Studies of the bacteriology of the blood were made in thirty-seven cases of lobar pneumonia. The pneumococcus was isolated from the blood in approximately 50 per cent. of the cases studied. The course of infection in individuals with pneumococcus in the blood was more severe than in those in which no organism could be cultivated from the blood. 77 per cent. of the patients with positive blood cultures died, and 79 per cent. of patients with negative blood cultures recovered. In fatal instances of pneumonia, where the pneumococcus was found in the blood, the number of organisms per cubic centimeter of blood was very high in the last stage of the disease. In individuals dying of pneumonia without blood infection, the disease was characterized by a rapid spread of the local process in the lungs. It is not unlikely that the symptoms of collapse, developing on the fifth or sixth day of lobar pneumonia, are often the expression of serious invasion of the blood by the pneumococcus. In other instances, they mark an extension of the local process in the lungs.

Strains of pneumococcus isolated from the blood of patients with lobar pneumonia were usually of high animal virulence. In a few instances where the organism isolated from the blood was of low virulence for animals, the patients recovered.

The protective power of a univalent antipneumococcus serum was tested against nineteen strains of typical pneumococcus and against four strains of closely allied organisms. The serum manifested some degree of protection against twelve out of nineteen strains of typical pneumococci. No protection was observed against the atypical organisms. In eight instances the degree of protection obtained was high, in three low, and in one there occurred only a prolongation of the period of life of the inoculated animal.

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