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°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°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.

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