In the preceding chapters statements have been made that “bacteria enter” at various places or they “pass through” different mucous membranes, skin, etc. Strictly speaking such statements are incorrect—bacteria do not “enter” or “pass through” of themselves. It is true that some of the intestinal organisms are motile, but most of the bacteria which are pathogenic are non-motile. Even the motile ones can not make their way against fluids secreted or excreted on free surfaces. Bacteria cannot pass by diffusion through membranes since they are finite particles and not in solution.
In the case of penetrating wounds bacteria may be carried mechanically into the tissues, but this is exceptional in most infections. Also after gaining lodgment they may gradually grow through by destroying tissue as they grow, but this is a minor factor. Evidently, there must be some mechanism by which they are carried through. The known mechanisms for this in the body are ameboid cells, especially the phagocytes. It is most probable that these are the chief agents in getting bacteria into the tissues through various free surfaces. The phagocytes engulf bacteria, carry them into the tissues and either destroy them, are destroyed by them, or may disgorge or excrete them free in the tissues or in the blood.
DISSEMINATION OF ORGANISMS.
Dissemination of organisms within the tissues occurs either through the lymph channels or the bloodvessels or both. If through the lymph vessels only it is usually much more restricted in extent, or much more slowly disseminated, while blood dissemination is characterized by the number of organs involved simultaneously.
PATHS OF ELIMINATION OF PATHOGENIC MICROÖRGANISMS.
I. Directly from the point, of injury. This is true in infected wounds open to the surface, skin glanders (farcy), black-leg, surface anthrax, exanthemata in man and animals (scarlet fever (?), measles (?), smallpox; hog erysipelas, foot-and-mouth disease): also in case of disease of mucous membranes continuous with the skin—from nasal discharges (glanders), saliva (foot-and-mouth disease), material coughed or sneezed out (tuberculosis, influenza, pneumonias), urethral and vaginal discharges (gonorrhea and syphilis in man, contagious abortion and dourine in animals), intestinal discharges (typhoid fever, “choleras,” “dysenteries,” anthrax, tuberculosis, Johne’s disease). Material from nose, mouth and lungs may be swallowed and the organisms passed out through the intestines.
II. Indirectly through the secretions and the excretions where the internal organs are involved. The saliva of rabid animals contains the ultramicroscopic virus of rabies (the sympathetic ganglia within the salivary glands, and pancreas also, are affected in this disease as well as the cells of the central nervous system). The gall-bladder in man is known to harbor colon and typhoid bacilli, as that of hog-cholera hogs does the virus of this disease. It may harbor analogous organisms in other animals, though such knowledge is scanty. The kidneys have been shown experimentally to excrete certain organisms introduced into the circulation within a few minutes (micrococci, colon and typhoid bacilli, anthrax). Typhoid bacilli occur in the urine of typhoid-fever patients in about 25 per cent. of all cases and the urine of hogs with hog cholera is highly virulent. Most observers are of the opinion, however, that under natural conditions the kidneys do not excrete bacteria unless they themselves are infected.
The milk both of tuberculous cattle and tuberculous women has been shown to contain tubercle bacilli even when the mammary glands are not involved. Doubtless such bacteria are carried through the walls of the secreting tubules or of the smaller ducts by phagocytes and are then set free in the milk.
SPECIFICITY OF LOCATION OF INFECTIVE ORGANISMS.
It is readily apparent that certain disease organisms tend to locate themselves in definite regions and the question arises, Is this due to any specific relationship between organism and tissue or not? Diphtheria in man usually attacks the tonsils first, gonorrhea and syphilis the external genitals, tuberculosis the lung, “choleras” the small intestine, “dysenteries” the large intestine, influenza the lungs. In these cases the explanation is probably that the points attacked are the places where the organism is most commonly carried, with no specific relationship, since all of these organisms (Asiatic cholera excepted) also produce lesions in other parts of the body when they reach them. On the other hand, the virus of hydrophobia attacks nerve cells, leprosy frequently singles out nerves, glanders bacilli introduced into the abdominal cavity of a young male guinea-pig cause an inflammation of the testicle, malarial parasites and piroplasms attack the red blood corpuscles, etc. In fact, most pathogenic protozoa are specific in their localization either in certain tissue cells or in the blood or lymph. In these cases there is apparently a real chemical relationship, as there is also between the toxins of bacteria and certain tissue cells (tetanus toxin and nerve cells). Whether “chemotherapy” will ever profit from a knowledge of such chemical relationships remains to be developed. It appears that a search for these specific chemical substances with the object of combining poisons with them so that the organisms might in this way be destroyed, would be a profitable line of research.