Sequential UVC Irradiation and Photocatalytic Treatment for Enhanced Gram-Positive Bacterial Inactivation

Sequential ultraviolet C (UVC) irradiation and photocatalytic treatment were investigated for Gram-positive bacteria using Lactobacillus casei as a biosafe and stress-tolerant model organism. Rapid and complete sterilization of Gram-positive bacteria remains an urgent challenge in human-made environments, where bacterial regrowth frequently compromises conventional disinfection strategies. Photocatalytic treatment was conducted using nano-sized magnetic TiO₂-SiO₂/Fe₃O₄ photocatalysts with high activity and facile magnetic separation. UVC irradiation alone failed to achieve complete sterilization due to bacterial regrowth, while photocatalytic treatment alone required prolonged irradiation times (>120 minutes). In contrast, the sequential combination achieved complete sterilization within a total of 50 minutes (10 minutes UVC irradiation and 40 minutes photocatalytic treatment). This reduction would arise from extensive UVC-induced DNA damage followed by reactive oxygen species–mediated disruption of DNA repair pathways and essential cellular functions, resulting in a repair-suppressed cellular state. This study establishes a new paradigm in disinfection of Gram-positive bacteria by demonstrating a repair-suppressed disinfection regime, in which bactericidal efficacy is governed not by the extent of primary damage but by the suppression of biological recovery pathways.