Light-activated amino-substituted dyes as dual-action antibacterial agents: Bio-efficacy and AFM evaluation

The present study aimed to explore the antibacterial activity of a group of nature-inspired and structurally similar amino-based flavylium dyes: 7-diethylamino-4′-dimethylaminoflavylium (7NEt₂4′NMe₂); 7-diethylamino-2-(dimethylaminostyryl)-1-benzopyrylium (7NEt₂st4′NMe₂); 7-diethylamino-4′-aminoflavylium (7NEt₂4′NH₂); 7-diethylamino-4′-hydroxyflavylium (7NEt₂4′OH). Except for 7NEt₂4′OH, all the other tested compounds showed inhibitory activity on the bacterial growth of Gram-positive bacteria (Staphylococcus aureus ATCC 29213 and S. epidermidis ATCC 14990) and Gram-negative bacteria (Pseudomonas aeruginosa ATCC 27853), with MIC values in the range of 2.61–24.4 μM and 359–669 μM, respectively. Besides their toxicity in the dark, remarkable light-induced killing effects were observed (light dose treatment of 22.5 J/cm²), causing significant morphological changes detected by atomic force microscopy. The photoinactivation effect was particularly effective against Gram-positive bacteria, where complete eradication of bacterial growth was achieved at concentrations in the range of 3–12 μM. Photochemical mechanistic studies suggested the occurrence of both type I (free radicals) and type II (singlet oxygen) reactions, although light-induced damages appear to be predominantly caused by singlet oxygen action. Altogether, these results highlight the dual-antibacterial action of this group of dyes, showing antibiotic activity in the dark and photosensitizer behavior under visible light exposure. These findings underline a new application for amino-based flavylium dyes in the management of microbial skin infections.