Breaking survival curves have been measured for Serratia marcescens on Millipore filters equilibrated with known oxygen concentrations and irradiated by single pulses at ultrahigh dose rates. This phenomenon is attributed to the radiochemical depletion of intracellular oxygen and provides a basis for double-pulse experiments conducted to measure diffusion of oxygen in irradiated cells. A first pulse with sufficient dose to deplete intracellular oxygen precedes a similar second pulse by an accurately known interpulse time, variable from 10-6 sec to 30 sec. The amount of oxygen diffusing to critical sites in the cell during the interpulse time is inferred by comparison to cellular response measured under various oxygen concentrations with single high-intensity pulses. Oxygen-diffusion curves obtained show that a significant amount of oxygen diffuses to these sites by 10-4 sec. This can be interpreted as an upper limit to the lifetime of the radiation-induced oxygen-dependent damage. This limit is in agreement with that previously obtained by this laboratory for E. coli B/r using the double-pulse technique, but it is five times shorter than the half-life of oxygen-dependent damage found for Serratia marcescens by another laboratory which employed a fast-mixing technique.

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