What Diseases Are Due To Bacteria?
( Originally Published 1897 )
It is, of course, an extremely important matter to determine to what extent human diseases are caused by bacteria. It is not easy, nor indeed possible, to do this to-day with accuracy. It is no easy matter to prove that any particular disease is caused by bacteria. To do this it is necessary to find some particular bacterium present in all cases of the disease; to find some method of getting it to grow outside the body in culture media; to demonstrate its absence in healthy animals, or healthy human individuals if it be a human disease; and, finally, to reproduce the disease in healthy animals by inoculating them with the bacterium. All of these steps of proof present difficulties, but especially the last one. In the study of animals it is comparatively easy to re-produce a disease by inoculation. But experiments upon man are commonly impossible, and in the case of human diseases it is frequently very difficult or impossible to obtain the final test of the matter. After finding a specific bacterium associated with a disease, it is usually possible to experiment with it further upon animals only. But some human diseases do not attack animals, and in the case of diseases that may be given to animals it is frequently uncertain whether the disease produced in the animal by such inoculation is identical with the human disease in question, owing to the difference of symptoms in the different animals. As a consequence, the proof of the germ nature of different diseases varies all the way from absolute demonstration to mere suspicion. To give a complete and correct list of the diseases caused by bacteria, or to give a list of the bacteria species pathogenic to man, is therefore at present impossible.
The difficulty of giving such a list is rendered greater from the fact that we have in recent years learned that the same species of pathogenic bacterium may produce different results under different conditions. When the subject of germ disease was first studied and the connection between bacteria and disease was first demonstrated, it was thought that each particular species of pathogenic bacteria produced a single definite disease; and conversely, each germ disease was supposed to have its own definite species of bacterium as its cause. Recent study has shown, however, that this is not wholly true. It is true that some diseases do have such a definite relation to definite bacteria. The anthrax germ, for example, will always produce anthrax, no matter where or how it is inoculated into the body. So, also, in quite a number of other cases distinct specific bacteria are associated with distinct diseases. But, on the other hand, there are some pathogenic bacteria which are not so definite in their action, and produce different results in accordance with circumstances, the effect, varying both with the organ attacked and with the condition of the individual. For instance, a consider-able number of different types of blood poisoning, septicemia, pyaemia, gangrene, inflammation of wounds, or formation of pus from slight skin wounds—indeed, a host of miscellaneous troubles, ranging all the way from a slight pus formation to a violent and severe blood poisoning--all appear to be caused by bacteria, and it is impossible to make out any definite species associated with the different types of these troubles. There are three common forms of so-called pus cocci, and these are found almost indiscriminately with various types of inflammatory troubles. More-over, these species of bacteria are found with al-most absolute constancy in and around the body, even in health. They are on the clothing, on the skin, in the mouth and alimentary canal. Here they exist, commonly doing no harm. They have, however, the power of doing injury if by chance they get into wounds. But their power of doing injury varies both with the condition of the individual and with variations in the bacteria them-selves. If the individual is in a good condition of health these bacteria have little power of injuring him even when they do get into such wounds, while at times of feeble vitality they may do much more injury, and take the occasion of any little cut or bruise to enter under the skin and give rise to inflammation and pus. Some people will develop slight abscesses or slight inflammations whenever the skin is bruised, while with others such bruises or cuts heal at 'once without trouble. Both are doubtless subject to the same chance of infection, but the one resists, while the other does not. In common parlance, we say that such a tendency to abscesses indicates a bad condition of the blood—a phrase which means nothing. Further, we find that the same species of bacterium may have varying powers of producing disease at different times. Some species are universal inhabitants of the alimentary canal and are ordinarily harmless, while under other conditions of unknown character they invade the tissues and give rise to a serious and perhaps fatal disease. We may thus recognise some bacteria which may be compared to foreign invaders, while others are domestic enemies. The former, like the typhoid bacillus, always produce trouble when they succeed in entering the body and finding a foothold. The latter, like the normal intestinal bacilli, are al-ways present but commonly harmless, only under special conditions becoming troublesome. All this shows that there are other factors in deter-mining the course of a disease, or even the existence of a disease, than the simple presence of a peculiar species of pathogenic bacterium.
From the facts just stated it will be evident that any list of germ diseases will be rather uncertain. Still, the studies of the last twenty years or more have disclosed some definite relations of bacteria and disease, and a list of the diseases more or less definitely associated with distinct species of bacteria is of interest.
Various wound infections, including septicce nia, pycemia, acute abscesses, ulcers, erysipelas, etc., are produced by a few forms of micrococci, resembling each other in many points but differing slightly. They are found almost indiscriminately in any of these wound infections, and none of them appears to have any definite relation to any special form of disease unless it be the micrococcus of erysipelas. The common pus micrococci are grouped under three species, Staphylococcus pyogenes aureus, Staphylococcus pyogenes, and Streptococcus pyogenes. These three are the most common, but others are occasionally found.
In addition to these, which may be regarded as demonstrated, the following diseases are with more or less certainty regarded as caused by distinct specific bacteria: Bronchitis, endocarditis, measles, whooping-cough, peritonitis, pneumonia, syphilis.
Still another list might be given of diseases whose general nature indicates that they are caused by bacteria, but in connection with which no distinct bacterium has yet been found. As might be expected also, a larger list of animal diseases has been demonstrated to be caused by these organisms. In addition, quite a number of species of bacteria have been found in such material as faeces, putrefying blood, etc., which have been shown by experiment to be capable of. producing diseases in animals, but in regard to which we have no evidence that they ever do produce actual disease under any normal conditions. These may contribute, perhaps, to the troubles arising from poisonous foods, but can not be regarded as disease germs proper.