Photosensitivity
P. acnes glows orange when exposed to Wood's light, possibly due to the presence of endogenous porphyrins. The bacterium is killed by ultraviolet light. P. acnes is also especially sensitive to light in the 405–420 nanometer (near the ultraviolet) range due to an endogenic porphyrin–coporphyrin III. A total irradiance of 320 J/cm² is found to inactivate this bacterium in vitro. This fact is used in phototherapy. Its photosensitivity can be enhanced by pretreatment with aminolevulinic acid, which boosts production of this chemical, although this causes significant side effects in humans, and in practice was not significantly better than the light treatment alone.
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