It must be remembered that there is an enormous number of bacteria which are not dangerous; some of them are entirely harmless even if introduced into the human body. Others are the bacteria of decomposition, or putrefaction, which are known as “saprophytic” bacteria. All of the harmless ones are known as “non-pathogenic,” that is, non-producers of disease. Those which produce disease are known as “pathogenic,” and those which produce suppuration as “pyogenic” or pus-producing bacteria.
All of these bacteria are plants, and not, as is very frequently supposed, animals of a low form. The danger from their introduction into the body can be best appreciated, perhaps, by the statement of Belfield, who estimated that a single bacterium which weighs, approximately, only the 1-40,000,000 part of a grain, if given plenty of food and plenty of “elbow room,” would so rapidly develop that in three days it would form a mass weighing 800 tons! It is the old story of the blacksmith who was to get a penny for the first nail, two for the second, four for the third, and so on till a set of shoes would cost more than Crœsus could pay for.
The effect of the bacteria has been determined by experiment to be proportionate to the dose. A cubic centimetre is a cube two-fifths of an inch on each side. One-tenth of such a cube of pure culture of one bacterium (Proteus vulgaris) contains 225,000,000 bacteria, and if injected under the skin of a rabbit will produce death. Less than 18,000,000 will produce no effect whatever. Of one kind of staphylococcus, if 250,000,000 are introduced under the skin of a rabbit there will be produced a small abscess, but it requires 1,000,000,000 to produce speedy death. On the other hand, of the bacillus of lockjaw it requires only 1000 to produce death, so virulent is this germ.
Moreover, their effect on tissues and persons in different states varies very much. Thus, it is found that when a certain number of bacteria are injected into the cavity of the abdomen of an animal, if the animal is healthy and the peritoneum (the thin lining membrane of the abdomen) is healthy, the animal will recover perfectly well; but if the peritoneum be scraped and torn (and it must be remembered that the healthy peritoneum is devoid of sensation), that the same dose which before was harmless will now produce a violent peritonitis and very likely death. The practical lesson from this experiment upon animals is very evident. Every surgeon who opens the abdomen is most careful, if possible, not to injure the peritoneum, but manipulates with the greatest care lest fatal results follow any serious injury to that membrane. So, too, if the general health be impaired, it is found that an injection from which a healthy animal would recover will be followed by fatal consequences if the general health is below par. Again, if an animal has a simple fracture of his thigh-bone, and that is the only injury that he receives, no infection from the exterior having occurred, he will make a good recovery; but if at the same time he receives a lacerated wound, it may be even in another part of the body, and this wound, not being cared for most scrupulously, becomes infected, the infection will fasten on the distant spot of least resistance, the broken thigh-bone, and will produce a most dangerous and very frequently fatal form of inflammation.
I need scarcely point out in this connection, as in fact throughout this entire consideration of bacteriology, how important a part in its development has been played by experiment upon animals. The experimental facts just stated are of vital importance in the treatment of surgical diseases, and evidently could not have been determined upon mankind. It is not too much to say that had vivisection been restricted or prohibited the surgery of to-day would be the barbarous surgery of thirty years ago.
Even granting that an enormous number of the bacteria are harmless, the wonder is that with so many foes on every hand we live an ordinary lifetime. Fortunately, however, in the human body there is not only a lack of food sufficient and “elbow room” enough for them to work their dire effects, but there is that which “makes for righteousness” in our physical organization as well as in our souls.
The moment that bacteria are introduced into the human body a certain number of cells hasten to destroy them. These are called “phagocytes” or devouring cells, because they eat up the bacteria. Whether the patient survives or dies depends on whether the bacteria get the upper hand of the phagocytes or the phagocytes the upper hand of the bacteria.
These statements are very easy to make, but the results have only been obtained by prolonged and laborious investigations in the laboratory and by experiments upon animals which have demonstrated these facts.
The bacteria are recognized by various methods: First, by form. Many which are identical in appearance, however, differ greatly in effects. A handful of turnip-seed and a handful of rape-seed look very much alike, but if they are planted the plants differ so greatly that we can recognize the difference in the seed by the difference in the crop; hence the second method of recognizing differences in bacteria is by planting them. Different methods have been practised. Some are sown on the raw surface of a potato; others on bread paste; others in certain jelly-like materials, such as gelatine or agar-agar. It was soon found as a result of these experiments that the bacteria flourished best, some in one soil, some in another. Again, the crops of mould which come from them differ greatly in color, some being black, some red, some white, some yellow, etc. A third method also is by staining them with various dyes, when it is found that some bacteria will take one stain best, others will take another, and so on through the whole list.
At first it was thought that these bacteria existed chiefly in the air, and hence in Lister’s early methods powerful spray-producing apparatus were used; but while it is true that they do exist in the air, it is found that this is not the principal source of infection. There is no substance (which has not been disinfected) that is not covered with the germs of these little plants. They exist in our food and drink; but the intestine is, one may say, a natural home in which many exist without harm to the body. For surgical purposes their existence is most important, first, in the earth, where, as I have already shown, the bacillus of lockjaw is most frequently found. So, too, the bacillus of wool-sorters’ disease (Anthrax) exists in the earth. If an animal dying of anthrax is buried, worms coming from the carcass up through the ground carry the infection, so that other animals grazing over this surface will become readily infected. The means by which we can avoid infection from the earth is very evident, viz., every person who has been run over by the cars or who has fallen on the ground and broken his leg, etc., must have the wound most carefully cleansed from all dirt. If this is scrupulously done the danger of tetanus or other similar earth-born bacterial disease is almost nothing.