The action of radiations on bacteria III—γ-rays on growing and on non-proliferating bacteria

The authors’ work (Lea, Haines and Coulson 1936)* on the action of ∝-rays and β-rays on bacteria was mainly concerned with tests of the validity of the target hypothesis. Experiments using γ-rays were commenced with the object of obtaining still further evidence on this question. While no essential difference is to be anticipated in the biological action of γ-rays and β-rays, the greater penetrating power of the former makes certain experiments possible which could not be carried out with β-rays. Thus, in a disinfection experiment lasting several hours, it is convenient to be able to expose a relatively large quantity of liquid and to follow the course of disinfection by taking samples at intervals. Further, it is possible by the same method to observe the influence of radiation upon the growth of bacteria in a liquid nutrient medium. This last type of experiment seemed particularly suitable as a test of the target theory, since on that theory one would expect growth in the medium, and death produced by the radiation, to take place independently and at their normal rates, a result not to be anticipated on other theories. Actually, however, the issue was found to be complicated by the fact that the radiation, in addition to its lethal action, had the effect of causing the production of organisms many times longer than the normal. The present paper, therefore, is concerned with an investigation of these long forms. Approximately 920 mg. of radium were available in various containers; the filtration used was sufficient to ensure that only the γ-rays reached the biological material. Owing to the necessity of using the radium in containers designed for other experiments, and of working at short distances in order to obtain a sufficiently high intensity, the radiation was by no means of uniform intensity throughout the tube of liquid exposed. Mixing by convection currents, and during the process of sampling, was probably sufficient to ensure that over an extended time each organism received a dose of radiation corresponding to the average intensity over the whole volume of liquid. This average intensity was measured with the aid of a cylindrical ionization chamber of graphite, having a size approximating to that of the exposed tube. The ionization current was measured with an instrument recently described (Lea 1937).