Modification of the oxygen effect when bacteria are given large pulses of radiation

MANY biological materials, including bacteria, have been shown to be more readily damaged when oxygen is available to the cell at the time of irradiation than when it is absent 1,2 . Serratia marcescens shows this enhanced sensitivity to radiation—the so-called ‘oxygen effect’—almost to the full extent when a suspension of them is saturated with a gas mixture containing only 1 per cent oxygen and 99 per cent nitrogen, provided the radiation is given at normal dose-rates of about 1,000 rads/min. Typical inactivation curves for this organism using 1.5 MV. X-radiation from a pulsed linear accelerator are given in Fig. 1. Using the intense electron beam from the same linear accelerator, however, a dose of some 10–20 kilorads can be delivered in 2 microsec. The results quoted below indicate that when such large single pulses are used, the same bacteria saturated with the same oxygen–nitrogen mixture show the lower sensitivity corresponding to anaerobic irradiation. This is thought to be due to the fact that the first few kilorads of the electron pulse remove dissolved oxygen from the interior of the bacterium by radiation-induced reactions. During the remainder of the pulse the bacterium will thus be receiving radiation under essentially anærobic conditions. Even though dissolved molecular oxygen may not be entirely removed by the radiation from the extracellular liquid, this cannot penetrate the cell by diffusion in the 2 microsec. that the pulse lasts.