Light tolerance of extended spectrum β-lactamase producing Escherichia coli strains after repetitive exposure to far-UVC and blue LED light

Aims: The aim of this study was to investigate dual far-UVC (Ultraviolet-C) (222 nm) and blue LED (Light Emitting Diode) (405 nm) light on the inactivation of extended spectrum β-lactamase-producing Escherichia coli (ESBL-Ec) and to determine if repetitive exposure to long pulses of light resulted in changes to light tolerance, and antibiotic susceptibility.

Methods and results: Antimicrobial efficiency of dual and individual light wavelengths and development of light tolerance in E. coli was evaluated through a spread plate method after exposure to light at 25 cm. Dual light exposure for 30 min resulted in a 5-6 log₁₀ CFU mL^-1 reduction in two ESBL-Ec and two antibiotic-sensitive control E. coli strains. The overall inhibition achieved by dual light treatment was always greater than the combined reductions (log₁₀ CFU) observed from exposure to individual light wavelengths (combined 222-405 nm), indicating a synergistic relationship between blue LED and far-UVC light when used together. Repetitive long pulses of dual and individual far-UVC light exposure resulted in light tolerance in two ESBL-Ec strains but not the antibiotic-sensitive E. coli strains. Subsequent passages of repetitive light-treated ESBL-Ec strains continued to exhibit light tolerance. Antibiotic susceptibility was determined through a standard disk diffusion method. No changes were observed in the antibiotic susceptibility profiles for any of the four strains after exposure to either dual or individual wavelengths.

Conclusions: Dual light exposure was effective in the disinfection of ESBL-Ec in solution; however, antibiotic-resistant E. coli were able to develop light tolerance after repetitive exposure to light.