MUTATION OF THE BACILLUS OF RABBIT SEPTICEMIA

Type G microbes, discovered in pure cultures of the rabbit septicemia bacillus, have been demonstrated to arise from the parent D form by mutation. The D --> G mutation takes place in broth cultures of pure-line strains of Microbe D, when these are kept for several days without transplant at 37 degrees C., or at room temperature, or in the ice box. The mutation is greatly inhibited by filtrates from 6 and 24 hour cultures of Microbe D, and to some extent by filtrates from 48 hour cultures. The process of transformation takes place to a very slight extent or not at all in undiluted rabbit serum, but Type G colonies subcultured to this medium do not revert to the parent D form. The D --> G change is strongly inhibited in cultures made in simple beef infusion, or in 5 per cent rabbit serum-beef infusion. Peptone would seem to be the constituent of plain broth which favors the process. In high concentrations of peptone, the mutation is rapid and may reach a degree of 90 per cent of the total organisms in 5 to 6 days. A distinct maximum of the relative number of Type G colonies as compared to the parent Type D is observable in plain broth and in some concentrations of peptone, when these are kept at 37 degrees C. for some days without transplant. Subsequent tests show the concentration of Type G microbes to diminish. The change in acid agglutination optimum exhibited by the mutant G forms implies a distinct change in bacterial protoplasm and would seem to be one of the most fundamental mutations so far described.