A Study of Bacterial Sensitivity to Ultraviolet Radiation'

Previous studies ( Bachem and Dushkin, 1932) on the reactions of bacteria to ultraviolet light have demonstrated that these organisms re spond with different sensitivities according to various experimental con ditions. Variables, such as the wave length utilized, the age of the bac terial culture, and the temperature at which the organisms are maintained will all be reflected in the bacterial sensitivity to radiations. The term “¿ bacterial sensitivity― never has been well defined, nor has a uniform method been established whereby it can be measured. We have attempted to provide a reliable method of study. One may arrive at such a definition by the following experiment. By exposing bacteria seeded on an agar plate to ultraviolet rays of definite intensity per square millimeter for different exposure times, and by determining the percentage destruction of bacteria, one attains figures that can he plotted as shown in Fig. 1. Different curves are obtained if various bacterial strains or bacteria kept at different environmental conditions are studied. Gates (1929, 1930) has shown that curves of nearly identical shape are obtained if they are plotted on a semilogarithmic scale. These curves approach straight lines over a considerable extent and are parallel to each other (Fig. 2). The tendency towards horizontal deviation from steep straight lines is most pronounced for minimal and maximal destruc tion. It is therefore difficult to determine exactly the energy density at which destruction of bacteria sets in and the one at which destruc tion is complete. The energy density at which 50 per cent of the bac teria are killed can be determined accurately by interpolation along an approximately straight line between points of lesser and greater de struction. This figure represents the average resistance of the bacteria towards ultraviolet exposure, since one-half of the bacteria exhibits more, and the other half, less resistance. We define the “¿ average resistance― of bacteria toward ultraviolet rays as that energy required to destroy 50 per cent of the bacteria in a unit field of 1 square milli