Bactericidal effects of in vitro 405 nm, 530 nm and 650 nm laser irradiation on methicillin-resistant Staphylococcus aureus , Pseudomonas aeruginosa and Mycobacterium fortuitum

Laser irradiation (LI) is an emerging antimicrobial strategy that may reduce public health reliance on antibiotics but previous studies reported results based on one wavelength-duration LI, typically on one bacterial species using solid culture. This study aimed to evaluate the effect of increasing duration of 405 nm, 532 nm and 650 nm LI on methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa and Mycobacterium fortuitum in liquid culture. Bacterial cultures of 1000 CFU/ml irradiated with 405 nm, 532 nm and 650 nm (200 mW) laser at 1, 2, 5 and 10 min were subcultured on nutrient agar; colony counts pre/post-LI were recorded and survival calculated. Antimicrobial sensitivity and cell morphology pre/post-LI were compared using scanning electron microscopy (SEM) and disc diffusion, respectively. The 405 nm (blue) LI had the highest bactericidal effect (0–6% survival) on all test bacteria compared to 532 nm (green) (12–49% survival) and 650 nm (red) (33–94% survival) LI. An inverse relationship between LI duration and survival was observed, except in 650 nm LI of P. aeruginosa, whereby survival increased with increasing duration. No change in antibiotic sensitivity was observed for subcultured bacterial colonies post-10 min LI. Although SEM micrographs of bacteria 10 minutes post-405 nm LI showed appreciable cell morphological changes, no significant cellular damage was observed. These encouraging results confirm bactericidal effects of 405 nm LI on bacteria such as P. aeruginosa and further expand findings on 532 nm and 650 nm LI on bacterial survival, highlighting the potential of LI as novel antimicrobial strategies on opportunistic pathogens such as MRSA, P. aeruginosa and M. fortuitum.