Expression patterns of oxyR induced by oxidative stress from Porphyromonas gingivalis in response to photo-activated disinfection Maryam Pourhajibagher,1–3 Roghayeh Ghorbanzadeh,4 Abbas Bahador1,3,5 1Department of Microbiology, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran; 2Dental Implant Research Center, Dentistry Research Institute, Tehran University of Medical Sciences, Tehran, Iran; 3Dental Research Center, Dentistry Research Institute, Tehran University of Medical Sciences, Tehran, Iran; 4Private Practice, Tehran, Iran; 5Laser Research Center, Dentistry Research Institute, Tehran University of Medical Sciences, Tehran, Iran Introduction: Porphyromonas gingivalis, an important endodontic pathogen, may be exposed to sublethal doses of photo-activated disinfection (sPAD) during root canal therapy. Such an exposure can affect bacterial survival and virulence features. In this study, we evaluated the effect of sPAD-related oxidative stresses on the expression of oxidative stress response gene (oxyR) in P. gingivalis clinical isolates surviving in vitro photodynamic treatment. Materials and methods: To determine the sPAD, 16 clinical P. gingivalis isolates photosensitized with toluidine blue O (TBO), methylene blue (MB), and indocyanine green (ICG) were irradiated with specific wavelength and energy density of diode laser corresponding to the photosensitizers following bacterial viability measurements. The effects of sPAD on the expression ratio of oxyR of 16 clinical P. gingivalis isolates were then assessed using quantitative real-time PCR (qRT-PCR) assay. Results: Maximum values of sPAD against P. gingivalis were 6.25, 15.6, and 25 µg/mL at fluencies of 171.87, 15.6, and 93.75 J/cm2, respectively, for TBO-, ICG-, and MB-sPAD (P>0.05). ICG-, MB-, and TBO-sPAD could increase the oxyR gene expression of the clinical P. gingivalis isolates 12.3-, 5.6-, and 8.5-fold, respectively. ICG-sPAD increased the expression of oxyR gene in clinical isolates of P. gingivalis ~1.5- and 2-fold higher than TBO- and MB-sPAD, respectively. Conclusion: Our results showed that upregulation of oxyR during sPAD may lead to better survival and increased pathogenicity of P. gingivalis isolates. Therefore, selection of appropriate photo-activated disinfection dosage should be considered for the successful treatment of endodontic infection. Keywords: antimicrobial photodynamic therapy, indocyanine green, methylene blue, oxidative stress, Porphyromonas gingivalis, toluidine blue O, low-level laser therapy