Effective combination of LED-based visible light, photosensitizer and photocatalyst to combat Gram (−) bacteria

•Inactivation of Gram (−) E. coli by chlorophyllin-based photosensitization is ineffective.•Photoactivated ZnO NPs are effective, but requires high photocatalyst concentration.•Combining these two treatments in a proper sequence is enough effective.•It enable reduce photocatalyst concentration 5 times.

The aim of this study was to investigate antimicrobial efficiency of combined treatments of visible light (405 nm), chlorophyllin (Chl) and ZnO nanoparticles (NPs). Results obtained indicate that incubation of the Gram (−) Escherichia coli with Chl (1.5 × 10−5 M) for 60 min as well as illumination alone (405 nm; 380 kJ/m2) has no impact on the viability of E. coli. Just photosensitization (60 min incubation with 1.5 × 10−5 M Chl and following illumination, 405 nm; 380 kJ/m2) slightly reduces their population (∼1.5 log). ZnO NPs (5 × 10−4 M) in the dark have negligible effect on E. coli viability. However, photocatalytic treatment (10 min incubation with ZnO NPs (5 × 10−4 M) and following illumination with light (λ = 405 nm; 380 kJ/m2) resulted in significant reduction of viable cell population (4.6 log). Scanning electron microscopy (SEM) images indicate that cell surface-adhered NPs as well as chlorophyllin after illumination trigger cell shrinkage and eventually death. Simultaneous (at the same time) incubation 30 min. with Chl and ZnO NPs and following illumination (380 kJ/m2) was less effective than every treatment separately. Interaction of bacteria with photosensitizer and photocatalyst in the sequence (photosensitizer → photocatalyst → light) reduced the viability of bacteria by 3 log. But applying interaction of bacteria with photosensitizer and photocatalyst in opposite order (photocatalyst → photosensitizer → light) looks more promising as this reduced the Gram (−) E. coli microbial population by 4.5 log using 5 times lower photocatalyst concentration in comparison with efficiency of photoactivated ZnO NPs (4.6 log). In this case ZnO NPs interact electrostatically with negatively-charged cell surface and afterwards bind negatively charged Chl, thus act as mediators between negatively-charged cell surface and negatively-charged photosensitizer.