A quantitative study of the action of ultra-violet light on bacteria

Abstract

Thin agar plates inoculated with different strains of bacteria were partially exposed to light of different wave lengths in the range 2300-5780 A. After incubation, the amount of growth in the exposed and unexposed areas was found by a photoelectric determination of opalescence. The bacteria were most resistant to damage by exposure between pH 7.2 and 7.6, increasing the concn. of agar in the plates had a protective effect. The time required to produce a given effect was inversely proportional to the intensity of the light. The shape of the percentage growth/time of exposure curves was constant for all wave lengths for any one organism, but varied with different species and with different strains of the same species, serologically indistinguishable. The bactericidal effect was greatest for all strains at 2655 A, then in diminishing order at 2536, 2804, 2482, 2700 A. The order of effectiveness of [lambda] 2967, 2378, 2300, 2350, 2320 and 3022 A varied with different strains. The effect was not proportional to the relative energies of the different wave lengths. When the logarithms of percentage growth were plotted against the time of exposure, the order of death was not exponential, nor were the curves obtained for different organisms of the same type. Experiments with lysol, formalin and phenol as bactericides showed stimulation of growth in the higher dilutions. Sensitization of the bacterial cultures with erythrosin was more easily effected with Gram-positive than Gram-negative organisms.