded bodies called spores. This spore is usually protected by a wall and may withstand very unfavorable conditions of dryness or heat; even boiling for several minutes will not kill some forms.

[Ill.u.s.tration: A figure to show the relative size and shape of (1) a green mold, (2) yeast cells, and (3) different forms of bacteria; _B_, bacillus; _C_, coccus; _S_, spirillum forms. The yeast and bacteria are drawn to scale, they are much enlarged in proportion to the green mold, being actually much smaller than the mold spores seen at the top of the picture.]

Where Bacteria are most Numerous.--As the result of our experiments, we can make some generalizations concerning the presence of bacteria in our own environment. They are evidently present in the air, and in greater quant.i.ty in air that is moving than quiet air. Why? That they stick to particles of dust can be proven by placing a little dust from the schoolroom in a culture dish. Bacteria are present in greater numbers where crowds of people live and move, the air from dusty streets of a populous city contains many more bacteria than does the air of a village street. The air of a city park contains relatively few bacteria as compared with the near-by street. The air of the woods or high mountains fewer still. Why?

Our previous experiment has shown that dirt on our hands, the mouth and teeth, decayed meat and vegetables, dirty money, the very hairs of our head are all carriers of bacteria.

Fluids the Favorite Home of Bacteria.--Tap water, standing water, milk, vinegar, wine, cider all can be proven to contain bacteria by experiments similar to those quoted above. Spring or artesian well water would have very few, if any, bacteria, while the same quant.i.ty of river water, if it held any sewage, might contain untold millions of these little organisms.

Foods preferred by Bacteria.--If bacteria are living and contain no chlorophyll, we should expect them to obtain protein food in order to grow.

Such is not always the case, for some bacteria seem to be able to build up protein out of simple inorganic nitrogenous substances. If, however, we take several food substances, some containing much protein and others not so much, we will find that the bacteria cause decay in the proteins almost at once, while other food substances are not always attacked by them.

[Ill.u.s.tration: Growth of bacteria in a drop of impure water allowed to run down a sterilized culture in a dish.]

What Bacteria do to Foods.--When bacteria feed upon a protein they use part of the materials in the food so that it falls to pieces and eventually rots. The material left behind after the bacteria have finished their meal is quite different from its original form. It is broken down by the action of the bacteria into gases, fluids, and some solids. It has a characteristic "rotten" odor and it has in it poisons which come as a result of the work of the bacteria. These poisonous wastes, called _ptomaines_, we shall learn more about later.

Conditions Favorable and Unfavorable to the Growth of Bacteria.--Moisture and Dryness.--_Experiment_.--Take two beans, remove the skins, crush one, soak the second bean overnight and then crush it. Place in test tubes, one dry, the second with water. Leave in a warm place two or three days, then smell each tube. In which is decay taking place? In which tube are bacteria at work? How do you know?

Moisture.--Moisture is an absolute need for bacterial growth, consequently keeping material dry will prevent the growth of germs upon its surface.

Foods, in order to decay, must contain enough water to make them moist.

Bacteria grow most freely in fluids.

Light.--If we cover one half of a petri dish in which bacteria are growing with black paper and then place the dish in a light warm place for a few days, the growth of bacteria in the light part of the dish will be found to be checked, while growth continues in the covered part. It is a matter of common knowledge that disease germs thrive where dirt and darkness exist and are killed by any long exposure to sunlight. This shows us the need of light in our homes, especially in our bedrooms.

Air.--We have seen that plants need oxygen in order to perform the work that they do. This is equally true of all animals. But not all bacteria need _air_ to live; in fact, some are killed by the presence of air. Just how these organisms get the oxygen necessary to oxidize their food is not well understood. The fact that some bacteria grow without air makes it necessary for us to use the one sure weapon we have for their extermination, and that is heat.

Heat.--_Experiment._--Take four cultures containing bouillon, inoculate each tube with bacteria and plug each tube with absorbent cotton. Place one tube in the ice box, a second tube in a dark closet at a moderate temperature, a third in a warm place (about 100 Fahrenheit), and boil the contents of the fourth tube for ten minutes, then place it with tube number two. In which tubes does growth take place most rapidly? Why?

Bacteria grow very slowly if at all in the temperature of an ice box, very rapidly at the room temperature of from 70 to 90 and much less rapidly at a higher temperature. All bacteria except those which have formed spores can be instantly killed as soon as boiling point is reached, and most spores are killed by a few minutes boiling.

Sterilization.--The practical lessons drawn from _sterilization_ are many.

We know enough now to boil our drinking water if we are uncertain of its purity; we sterilize any foods that we believe might harbor bacteria, and thus keep them from spoiling. The industry of canning is built upon the principle of sterilization.