Low-level light/low concentration of reactive oxygen species (ROS) may trigger some biochemical pathways that lead to cell proliferation. Thus, there is a risk of stimulation of bacterial cell proliferation during photodynamic therapy (PDT). In this study, PDT with different doses of 809-nm laser and indocyanine green (ICG) was investigated invitro for safe bactericidal application. The combined effect of laser doses with ICG concentrations were examined on Pseudomonas aeruginosa invitro. Data showed that low energy dose and ICG concentration caused bacterial cell proliferation. When these parameters were increased high enough, photoinactivation of the bacteria was achieved. Energy dose and photosensitizer concentration ranges at which proliferation, cell death or neither observed were determined. Furthermore, l-histidine was used as a scavenger of ROS to block the mechanism of biostimulation and cell killing. It inhibited proliferation when laser dose and ICG concentrations were low. It also inhibited cell killing when dose andconcentration were high. Data showed that mechanisms of proliferation and cell killing depend on the amount of ROS and antibacterial photodynamic treatment have serious biostimulative risk. Effective range might need to be determined before any therapeutic usage. The risk seems to exist specifically at lower energy doses and photosensitizer concentrations.