Two types of organism have been shown to exist in cultures of the bacillus of rabbit septicemia, recently isolated from spontaneous infections.

One, Microbe D, grows diffusely in serum and plain broth, forms rather opaque, fluorescing colonies on serum agar, and is highly virulent for rabbits. These characters are retained throughout many passages in serum or plain broth.

The other type, Microbe G, flocculates rapidly in fluid media, forms translucent) bluish colonies with little fluorescence, and exhibits extremely low virulence for rabbits. Like Microbe D, its distinguishing characters persist throughout many passages in artificial media.

Two methods for the dissociation of these varieties from the parent culture have been described.

The two types are morphologically indistinguishable and possess identical fermentation reactions.

Rabbits surviving inoculation with Type G are resistant to multiple lethal doses of Type D. The agglutination reactions bear out this suggestion of the antigenic identity of the varieties. Community of antigenic character is rendered certain by the results of absorption reactions.

Microbe D, in contact with immune serum, flocculates well at 55°, but poorly or not at all at 37°C. Microbe G, on the other hand, agglutinates easily at both temperatures.

Microbe D, after being carried through twenty-five passages in serum and in plain broth, retains perfectly its characteristics of diffuse growth and of virulence, in the plain as well as in the serum broth.

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