The work carried on under the auspices of the Medical Commission for the Investigation of Acute Respiratory Diseases, at the Bacteriological Laboratory of the College of Physicians and Surgeons, Columbia University, has consisted principally of a comparative study of the morphology, growth characters, fermentative activities, and agglutination reactions of pneumococci and allied organisms isolated by ourselves and the various workers under the Commission.

These organisms were from two chief sources: (a) from the mouth and naso-pharynx of supposedly normal persons and persons suffering from minor inflammations of the naso-pharynx, and (b) from definitely pathologic sources, such as pneumonic sputum, pneumonic lungs, empyæma, the circulating blood of pneumonia patients, septicaemias, meningitis, and various minor lesions, usually due to infection with pneumococci.

One of the principal objects of the study was to make a careful comparison, in the light of the most recent knowledge of the biology of the pneumococcus and by the aid of the latest biological and technical methods, of the series of organisms from these two sources, and thus to determine definitely the true nature of pneumococcus-like organisms occurring in the mouths of normal persons.

The second and equally important object, ultimately dependent, however, upon the solution of the first, was the determination of the frequency of occurrence of typical pneumococci in the mouths of healthy individuals.

The investigations detailed in the present paper have dealt chiefly with the first problem and incidentally with the second, and lead to the following conclusions:

(a) That organisms, not to be distinguished by morphological characters or by any physiological peculiarities from true pneumococci derived from pathologic sources, occur with frequency in the mouths of healthy persons and those suffering from slight inflammations of the naso pharynx, and that the only permissible and legitimate conclusion is that these organisms are true pneumococci.

(b) That there are other organisms in normal mouths and from pathologic sources that morphologically or by staining reactions are not definitely to be distinguished from pneumococci, and can only be recognized by a careful study of their fermentative activities and agglutination reactions. These organisms are non-inulin fermenters.

(c) That the organism known as Streptococcus mucosus is at times found in cultivations from the mouths of apparently healthy individuals, and that, although it shows certain peculiarities distinguishing it from the typical pneumococcus, it is probably very closely related to, and a variety of, this species.

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