EFFECT OF SUCCESSIVE X-IRRADIATION ON NOCARDIA CORALLINA

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Abstract
Cultures of N. corallina which had survived an 18,000 r dose of X-ray were more sensitive to X-radiation than the parent cells. Maximum sensitivity was obtained after 4 successive irradiations of survivors, after which the radiation response became more resistant. This was due to the selection of a normally occurring resistant coccoidal cell which was initially present in a very small percentage of the total population. A culture of Micrococcus aureus treated in a similar manner showed no change in survival response on successive irradiation. These results conform with the unpaired-defect theory of Tobias applied to a diploid cell. Although induced segregation of a diploid would cause similar results, experimental findings favor the theory of recessive lethal formation. Regardless of which theory is found to be more correct in interpreting the results, it appears probable that only a diploid or mixoploid population would yield the results obtained.

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