Ultrafast-light driven plasmonic inactivation of S. epidermidis: mechanistic insights

Biofilm infections and antimicrobial resistance are growing problems that necessitate alternative strategies to supplement or replace antibiotic drugs. Plasmonic antimicrobials utilize localized thermal, chemical and photophysical responses to the optical excitation of localized plasmons in noble metal nanostructures for microbial inactivation. High fluence femtosecond pulsed laser excitation of noble metal nanoparticles allows for highly non-linear physics and gives rise to a wide range of physico-chemical phenomena, such as plasma, reactive oxygen species generation, and nanocavitation, which also have antibacterial implications. In this work, we irradiate lipid functionalized gold nanorods that have a longitudinal plasmon resonance at 710 nm bound to Gram-positive Staphylococcus epidermidis with 5-9 mJ cm^-2 85 fs pulses centered at 812 nm and demonstrate a bacterial inactivation of up to 50%. We show that this inactivation is driven by ROS generation and possibly cell permeabilization through shockwaves. The moderate fluence and off-resonant nanorod excitation is found to provide a tradeoff between ROS generation and minimizing nanorod deformation, laying the foundation for future multi-modal plasmonic therapies and a more complete understanding of their antimicrobial mechanisms.