Journal of Theoretical and Applied Physics (JTAP) Photodynamic Activation using Herbal Extract and Low-Power Laser against Gram-Positive (Streptococcus mutans) and Gram-Negative (Klebsiella pneumoniae) Bacteria: Threats and Resistance Mechanisms Volume 18 (2024) Issue 4, July & August 2024 2024 08 13 Photodynamic Activation using Herbal Extract and Low-Power Laser against Gram-Positive (Streptococcus mutans) and Gram-Negative (Klebsiella pneumoniae) Bacteria: Threats and Resistance Mechanisms 10.57647/j.jtap.2024.1804.46 EN Zahra AghaEbrahimi Department of Physics, Central Tehran Branch, Islamic Azad University, Tehran, Iran Jamshid Sabaghzadeh Department of Physics, Central Tehran Branch, Islamic Azad University, Tehran, Iran 0000-0002-0055-7875 Sasan Soudi Department of Medical Engineering, Faculty of Health and Medical Engineering, Tehran Medical Sciences 0000-0002-5558-2867 MohamadReza Tanhayi Ahari Department of Physics, Central Tehran Branch, Islamic Azad University, Tehran, Iran 0000-0003-0907-4842 Journal Article 2024 08 13   This paper investigates the efficacy of photodynamic activation (PDA) using herbal extract in combination with low-power laser treatment against both Gram-positive (Streptococcus mutans) and Gram-negative (Klebsiella pneumoniae) bacteria. The study explores the threats posed by these bacteria and highlights their resistance mechanisms. Various experiments, including colony formation assays, minimal inhibitory concentration tests, and bacterial viability assays, were conducted to assess the antibacterial effects of PDA in the presence and absence of laser irradiation. Additionally, spectroscopic analysis of the herbal extract was performed to elucidate their composition and potential synergistic interactions with PDA.