X-RAY IRRADIATION OF COCCIDIOIDES IMMITIS ARTHROSPORES: SURVIVAL CURVES AND AVIRULENT MUTANTS ISOLATED

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Abstract
X-ray survival curves obtained by irradiating saline suspensions containing approximately 1 X 106 spores of C. immitis per ml with doses ranging from 20 to 100 kiloroent-gens (kr) of X-rays from the gold target of a 2 MEV Van de Graff accelerator, at a rate of approximately 1 kr per min., are sigmoidal rather than exponential in shape, and the LD50 from the curves is 14 kr. The shape of the curves may be partially explained by the existence of a 1:1 ratio of uninucleate to binucleate spores in the irradiated suspensions, as demonstrated by nuclear stains. Two different riboflavin-requiring mutants were the only auxotrophs isolated from 1000 survivors in suspensions receiving 80 to 100 kr. Both are avirulent in mice in dosages of up to 104 spores per mouse under conditions where the parent strain (Silveira) causes 92% mortality at a dosage of 102 spores per mouse. Mice previously infected with the mutants survive challenge with a lethal dose of the parent strain. The existence of these mutants extends the possibilities of using live avirulent strains of Coccidioides for the purpose of immunization.