Fluorescein-Metal Hybrid Surfactant Conjugates as a Smart Material for Antimicrobial Photodynamic Therapy against Staphylococcus aureus.

Photodynamic therapy (PDT) has been extensively used as an effective alternative for the treatment of bacterial infection using photosensitizers (PSs) in the presence of appropriate light. However, the limitation in the effectiveness of PDT is because of the low yield of singlet oxygen from existing PSs because of their low solubility. Thus, we have developed a platform to enhance the solubility and the photodynamic activity of PSs against bacterial cells using metallosurfactants. Herein, we have used manganese metal-containing single- (MnC I) and double-chain metallosurfactants (MnC II) which show an interplay of electrostatic/hydrophobic interactions with fluorescein (FL) dye (as a PS) and when used in the presence of light enhances the PDT. These interactions play a significant role in enhancing the singlet oxygen generation efficiency of FL. MnC I and MnC II have shown good antimicrobial activity against Gram-positive Staphylococcus aureus (S. aureus) bacteria. More interestingly, these metallosurfactants when combined with FL significantly enhanced the affectivity against S. aureus, wherein 100% killing was achieved. As compared to experiments performed in the dark, the metallosurfactant, by enhancing the solubility of FL, increases the formation of singlet oxygen upon light irradiation and thus increases cell death. Therefore, the synergistic effect of FL (light toxicity) and metallosurfactants (dark toxicity) defined excellent reduction in the colony formation of bacteria. These results were corroborated through field-emission scanning electron microscopy and optical microscopy, where the rupturing of the cell wall of bacterial cells was observed during this therapy. Moreover, the binding of metallosurfactants to the genomic DNA of S. aureus was also evaluated by gel retardation analysis and UV-visible spectroscopy. The outcomes from this study will deliver formulations for PDT which can be used in clinical medical applications and cancer therapy in the future.