Low Level Light irradiation (LLLI) may proliferate cell growth at certain conditions during a set of photochemical reactions called biostimulation. However, phototoxic inhibitory reactions after irradiating natural or artificially inoculated cells are possible. The purpose of this study was to determine these effects on Pseudomonas aeruginosa and Staphylococcus aureus. Ar ion laser at 514 nm was used to determine the effect of various energy densities of green light on these bacteria. The most effective energy densities of Ar ion laser were chosen to irradiate both bacteria with He-Ne (633 nm) and SHG Nd:YAG (532 nm) lasers to compare the effect of red and green lights on the growth. Irradiation of Pseudomonas aeruginosa for both He-Ne and SHG Nd:YAG lasers in the presence of toluidine blue O or safranine O as photosensitizers was also studied. All energy densities of Ar ion laser showed a proliferative effect on Pseudomonas aeruginosa and inhibitory effect on Staphylococcus aureus. Similarly, SHG Nd:YAG and He-Ne lasers with chosen energy densities were again proliferating for Pseudomonas aeruginosa and inhibitory for Staphylococcus aureus and SHG Nd:YAG was more effective than He-Ne in both cases. Irradiation of Pseudomonas aeruginosa in the presence of both photosensitizers led to the decrease of the cell population compared to the control.