Lethal photosensitization of bacteria with 8-methoxypsoralen to long wave length ultraviolet radiation.

Severe, or even lethal, cellular damage can follow the conjoint action of a chemical photosensitizer and effective frequencies of the electromagnetic spectrum. The fact that bacteria, as well as other cellular systems, are subject to photosensitization by such compounds as methylene blue was known fifty years ago (Reitz, 1908). The relatively limited number of studies of bacterial photosensitization over the years since then has been concerned primarily with the photodynamic action of methylene blue. Recently, Sistrom et al. (1956) demonstrated a lethal photosensitizing effect of bacteriochlorophyll in the absence of carotenoids in Rhodopseudomonas spheroides. The photosensitizing action of both dyes and bacteriochlorophyll is dependent primarily on wave lengths in the red region of the visible spectrum. Another group of photosensitizing compounds was disclosed by the finding that certain furocoumarins of plant origin, particularly 8-methoxypsoralen, are useful in the deliberate photosensitization of the skin of patients with vitiligo (Fahmy and Abu-Shady, 1947), and indeed, the compounds were present in certain folk remedies used for this skin disease in India for at least 3000 years and in Egypt almost as long (Pathak, 1958). While early studies with 8-methoxypsoralen used sunlight as the radiation source, Musajo et al. (1954) demonstrated with an artificial light source the effectiveness of frequencies in the long wave length ultraviolet region in producing typical skin responses of erythema and edema. Clinical studies on the use of 8-methoxypsoralen to augment the normal tanning process 1 Supported in part by U. S. Public Health Service grant C-2837 and by the Medical Research Foundation of Oregon. 2 Present address: Department of Ophthalmology, College of Medical Science, University of Minnesota, Minneapolis. of light sensitive skin have also been reported (Fitzpatrick et al., 1955). That the photosensitization effect of psoralens is not peculiar to human skin was clearly shown by the strikingly increased susceptibility of several bacterial species to long wave length ultraviolet irradiation in the presence of these compounds, as measured by paper-disk diffusion assay (Fowlks et al., 1958). Such irradiation normally requires far more incident energy to kill bacterial cells than does short wave length ultraviolet (Hollaender, 1943); the obvious effect of the psoralens appeared to be a reduction in the amount of energy required for killing bacterial cells with long wave length ultraviolet radiation.