This paper was postulating that some of the benefits of PDT could be attributed to the change in skin flora (bacteria) as a result of the topical pre-treatment or the light irradiation used by this form of treatment. Their results show that the reduction in flora was not significant, so some other mechanism of benefit is at work. (See also: Lasers reduce `Substance P’ and PGP 9.5)

The bacterial flora of the skin surface following routine MAL-PDT, J Dermatolog Treat. 2006;17(4):222-3. Bryld LE, Jemec GB.

Photodynamic therapy using methylated 5-aminolevulate (MAL-PDT) appears to have an effect on non-neoplastic skin diseases, for example acne vulgaris and rosacea, for which antibiotics are sometimes used, and a possible antibiotic effect of PDT has previously been suggested. It does, however, also cause local immunosuppression and post-treatment barrier defects, which may promote infection. At the same time, PDT-induced therapeutic skin damage is sometimes confused with secondary bacterial infection by non-dermatologists. The possible changes in bacterial flora associated with MAL-PDT were therefore studied in 47 patients undergoing treatment. Skin swabs were taken immediately before applying the MAL and instantly after light irradiation. Bacterial growth was identified in 18 cases. No statistically significant changes were seen, either in the specific species found or the estimated bacterial density on the skin surface. The observations do not support the notion that routine MAL-PDT affects the bacterial flora of the skin in a clinically significant manner. Therefore, the possible antibacterial effect of routine MAL-PDT is probably not the main explanation of its apparent effect on non-neoplastic skin disease

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