Enhanced blue-light antibacterial action of a neutral vanillin-derived polyimine against Staphylococcus aureus

Microbial contamination remains a critical concern in water and biomedical contexts, motivating the search for sustainable disinfection strategies that minimize chemical inputs and energy demand. Blue light (400-470 nm) can inactivate bacteria via endogenous porphyrin photoactivation and reactive oxygen species (ROS) generation; however, significant antimicrobial effects typically require high fluences (>100 J cm-2), particularly at longer wavelengths such as 470 nm, limiting practical applications. Therefore, strategies that lower the required dose without introducing exogenous photosensitizers are highly desirable. Here, we investigate the previously reported vanillin-derived polyimine (VP) as a photo-inert interfacial modulator of bacterial susceptibility to blue light. Although VP is not itself a photosensitizer, it enhances blue-light antibacterial efficacy against Staphylococcus aureus by inducing sublethal stress that increases susceptibility to endogenous photo-oxidative pathways. VP films alone did not reduce bacterial viability after 90 min of dark incubation, but colony morphology, growth kinetics, and scanning electron microscopy revealed the induction of sublethal cellular stress. When combined with 470 nm irradiation at moderate fluences (18-54 J cm-2), selected to provide a weak light-only antibacterial action to reveal possible VP-mediated priming effects, VP films exhibited a reuse-dependent enhancement of antibacterial activity, with limited effect at first use and progressively stronger killing upon repeated reuse, ultimately approaching a 2-log reduction. Structural analyses (BET/BJH) showed that reuse-induced micro fracturing increased the specific surface area (12.9 m2 g-1 for pristine films vs. 21.8 m2 g-1 after reuse) without altering pore size distribution, thereby enhancing polymer-cell interactions. Importantly, no measurable generation of singlet oxygen, radical species, or hydrogen peroxide by the polymer was detected under the tested conditions, indicating that antibacterial activity arises from polymer-mediated sensitization of bacterial endogenous photo-oxidative pathways. Overall, these results support a priming-plus-light mechanism, in which a neutral polyimine induces sublethal physiological stress that amplifies bacterial susceptibility to blue-light irradiation under deliberately mild irradiation conditions.