When intrabronchial insufflation of pure cultures of the streptococcus or of the influenza bacillus is properly carried out, it produces without fail a pneumonic lesion. This lesion is similar in its nature to the one known in human pathology as bronchopneumonia, and differs materially from the pneumonic lesion produced experimentally by the intrabronchial insufflation of pure cultures of the pneumococcus.
Considering the fact that none of the dogs used in the experiments with the pneumococcus and none of those used in the present investigation were selected or prepared in any way, the conclusion seems to be unavoidable that the proper invasion of the microörganism is the determining factor in the development of pneumonia, the condition of the animal being only a minor element in this regard.
Furthermore, since different organisms introduced in the same way and under conditions which are apparently the same produced distinctly different pneumonic lesions in animals of the same species, the further conclusion presents itself that the different types of pneumonia are produced by specifically different bacteria.
However, further investigation may show that the differences in the nature of the lesion are due rather to the degree of virulence of the causative microörganism than to differences in the species; that is, that different lesions may possibly be produced by organisms of the same species, provided they possess different degrees of virulence. Further experimentation may also show that the condition of the animal and of the affected organ which, in the onset and development of the pneumonic disease, is, perhaps, unimportant, may be the leading factor in determining the course and outcome of the disease.