Sporicidal effects of high-intensity 405 nm visible light on endospore-forming bacteria.

Resistance of bacterial endospores to treatments, including biocides, heat and radiation is a persistent problem. This study investigates the susceptibility of Bacillus and Clostridium endospores to 405 nm visible light, wavelengths which have been shown to induce inactivation of vegetative bacterial cells. Suspensions of B. cereus endospores were exposed to high-intensity 405 nm light generated from a light-emitting diode array and results demonstrate the induction of a sporicidal effect. Up to a 4-log(10) CFU mL(-1) reduction in spore population was achieved after exposure to a dose of 1.73 kJ cm(-2). Similar inactivation kinetics were demonstrated with B. subtilis, B. megaterium and C. difficile endospores. The doses required for inactivation of endospores were significantly higher than those required for inactivation of B. cereus and C. difficile vegetative cells, where ca 4-log(10) CFU mL(-1) reductions were achieved after exposure to doses of 108 and 48 J cm(-2), respectively. The significant increase in dose required for inactivation of endospores compared with vegetative cells is unsurprising due to the notorious resilience of these microbial structures. However, the demonstration that visible light of 405 nm can induce a bactericidal effect against endospores is significant, and could have potential for incorporation into decontamination methods for the removal of bacterial contamination including endospores.

Bactericidal effects of 405 nm light exposure demonstrated by inactivation of Escherichia, Salmonella, Shigella, Listeria, and Mycobacterium species in liquid suspensions and on exposed surfaces.

The bactericidal effect of 405 nm light was investigated on taxonomically diverse bacterial pathogens from the genera Salmonella, Shigella, Escherichia, Listeria, and Mycobacterium. High-intensity 405 nm light, generated from an array of 405-nm light-emitting diodes (LEDs), was used to inactivate bacteria in liquid suspension and on exposed surfaces. L. monocytogenes was most readily inactivated in suspension, whereas S. enterica was most resistant. In surface exposure tests, L. monocytogenes was more susceptible than Gram-negative enteric bacteria to 405 nm light when exposed on an agar surface but interestingly less susceptible than S. enterica after drying onto PVC and acrylic surfaces. The study findings, that 405 nm light inactivates diverse types of bacteria in liquids and on surfaces, in addition to the safety advantages of this visible (non-UV wavelength) light, indicate the potential of this technology for a range of decontamination applications.