Laser NIR irradiation enhances antimicrobial photodynamic inactivation of biofilms of Staphylococcus aureus

Photodynamic inactivation (PDI) utilizes a photosensitizer (PS) activated by visible light to generate reactive oxygen species (ROS), to kill bacteria. PDI is effective against planktonic microorganisms, but biofilms are less sensitive due to limited PS and oxygen penetration. Near-infrared treatment (NIRT), involve the use of near-infrared light to kill bacteria either via thermal effects or ROS production. Our objective was to enhance S. aureus biofilm’s sensitivity to PDI by pre-treating with NIR irradiation before visible light exposure. In an in vitro biofilm model, laser NIRT (980 nm) followed by exposure to PDI, showed a synergistic effect on bacterial viability loss (4-log CFU vs 1-log loss with individual treatments). Interestingly, pre-heating liquid medium had no significant impact on PDI efficacy, suggesting that both thermal and non-thermal effects of NIR may be involved. NIRT increased PS uptake, induced clefts in the biofilm matrix, and released bacterial cells from the biofilm. NIRT induced a transient increase in the temperature to 46°C of in vitro cultures, however under the same conditions, when mice were irradiated, skin temperature rose to 37°. Our findings suggest that NIR irradiation serves as a complementary treatment to PDI, allowing reducing PS concentration, and highlighting its potential as an effective and resource-efficient antibacterial approach. * ALI : air-liquid interface CFUs : colony forming units CSLM : confocal scanning laser microscopy PBS : Phosphate-buffered saline NIRT : Near Infra-red Treatment NIRTc : NIRT in continuous mode NIRTf : fractionated NIRT NIRTp : NIRT in pulsed mode PDI : Photodynamic Inactivation PS : photosensitizer, photosensitizers SEM : scanning electronic microscopy TAPP : 5,10,15,20-tetrakis[4-(3- N , N- )dimethylaminopropoxy)phenyl]porphyrin TAPP-PDI : TAPP mediated photodynamic inactivation TSA : Trypticase Soy agar TSB : Trypticase Soy Broth.