Those which are spherical in shape are known as micrococci, the rod-shaped as bacilli, and the spirals as spirilla.

Bacteria which live best with air are known as aërobic, and those which live best without air are

FIG. 2.
Micrococci.

Size.

anaërobic.

Bacteria are also classified as those which live upon dead organic matter, called saprophytes, and those which may exist in living organic matter, called parasites or pathogenic bacteria.

All bacteria are extremely minute and are never visible to the naked eye. It is said their size averages .00001 inch, and

they are the smallest living organisms the microscope has revealed.

Multiplication. Bacteria multiply by fission, or splitting into two parts, each part in turn dividing again in the same manner. It is this power of multiplication by fission which makes bacteria

b

FIG. 3.- - Method of multiplication of bacteria a and b, Bacteria dividing by fission; c, A yeast multiplying by bud

ding.

of such significance; their minute size would make them ineffectual or harmless were it not for their extraordinary power of multiplication, which is almost incredible in its rapidity.

Some of the species which have been watched under

the microscope under favorable conditions have been found to divide every half hour or in less time, and thus a single bacterium might produce millions in twentyfour hours; and if their multiplication was not checked, at the end of a year their numbers would be beyond our powers of expression. These figures interest us because they make us realize that in bacteria there is a force of mighty strength.

However, long before the offspring have reached even into the millions their multiplication is checked, either by lack of proper food, or by the effects of their own excreted products, which

are injurious to them.

Food. Bacteria differ from most other vegetable growths in the food they live upon. Most plants make their own

FIG. 4. Rod-forms united to form chains.

food by extracting the substances they need from the soil and air, but bacteria subsist upon the same substances which are already prepared by the life of the animal or plant, and for this reason the bacteria can grow much more rapidly than other vegetation, as they are not obliged to make their own food as other plants, nor to hunt for it as the animals, but living in the midst of prepared food, this multiplication is limited only by their power to assimilate it.

Chemical Changes. As bacteria grow in masses (called colonies) in food, they cause certain chemical changes to take place in it, and these chemical changes and the marvelous rapidity of their multiplication make it easy for us to understand the transformation they

may bring about wherever they gain a favorable foothold and begin to grow.

Where Found. Bacteria are found everywhere; no other vegetables or animals are so universal. They are in the soil to a depth of about four feet, the amount varying from a few hundreds to hundreds of millions when the soil is moist and full of organic matter; they are found at a considerable depth in the ocean; all fresh water contains them; they are in the air, particularly in cities and towns, their greatest number being near the surface of the earth and decreasing in the higher altitudes. Anything which raises the dust increases the bacteria in the air, while the dust and emanations from the clothing and bodies of persons in a close room fill the air with bacteria. It is supposed that bacteria are carried through the air only by becoming adherent to particles of dust, hence the great danger of any accumulation of dust which might carry the bacteria of disease.

Bacteria are found in greatest abundance in decaying vegetable and animal matter wherever it may be, in decaying wood and foliage, in dead bodies of all animals, in manure heaps, and in filth and slime they find their best nourishment. The bodies of both men and animals contain bacteria, they being found especially in the mouth, stomach, and intestines. On the surface of the body in the minutest crevice of the skin, upon the hair, under the finger nails, and upon the clothing they cling in great numbers, but they are never found in the tissue of a healthy person, either in the blood, muscle, gland, or any other organ.

The secretions, such as saliva, contain them; for while

the healthy gland itself contains no bacteria, they do exist in the ducts which convey the secretions to the exterior of the body, and thus the secretions are always contaminated by them.

The bodies of insects and other lower animals are more or less covered with bacteria; in fact, wherever there is a lodging place for dust, bacteria will be found. Although bacteria are found in so many places they are usually dormant or growing very little; when dried as dust, they are wholly dormant, and those contained in pure water multiply very slowly; but every single bacterium or its spore has the power of multiplication as soon as it falls into a favorable place where there is moisture and food for it. Their inconceivable power of multiplication being held in check by lack of food, then when favorable food, mois

FIG. 5. Various types of spiral bacteria.

ture, and temperature are furnished them, their possible powers at once become a mighty reality. Such food is provided by the dead bodies of animals, plants, and by animal secretions and some other sources. We have learned that bacteria are present everywhere except in the tissues of healthy men and animals, but there are a few species which are capable of existing and growing

for some time in the living tissues, and these are known as the disease-producing bacteria.

The large majority of bacteria are capable of living only upon dead organic matter. These are known as saprophytes, and by digestion-like processes, known as putrefaction, they break up the complex chemical molecules of the dead animal or plant bodies into simpler molecules capable of again being used by growing plants to form the starches, sugars, etc., of the plant tissues, which in turn serve for food for men and animals.

This transfer of the simpler chemical substances, such as water, carbon dioxide, and the nitrates into the complex chemical substances of plant tissues and from them into the more complex substances of the animal tissues, and back into the simpler compounds which may again be used by plants for food, is known as the food cycle, and it is in the completion of this cycle that bacteria play such an important part, a rôle so important that the continuance of life upon the surface of the earth would be impossible if bacterial activity were checked for any length of time.

Scavengers. Bacteria act as scavengers; by their action upon dead organic matter, as described in the food cycle, foul, offensive materials are transformed into harmless, simple substances which are taken up by the soil and thus furnish food for vegetation. Knowing this, we can easily understand how life would be impossible if all dead animals and plants did not succumb to these changes, but were left to accumulate upon the face of the earth.

It was formerly thought these changes, called putre