After a time the culture fluid or soil becomes so exhausted of its needed constituents, by the immense number of plants living in it, that it is unfit for their life and development. Then this particular form will no longer thrive; but some other form of bacterium may find in it the properties required for functional activity, and may grow vigorously. It is probable that exhaustion or absence of proper soil is an important agent in protecting man from sickness due to infection from bacteria. The ever-present bacteria often gain access to man's blood through external wounds, or through the lungs and digestive tracts; but unless a soil suited for their development is found in its fluids, the plants will not grow. If they do not grow and increase in numbers, they can do little harm.

Again, there are certain bacteria which are so antagonistic to each other that it is impossible to make them grow in company, or to co-exist in the blood of the same individual. For example, an animal inoculated with erysipelas germs cannot be successfully inoculated immediately afterward with the germs of malignant pustule. This antagonism is illustrated by the impossibility of having a good crop of grain in a field overrun with daisies.

On the other hand, however, there are some micro-organisms which flourish luxuriantly when planted together in the same fluid, somewhat after the manner of pumpkins and Indian corn growing between the same fence rails. Others seem unwilling to grow alone, and only flourish when planted along with other germs. It is very evident, therefore, that bacteriology is a branch of botany, and that nature shows the same tendencies in these minute plants as it does in the larger vegetable world visible to our unaided eyes.

As the horticulturist is able to alter the character of his plants by changing the circumstances under which they live, so can the bacteriologist change the vital properties and activities of bacteria by chemical and other manipulations of the culture substances in which these organisms grow. The power of bacteria to cause pathological changes may thus be weakened and attenuated; in other words, their functional power for evil is taken from them by alterations in the soil. The pathogenic, or disease producing, power may be increased by similar, though not identical, alterations. The rapidity of their multiplication may be accelerated, or they may be compelled to lie dormant and inactive for a time; and, on the other hand, by exhausting the constituents of the soil upon which they depend for life, they may be killed.

It is a most curious fact, also, that it is possible by selecting and cultivating only the lighter colored specimens of a certain purple bacterium for the bacteriologist to obtain finally a plant which is nearly white, but which has the essential characteristics of the original purple fungus. In this we see the same power which the florist has to alter the color of the petals of his flowers by various methods of selective breeding.

The destruction of bacteria by means of heat and antiseptics is the essence of modern surgery. It is, then, by preventing access of these parasitic plants to the human organism (aseptic surgery), or the destruction of them by chemical agents and heat (antiseptic surgery), that we are enabled to invade by operative attack regions of the body which a few years ago were sacred.

When the disease-producing bacteria gain access to the tissues and blood of human and other animals by means of wounds, or through an inflamed pulmonary or alimentary mucous membrane, they produce pathological effects, provided there is not sufficient resistance and health power in the animal's tissues to antagonize successfully the deleterious influence of the invading parasitic fungus. It is the rapid multiplication of the germs which furnishes a continuous irritation that enables them to have such a disastrous effect upon the tissues of the animal. If the tissues had only the original dose of microbes to deal with, the warfare between health and disease would be less uncertain in outcome. Victory would usually be on the side of the tissues and health. The immediate cause of the pathogenic influence is probably the chemical excretions which are given out by these microscopic organisms. All plants and animals require a certain number of substances to be taken into their organisms for preservation of their vital activities. After these substances have been utilized there occurs a sort of excretion of other chemical products. It is probably the excretions of many millions of micro-organisms, circulating in the blood, which give rise to the disease characteristic of the fungus with which the animal has been infected. The condition called sapraemia, or septic intoxication, for example, is undoubtedly due to the entrance of the excretory products of putrefaction bacteria into the circulation. This can be proved by injecting into an animal a small portion of these products obtained from cultures of germs of putrefaction.

Characteristic symptoms will at once be exhibited.

Septicaemia is a similar condition due to the presence of the putrefactive organisms themselves, and hence of their products, or ptomaines, also in the blood. The rapidity of their multiplication in this albuminous soil and the great amount of excretion from these numerous fungi make the condition more serious than sapraemia. Clinically, the two conditions occur together.

The rapidity with which symptoms may arise after inoculation of small wounds with a very few germs will be apparent, when it is stated that one parasitic plant of this kind may, by its rapidity of multiplication, give rise to fifteen or sixteen million individuals within twenty-four hours. The enormous increase which takes place within three or four days is almost incalculable. It has been estimated that a certain bacillus, only about one thousandth of an inch in length, could, under favorable conditions, develop a brood of progeny in less than four days which would make a mass of fungi sufficient to fill all the oceans of the world, if they each had a depth of one mile.