Er:YAG Laser Energy Optimization for Reducing Single-Species Microbial Growth on Agar Surfaces In Vitro.

BACKGROUND: Standardized Er:YAG laser settings for microbial reduction remain undefined, and existing studies rarely compare multiple species under identical conditions. This work aimed to characterize susceptibility across selected microorganisms using a controlled agar-based surface growth model. METHODS: Six reference strains (E. coli, S. aureus MSSA, S. aureus MRSA, E. faecalis, P. aeruginosa, and C. albicans) were cultured on agar and exposed to Er:YAG irradiation. Two experimental phases were conducted: (1) inhibition zone mapping using energies between 30 and 400 mJ at 1 Hz, with tapered and flat laser tips; and (2) quantification of viable surface coverage after irradiating mature 96 h cultures with 80, 130, 180, and 230 mJ at 10 Hz in contact mode. ImageJ analysis was used to measure inhibition diameters and remaining coverage. Data were evaluated using two-way ANOVA. RESULTS: All microorganisms showed measurable inhibition at every tested energy level, with diameter increasing proportionally to energy. E. coli and E. faecalis produced the largest inhibition zones in the mapping phase, while P. aeruginosa and C. albicans required higher energies to reach comparable levels. Mature surface cultures showed progressive reductions in viable coverage; the strongest effects occurred at 230 mJ. The tapered tip generated broader inhibition zones at lower energies compared with the flat tip. CONCLUSIONS: Er:YAG laser irradiation produces consistent, energy-dependent antimicrobial effects on single-species agar-based surface growth, with clear differences in species susceptibility and tip performance. The identified parameter ranges provide a quantitative foundation for future in vitro studies aiming to refine Er:YAG-based microbial reduction strategies.