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 mu M and 359-669 mu M, 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 mu M. 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.