Microplate-based assays for the evaluation of antibacterial effects of photocatalytic coatings

Three microplate-based viability assays for assessing the antibacterial effects of photocatalytic coatings were compared to the conventional colony count method. In the experimental design, cultured Escherichia coli were exposed to photocatalysis on various TiO2 films in the presence of either UVA or visible light. The photocatalytic effects on the bacterial physiology were determined by real-time measurements of metabolic activity (XTT assay), biomass formation in the liquid medium (growth assay), and by assessing membrane integrity (with propidium iodide and SYTO 9 fluorescent nucleic acid binding dyes—BacLight assay). All three methods proved to be more sensitive and reproducible than colony count for the evaluation of the bactericidal effect of photocatalysis, XTT, and growth assay succeeded in detecting differences in both UVA and visible light-activated photocatalytic coatings. BacLight could efficiently detect the visible light-dependent photocatalytic effect on bacteria and identify membrane damage, but resulted inadequate for evaluating the UVA-dependent antibacterial effects. The described microplate-based evaluation methods proved being more effective and rapid than the colony count assay for assessing the antibacterial effect of various photocatalytic coatings.