different species, and for members of the same species under different conditions. It is confined mainly to tlie bacilli or rod-shaped bacteria, but has been observed in a few species of micrococci. Such motile micrococci possess fiagelæ. Like all living things, bacteria nourish themselves from their surrounding media, taking up external substances as food and building them up into their own cell substance, at the same time giving off waste-products. Like other plants, bacteria differ from one another as regards suitable foods, some growing best in one soil, others in a soil quite different. Lender the influence, apparently, of unfavorable nutritive conditions, certain bacilli have the property of passing from their ordinary condition into that known as 'spore formation.' This consists in the formation within the bacillus of a clear round or oval body; the spore, which, when fully developed, replaces completely the bacillus. The spore does not represent a phase of reproduction. A single
bacillus can form but one spore. It seems to be a defense metamorphosis which the organism undergoes when its environment becomes unfavorable. When proper nutritive conditions again obtain, the organism returns to its more common form. When this occurs, a single spore produces but a single bacillus. The spore represents an inactive condition, inasmuch as there is no reproduction of spores. A most important significance of this spore-formation lies in the
fact that being a defense-form, spores are much more resistant to the methods usually employed for destroying micro-organisms than are the bacilli from which they develop; and it was the failure to destroy spores that kept alive, for so long a time, the theory of spontaneous generation. Bacteria multiply by simple fission. They elongate, an indentation appears near the middle, and this is followed by complete division, forming two daughter-cells similar to
the parent-cell. The different species of bacteria preserve their identity as carefully as do the higher vegetable organisms, a given species of bacteria never producing any but the same species. This reproduction of bacteria is, under favorable circumstances, exceedingly rapid—a bacterium reproducing itself in from 15 to 40 minutes. This would, in 24 hours, result in the production of many millions of bacteria from a single individual. Woodhead says, "Fortunately for us, they can seldom find food enough to
carry on this wonderful reproduction. A large number die from want of food, and because of other conditions unfavorable to their existence."
Drying kills many species of bacteria; others simply remain inactive. Cold destroys many bacteria, though some are not killed by low temperatures. The typhoid bacillus can exist for many months frozen in a cake of ice, to become active and dangerous again when the ice is melted and used. Heat, especially moist heat, above a certain point, kills all bacteria: the application of heat to the destruction of bacteria is known as sterilization. Thus, boiling milk or water, or cooking meats or vegetables, destroys any bacteria that may have been present. The same applies to surgical dressings, such dressings being spoken of as sterile, or, because of the absence of infectious material, as aseptic. Certain chemical substances known as germicides, disinfectants, etc., are very inimical to bacteria, e.g. solutions of mercuric chloride, of carbolic acid, etc., and surgical dressings in which such chemicals are used are known as antiseptic dressings.
As has already been noted, those bacteria which live on dead animal and vegetable matter, the saprophytes, are conservative in their action. Upon them depends all destruction of dead animal and plant life. By the destructive action of bacteria, these dead bodies are broken down and their component elements returned to the soil, again to become food for plant-life. Were it not for the action of bacteria, the world would have long since been so filled with the remains of the dead, that there would be no room for the living. Thus the saprophytic forms of bacteria are absolutely essential to life.
It is, however, because of the relation of bacteria to certain diseases that these organisms have within the last few years come to occupy so much general public attention. The parasitic bacteria, in contradistinction to the saprophytic, live upon live animal or vegetable matter, and are therefore destructive to living organisms. The disease-producing bacteria are known as pathogenic, many infectious diseases are positively known to be caused by bacteria, and there is excellent reason to think that all other infectious diseases are likewise due to bacteria, although measles, scarlet -fever, and other diseases have as yet baffled all efforts to discover their specific germs. The effects of disease-producing bacteria upon the body tissues may be grouped in three classes: (1) Effects due to the direct local action of the bacteria on the tissues, as the membrane in diphtheria. (2) Mechanical effects, as when a clump of bacteria get into a blood-vessel and block it (infectious embolus). (3) Effects due to the production in the body, under the influence of the bacteria, of certain chemical substances which act as poisons to the tissues. These poisons are in solution in the body fluids, and each poison is peculiar to the species of bacteria which produces it: e.g. the poison, or toxin, as it is called, of diphtheria. is different from the toxin of typhoid fever.
The question presents itself as to why, when bacteria once start to grow on such a good culture-medium as the body, do they ever stop growing; or, in other words, why does one ever recover from an infectious disease? Our present belief is that there is produced in the body, as the result of the action of the bacteria, not only a toxin, but another chemical substance, which is antagonistic to the proliferation of the bacteria, and which is known as antitoxin. To the persistence in the body of this antitoxin is ascribed the immunity which a person who has had an infectious disease often has from that disease. Upon this principle is based the well-known diphtheria antitoxin. A fresh culture in bouillon of the diphtheria bacillus is allowed to stand until toxins are formed in the bouillon. It is then strained through a Pasteur filter, and a small amount is introduced into the body of a horse. Some days later a larger dose is given, and then constantly increasing quantities are injected during some months. The animal is thus rendered immune to the action of the toxin, and the fluid part, or serum, of its blood is the antitoxin now so successfully used in the treatment of diphtheria. See Diphtheria.
Bacteria may be grown artificially on various
