Fast and effective photodynamic inactivation of 4-day-old biofilm of methicillin-resistant Staphylococcus aureus using methylene blue-conjugated gold nanoparticles

We aimed to determine whether gold nanoparticles (GNPs) could enhance the efficiency of methylene blue (MB)-induced photodynamic inactivation (PDI) of methicillin-resistant Staphylococcus aureus (MRSA) in mature biofilm. MB was immobilized on to the negatively-charged GNPs through electrostatic interaction. Four day-old biofilms of MRSA were treated with MB-conjugated GNPs and subsequently exposed to laser light (650 nm). Phototoxicity of MB-conjugated GNPs was also assessed on human dermal fibroblasts. MB-conjugated GNPs showed significant photoinactivation against 4-day-old biofilm of MRSA (>5 log10 CFU reduction) while MB-mediated PDI resulted in less than 1 log10 CFU reduction. Also, only 25.8% of the fibroblasts were photoinactivated. Our findings showed that GNPs enhanced the anti-biofilm efficacy of MB-PDI on MRSA; it may be due to the significantly elevated reactive oxygen species (ROS) formation (by localized surface plasmon resonance (SPR) of GNPs) and delivery of MB into the deeper part of the mature biofilm (by preventing MB aggregation).