SUMMARY: Single organisms of were repeatedly selected for resistance on streptomycin agar yielding a population with increased resistance clearly different from that of the parent strain. These two strains were grown together and when retested a uniform population was found to be present with streptomycin resistance intermediate between the initial two.

Two strains of one the type-strain, the other a chloramphenicol-resistant mutant with a different colonial appearance, were grown together and the mixture sampled at intervals. The two populations became progressively more alike until within six hours they were indistinguishable. The resulting uniform population was intermediate in sensitivity to chloramphenicol and the colonies could no longer be assigned easily to either parental type on morphological grounds. When the two strains were separated by a collodion membrane, this diminution of variation did not occur.

The results cannot be explained by overgrowth of one strain by the other. It is suggested that something analogous to inbreeding is taking place.


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