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Summary
The effect of x-irradiation at various oxygen-concentrations has been examined for a strain of Pseudomonas aeruginosa. The maximum ratio for enhancement of the effect by oxygen is 3·0.
The data confirm the relationship between the slope of the dose–survival curve and the oxygen-concentration established by Howard-Flanders and Alper (1957) for Escherichia coli and Shigella flexneri.
The fractions of bacteria surviving a specified dose, plotted against the logarithm of the oxygen-concentration, also follow approximately a cumulative normal curve. The exponential function corresponding with this curve and the relationship established by Howard-Flanders and Alper fit the data equally well, as is demonstrated by statistical analysis. The 50 per cent activity value for oxygen decreases with increasing radiation dose.
The experimental evidence indicates that the sensitivity of the bacteria to x-irradiation increases with the dose. It is proposed that products formed inside the cell by irradiation in the presence of oxygen interact with a receptor site which is sensitized by the irradiation. The assumed sensitive site persists for at least 3 min after cessation of irradiation, causes no lethal damage to the organism, and does not react with oxygen.