A photosensitizer is a light-activated molecule that can generate reactive oxygen species or directly interact with nucleic acids. Both consequences can be applied to reduction of pathogens in various media and to selectively attack tumor cells. Numerous natural and synthesized photosensitizers have been identified for pathogen reduction. The photosensitizers of vitamins K3 (VK3), B1 (VB1), B6 (VB6) and benzophenone (BP) were prepared in 100-200 μM of PBS solution, irradiated with UVA at 0-48 J/cm2 for absorption spectrum alterations analysis. Bacteria species of E. coli, B. cereus, S. aureus and K. pneumoniae were mixed with 0-200 mM concentration of compounds and exposed to UVA irradiation of different dose at 6, 12 or 18 J/cm2 to assess the bactericidal effects. Over six logs CFU/ml reduction of E. coli suspended in PBS occurred after treatment with either VB1, VB6, VK3 or BP combined with UVA irradiation. When bacteria were suspended in plasma, two to seven logs reduction occurred depending on the UVA dose, photosensitizer concentration, and bacteria species. Among these photosensitizers, BP had the most potent bactericidal effect and is a promising UVA photosensitizer for pathogen reduction. The level of absorption spectrum alteration after UVA irradiation was profound for VK3 and VB6 but minimal for BP and VB1. The UV-vis absorption spectrum changes did not correlate with the bactericidal effect indicating that molecule modification by UVA light is not required for the bactericidal activity.