Abstract
The micro-organism of the pasteurella group associated with rabbit snuffles and its complicating pneumonias, septicemias, etc., is known as Bacterium lepisepticum. Quite recently, De Kruif noted that a mutation of freshly isolated strains of this microbe, which took place in extract broth cultures, was favored by high concentrations of peptone, and was inhibited by undiluted serum or beef infusion. The recently isolated virulent strain which he designated as Type D was found by him to grow diffusely in serum and plain broth, to form rather opaque, fluorescing colonies on serum agar, and to have an acid agglutination zone of pH 3 to 3.5; while the mutant, which he called Type G, exhibited granular growth in fluid media, appeared as translucent, non-fluorescing colonies on serum agar, showed an acid agglutination optimum between 3.5 and 5, and was of low virulence.
This apparent physical change in the organism, together with the general distinguishing characteristics of each type, has been noted by us in many of our strains derived from both normal and infected rabbits. So far the pure Type D strains have shown a similar high degree of virulence for rabbits and mice, and all pure G strains have shown a similar low degree of pathogenicity for these same animals. Present studies on growth activities described in this paper, however, necessitate a different interpretation of the D to G transformation and the accompanying drop in virulence.
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