LAUSR

To the Editor, Being involved in research on new laser therapy protocols in medicine, we noticed that the photodynamic treatment (PDT)—intended as light-induced activation of photosensitive drugs—is increasingly studied as an effective antimicrobial approach alternative to antibiotics and antiseptics, particularly for the management of diseases due to pathogens resistant to conventional drugs [1]. In this context, we have read with great interest the recent article by Dr. Gomez and coworkers [2], in which the authors have analyzed the in vitro bactericidal effect of methylene blue photo-activated with low-level lasers on Escherichia coli isolated from a pressure ulcer. In principle, the reported results can expand the current knowledge and help improve anti-bacterial PDT. However, we deem that a key methodological issue requires to be critically reappraised and further discussed. As reported by the authors, in this study: Bmethylene blue was used at concentrations of 0.001 and 0.005%,^ and Birradiated with a galliumaluminum-arsenide laser, 670, 830 and 940 nm, with energy density of 4, 8, 10 and 14 J/cm.^ Although the choice to test multiple laser wavelengths and power outputs is appropriate and meaningful, the reported conclusions obtained with laser at 830 and 940 wavelengths are conceptually biased, since these results cannot be interpreted as an effective PDT and may be misleading if transferred to clinical practice, because they may lead to erroneous evaluation of their actual antibacterial efficacy in pressure ulcers. In fact, the rationale for using methylene blue as a photoactivatable agent in PDT is based on the fact that, when excited with light at appropriate wavelength, it releases reactive oxygen species (ROS) endowed with potent anti-bacterial effects [3]. There are two photochemical pathways for the anti-bacterial effects of methylene blue-based PDT: the first step (type II reaction) involves generation of singlet oxygen, which readily reacts to generate hydroxyl radicals (OH–) and other ROS (type II reaction), which are considered the main effectors of bacterial photodamage and cytotoxicity [3]. However, as shown in Fig. 1, methylene blue has two absorption peaks at 635and 670-nm wavelengths and its complete absorption spectrum ranges from of 609to 690-nm wavelengths, with a low slope trend of the ascending part of the absorption curve and a sudden drop of the descending part [4]. On these grounds, it is clear that the low-level diode laser at 830and 940-nm wavelength used in the reported experiments was unsuitable to actually achieve methylene blue photoactivation and effective antibacterial PDT. Rather, it may be postulated that the observed bacterial killing in the presence of methylene blue under laser irradiation at 830and 940-nm wavelength