Page:Popular Science Monthly Volume 41.djvu/637

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619
INFECTIOUS DISEASES.
The constant presence of this micrococcus in the buccal secretions of healthy persons indicates that some other factor is required for the development of an attack of pneumonia; and it seems probable that this other factor acts by reducing the vital resisting power of the pulmonary tissues, and thus making them vulnerable to the attacks of the microbe. This supposition enables us to account for the development of the numerous cases of pneumonia which can not be traced to infection from without. The germ being always present, auto-infection is liable to occur when, from alcoholism, sewer-gas poisoning, crowd-poisoning, or any other depressing agency, the vitality of the tissues is reduced below the resisting point. We may suppose, also, that a reflex vaso-motor paralysis, affecting a single lobe of the lung, for example, and induced by exposure to cold, may so reduce the resisting power of the pulmonary tissues as to permit this micrococcus to produce its characteristic effects. Again, we may suppose that a person, whose vital resisting power is reduced by any of the causes mentioned, may be attacked by pneumonia from external infection with material containing a pathogenic variety of the micrococcus having a potency, permanent or acquired, greater than that possessed by the same organism in normal buccal secretions."

Investigations made since the above was written show that this micrococcus does vary greatly in its pathogenic power when obtained from different sources, and that virulent cultures obtained from the blood of inoculated animals become attenuated when they are kept for a short time. This, indeed, is a general rule as regards the best-known pathogenic bacteria; which usually acquire increased virulence when cultivated in the bodies of susceptible animals, and become attenuated as regards their pathogenic potency when they are cultivated for a certain length of time in artificial media. My own experiments with pneumonic sputum were made in January, 1885, and led me to the identification of the oval coccus, commonly in pairs, which is found in this material, with the coccus which I had previously found in my own saliva (September, 1880), and which was subsequently the object of extended experimental researches made by me in 1881-1884.

In my paper read before the Pathological Society of Philadelphia, in April, 1885, I say: "It seems extremely probable that this micrococcus is concerned in the etiology of croupous pneumonia. . . . But this can not be considered as definitely established by the experiments which have thus far been made upon the lower animals." The extended researches of Frankel, Weichselbaum, Netter, Gamaleïa, G. and F. Klemperer, and others, which have been published since, have fully established the etiological rolé of the micrococcus in question.

In 1882 Fehleisen isolated the so-called streptococcus of erysipelas and proved by experiment that it is the etiological agent in the production of erysipelatous inflammations. At a later date (1884) Rosenbach isolated the micro-organisms commonly concerned in traumatic infections and in the production of acute abscesses. Among these was a streptococcus, called by him Strep-