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 (lead 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 bac teria 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 posi tively known to be caused by bacteria, and there is excellent reason to think that all other in fectious diseases are likewise due to bacteria, although measles, searlet-fever, and other dis eases have as yet baffled all efforts to discover their specific germs. The effects of disease-pro clueing 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 sub stance, 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 cul ture 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 clays 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 Dint