The Influence of Pretreatments and Posttreatments on Bacterial Inactivation by Ionizing Radiations

Abstract

An evaluation of the available information concerned with the mode of lethal action of ionizing radiations on bacterial cells leads to the conclusion that alteration of relatively few molecules within the cell must ultimately bring about its inactivation. I t is possible that a chain reaction is initiated by the radiation that results in the destruction of a large number of biologically important molecules. The previous paper has discussed the improbability that a radiochemical chain reaction is of importance in the inactivation of bacterial cells. I t has been proposed in the past that the effect we study, usually the destruction of the ability of a bacterium to form a visible colony, may be the consequence of a biochemical chain reaction, in that some key synthetic system may be disrupted by the radiation, which leads to a general loss of synthetic ability of the cell. The final answer to what happens between the initial physical events and the biological effect that we study may be forthcoming by an orderly bringing together of the results of the research based on the diversified approaches now being used.The general conclusion from the radiobiological experimentation involving the use of bacterial cells as test organisms is that the inactivation results chiefly, if not entirely, from energy absorption within the cell. This conclusion is based primarily on the general finding that the fraction rather than the number of cells inactivated per unit dose is constant, independent of the concentration of bacteria in the irradiated suspension, and also independent of added protein in the suspension. Both lines of evidence indicate that the absorption of energy in the surrounding medium contributes very little to the lethal effect.It is well to indicate, at this point, that a variety of agents which may be added to a suspension of bacteria before and during irradiation can alter the sensitivity of the bacteria to ionizing radiation.6 Changes in the conditions surrounding the bacteria during irradiation can also modify the bactericidal effect. These treatments may be divided into a t least two classes, depending on when they may be effectively used with respect to the irradiation period. This division is somewhat arbitrary, since many techniques for measuring the damaging effect (for example, viable cell count) do not usually permit a decision as to whether the modifying system had its effect before, during, or after the radiation exposure.Most of the data to be presented in this paper will be concerned with the lethal action of ionizing radiations on bacterial suspensions. In most cases, this action has been measured by the radiation-induced loss of ability of the cells to reproduce or form colonies'on a growth medium. In all cases the bacteria, unless otherwise specified, are stationary-phase Escherichia coli, usually from a continuously aerated nutrient broth culture about 18 hours old. The irradiation was usually carried out with 250-kvp. X rays at a dose rate of about 2000 r/minu te. 604

The Influence of Environmental Factors on the Microbicidal Effect of Ionising Radiations

“…It is a common experience among workers in this field that radiation survivors may need 2-3 weeks incubation in broth before growth occurs, and the work of Stapleton (1955) and others has shown that certain bacteria may recover from the effects of irradiation if they are incubated a t suboptimal temperatures or in the presence of certain metabolites. It would therefore seem desirable that further work should be done on this topic.…”

On the Relationship Between the Viability of Bacteria Damaged by Antiseptics and Their Responsiveness to Modifications of the Counting Medium

“…Roepke & Mercer (1947) counted X-irradiated E. coli on three different media and found that all gave similar counts a t low mortalities, but significantly different counts a t high mortalities; Alper & Gillies (1958) have reported similar results. Stapleton (1955) incubated E. coli a t different temperatures following X-ray exposure and found that 18" was optimal for recovery. Inspection of these data shows that the greater the dose, and therefore the mortality, the greater the relative beneficial effect of incubation a t 18".…”

Radioprotection inE. Coliby an Agent fromM. Radiodurans

“…That the action of M .r . extract under these conditions may be more closely related to procedures such as liquid-holding recovery (Stapleton 1955) was shown by the observation that, on incubation in M .r . extract for 5 or 15 min after irradiation in buffer, E. coli B/r showed increased survival, comparable to a dose modification factor (d .m .f.)…”