Phototoxicity of skin microorganisms tested with a new model.

A new standardized method for testing phototoxicity of chemicals against microorganisms is described. The inoculum size of the microorganism, application of test chemicals, prediffusion time, incubation time and incubation period are defined. Staphylococcus aureus, S. epidermidis, Candida albicans, and Pityrosporum orbiculare were studied. Both 8-methoxypsoralen and trimethylpsoralen were phototoxic against all microorganisms tested, while tetracycline and doxycycline were not phototoxic. C. albicans may be chosen for phototoxicity testing because it has been used earlier, it is easy to maintain in culture, it grows easily when tested, and its pathogenicity is low. The phototoxicity of S. aureus, S. epidermidis, Pseudomonas aeruginosa, Propionibacterium acnes, C. albicans, and P. orbiculare against each other were also investigated. Only P. orbiculare was inhibitory. It inhibited the growth of S. aureus, S. epidermidis, and Ps. aeruginosa - both in the dark and after irradiation. The growth inhibition was markedly enhanced after UVA irradiation, indicating phototoxicity. The phototoxic effect of P. orbiculare may play a role in the ecology of the human skin flora.