Photoinactivation of bacteria by hypocrellin-grafted bacterial cellulose

Abstract

With the aim of developing self-disinfecting materials to prevent pathogen transmission from surfaces to new hosts, here we report a simple and eco-friendly way to prepare photodynamic bacterial cellulose (BC) onto which the naturally-occurring photosensitizer hypocrellin (Hc) has been covalently appended. The resultant hypocrellin-grafted BC membrane (Hc-BC) was characterized by both physical (SEM, TGA, XRD) and spectroscopic (IR, diffuse reflectance UV–visible) methods, and the photosensitizer loading was found to be 155 nmol Hc/mg membrane. Indirect cytotoxicity tests employing mouse skin fibroblast (L929) cells showed no changes in cell viability, demonstrating that the Hc-BC membrane lacked any leachable components (e.g., unreacted coupling agent or hypocrellin) that could be cytotoxic to mammalian cells. The photodynamic antibacterial activity of Hc-BC was evaluated against gram-positive S. aureus (ATCC-6538) and gram-negative E. coli strain 8099. Our results demonstrated a 99.5 + % (2.7 log units) reduction in S. aureus upon illumination (Xe lamp, 65 ± 5 mW/cm2, 420–780 nm; 30 min), however, no statistically significant inactivation of E. coli was observed. Potentiation with potassium iodide was found to increase the antibacterial efficacy of Hc-BC against S. aureus to 99.997% (4.8 log units) at 10 mM KI, while E. coli was inactivated by 99.1% (2 log units) at 100 mM KI, with the increase in inactivation being attributable to short-lived reactive iodide radicals that are the major biocidal agents in the potentiation of Hc-BC by KI. Taken together, our findings demonstrated that hypocrellin-grafted bacterial cellulose is a sustainable material from which potent photodynamic antibacterial materials may be derived.