Enhanced antimicrobial efficacy and energy efficiency of low irradiance 405-nm light for bacterial decontamination

Abstract

Due to its increased safety over ultraviolet light, there is interest in the development of antimicrobial violet-blue light technologies for infection control applications. To ensure compatibility with exposed materials and tissue, the light irradiances and dose regimes used must be suitable for the target application. This study investigates the antimicrobial dose responses and germicidal efficiency of 405 nm violet-blue light when applied at a range of irradiance levels, for inactivation of surface-seeded and suspended bacteria. Bacteria were seeded onto agar surfaces (10¹-10⁸ CFUplate^-1) or suspended in PBS (10³-10⁹ CFUmL^-1) and exposed to increasing doses of 405-nm light (≤ 288 Jcm^-2) using various irradiances (0.5-150 mWcm^-2), with susceptibility at equivalent light doses compared. Bacterial reductions ≥ 96% were demonstrated in all cases for lower irradiance (≤ 5 mWcm^-2) exposures. Comparisons indicated, on a per unit dose basis, that significantly lower doses were required for significant reductions of all species when exposed at lower irradiances: 3-30 Jcm^-2/0.5 mWcm^-2 compared to 9-75 Jcm^-2/50 mWcm^-2 for low cell density (10² CFUplate^-1) surface exposures and 22.5 Jcm^-2/5 mWcm^-2 compared to 67.5 Jcm^-2/150 mWcm^-2 for low density (10³ CFUmL^-1) liquid exposures (P ≤ 0.05). Similar patterns were observed at higher densities, excluding S. aureus exposed at 10⁹ CFUmL^-1, suggesting bacterial density at predictable levels has minimal influence on decontamination efficacy. This study provides fundamental evidence of the greater energy efficacy of 405-nm light for inactivation of clinically-significant pathogens when lower irradiances are employed, further supporting its relevance for practical decontamination applications.

Keywords: 405 nm light; Antibacterial; Antimicrobial photoinactivation; ESKAPE pathogens; Low irradiance; Violet-blue light.

Fig. 1

Light sources for exposures of ESKAPE pathogens: a small-scale bench top system, b ceiling-mounted system, c 405-nm LED array and d emission spectra of the 405-nm light output of the array. All emission spectra data was captured using an HR4000 spectrometer (Ocean Optics, Germany) and Spectra Suite software version 2.0.151

Fig. 2

Inactivation of surface-seeded (~ 10² CFUplate^-1) ESKAPE pathogens, a E. faecium, b S. aureus, c K. pneumoniae, d A. baumannii, e P. aeruginosa and f E. cloacae, upon exposure to increasing doses of 405-nm light at irradiances of 0.5, 5 and 50 mWcm⁻². Surviving bacterial populations are presented as percentages with respect to equivalent non-exposed control populations. In all instances, GE was calculated at the dose at which complete or near complete (≥ 95%) inactivation was achieved. Each data point represents the mean value ± SD (n = 3). Asterisks (*) represent data points in which there was a significant reduction in the surviving bacterial population in comparison to the equivalent non-exposed control population (P ≤ 0.05)

Fig. 3

Appearance of a S. aureus and b P. aeruginosa at 10⁸–10¹ CFUplate^-1, after exposure to 0.5 mWcm^-2 405-nm light for 16 h and 24 h (28.8 and 43.4 Jcm^-2, respectively). CG represents plates with confluent growth of bacteria

Fig. 4

Inactivation of a S. aureus and b P. aeruginosa suspended in PBS upon exposure to 405-nm light up to a dose of 180 Jcm⁻² at irradiances of 5, 10, 50, 100 and 150 mWcm⁻². Surviving bacterial populations are presented as percentages with respect to equivalent non-exposed controls. In all instances, GE was calculated at the dose at which complete or near-complete (≥ 95%) inactivation was achieved. Each data point represents the mean value ± SD (n = 6); LOD = 10 CFUmL⁻¹. Asterisks (*) represent data points in which there was a significant reduction in the surviving bacterial population in comparison to the equivalent non-exposed control population (P ≤ 0.05)

Fig. 5

Inactivation kinetics of S. aureus suspended in PBS at initial population densities of a 10³ CFUmL⁻¹, c 10⁵ CFUmL⁻¹, e 10⁷ CFUmL⁻¹ and g 10⁹ CFUmL⁻¹ and P. aeruginosa at initial population densities of b 10³ CFUmL⁻¹, d 10⁵ CFUmL⁻¹, f 10⁷ CFUmL⁻¹ and (h) 10⁹ CFUmL⁻¹ upon exposure to increasing doses of 405-nm light at irradiances of 5, 50 and 150 mWcm⁻². Surviving bacterial populations are presented in log₁₀ CFUmL⁻¹. Each data point represents the mean value ± SD (n = 3); LOD = 10 CFUmL⁻¹. Asterisks (*) represents levels of inactivation significantly different to other irradiances at a particular applied dose (P ≤ 0.05): *a, significantly different to all other irradiances; *b, significantly different to 150 mWcm⁻²; *c, significantly different to 5 mWcm⁻²