The lipids of the skin after exposure to ultraviolet light are bactericidal. Since other fats and oils which have been irradiated are bactericidal on account of the active oxygen released on contact with bacteria, the mechanism of the bactericidal action of irradiated lipids of the skin must be similar because the lipids have the properties of other irradiated fats and oils. Irradiation increases the active oxygen content of dried skin markedly but little increase occurs if the lipids have been extracted. Although the normal lipids extracted from the skin contain some active oxygen, the active oxygen content is much increased by irradiation. The vapor from lipids exposed to ultraviolet light fogs photographic plates intensely and retards the growth of hemolytic streptococcus. When emulsified in salt solution, the irradiated lipids kill hemolytic streptococcus promptly in comparison with emulsions of lipid which have not been irradiated. The addition of neutralized cysteine HCl to the emulsions of the lipid, normal or irradiated, prolongs the life of bacteria suspended in the emulsions. This protective effect is due to the reducing action of the cysteine, Normal non-irradiated lipid, extracted from the skin under conditions which permit oxidation, kills bacteria more quickly than that used in these experiments, where precautions were taken to prevent oxidation (unpublished data). Even though these precautions were taken some oxidation occurred, because lipid so extracted contained some active oxygen, and bacteria lived longer in emulsions of this normal lipid if cysteine were added.

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