In this paper are reported the results of a study of nine strains of pneumococci agglutinating with antipneumococcus sera of all three types (Nos. I, II, and III). Seven of the strains were the cause of serious or fatal infections in human beings.

Morphologically they were typical pneumococci with characteristic growth on ordinary media. Most of the strains were soluble in bile, fermented inulin, and caused no precipitation on glucose ascitic fluid agar. Two of the strains, however, resembled streptococci in these three cultural characteristics, but have been regarded as pneumococci on account of their serological reactions. Variations in the cultural reactions occurred with several strains while they were under observation.

The virulence of the strains varied greatly, some strains being almost non-pathogenic, and others killing mice in doses of 0.000001 cc. of a 24 hour broth culture.

Antipneumococcus Sera I, II, and III agglutinated all the strains in fairly high dilution (1:8 to 1:64 or higher), while normal horse serum caused no agglutination.

Antipneumococcus Sera I, II, and III stimulated active phagocytosis of all the strains, while no phagocytosis occurred in control preparations with normal horse serum.

These strains elaborated a soluble substance in the body of inoculated mice which caused the formation of a precipitate when the peritoneal washings, cleared by centrifugalization, were added to the antipneumococcus sera of all three types.

Antipneumococcus Sera I, II, and III protected mice equally well against 1,000 to 10,000 times the minimal lethal dose of the two strains with which protection tests could be carried out.

Absorption of serum of Types I and II with the homologous pneumococcus removed the agglutinins and the bacteriotropins for all these strains. Absorption of these sera with Strains T and N removed the agglutinins and the bacteriotropins for the homologous strain only, and not for typical members of Type I or II, or for the other atypically agglutinable strains reported in this paper. The agglutinins concerned in the agglutination of these peculiar strains are therefore minor agglutinins.

As shown not only by agglutination tests, but also by protection tests and agglutinin absorption tests, these organisms bear the same relation to Types I, II, and III, as do atypical Type II strains to Type II.

Immune sera were prepared with these strains, and each strain was tested with all the immune sera by means of phagocytic and agglutinative reactions. In general, the strains were found to be serologically distinct, though some interrelationships existed between Strains V and R, and between Strains H, F, and N. These sera had no activity towards strains belonging to Type I or II, or atypical Type II.

A mutation occurred in one of the strains, B, while it was under observation. On isolation this strain had the cultural reactions of a typical pneumococcus, and had the phagocytic and agglutinative reactions of an atypical Type II. After 6 months cultivation on blood agar its serological reactions changed, and it became actively phagocyted and agglutinated in antipneumococcus sera of Types I, II, and III. Its cultural characteristics also changed, and it became bile-insoluble, did not ferment inulin, and caused precipitation in glucose ascitic fluid agar. At this time it caused an intense green discoloration at the base of the blood agar slants around the water of condensation. By repeated animal passages this strain was three times made to revert abruptly to its original form (atypical Type IIa), both in cultural and serological reactions. An immune serum was prepared to each form of the strain, and each serum acted strongly on the homologous form, but was without action on the heterologous form of the strain.

This mutation suggests that these pneumococci reacting with all three types of antipneumococcus sera may represent primitive, relatively undifferentiated forms from which the fixed types may have arisen.

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