Inactivation of dried cells and biofilms of Listeria monocytogenes by exposure to blue light at different wavelengths and the influence of surface materials

Antimicrobial blue light (aBL) in the 400–470 nm wavelength range has been reported to kill multiple bacteria. This study assessed its potential for mitigating an important foodborne pathogen, Listeria monocytogenes (Lm), focusing on surface decontamination. Three wavelengths were tested, with gallic acid as a photosensitizing agent (Ps), against dried cells obtained from bacterial suspensions, and biofilms on stainless-steel (SS) coupons. Following aBL exposure, standard microbiological analysis of inoculated coupons was conducted to measure viability. Statistical analysis of variance was performed. Confocal laser scanning microscopy was used to observe the biofilm structures. Within 16 h of exposure at 405 nm, viable Lm dried cells and biofilms were reduced by approx. 3 log CFU/cm(2) with doses of 2,672 J/cm(2). Application of Ps resulted in an additional 1 log CFU/cm(2) at 668 J/cm(2), but its effect was not consistent. The highest dose (960 J/cm(2)) at 420 nm reduced viable counts on the biofilms by 1.9 log CFU/cm(2). At 460 nm, after 800 J/cm(2), biofilm counts were reduced by 1.6 log CFU/cm(2). The effect of material composition on Lm viability was also investigated. Irradiation at 405 nm (668 J/cm(2)) of cells dried on polystyrene resulted in one of the largest viability reductions (4.0 log CFU/cm(2)), followed by high-density polyethylene (3.5 log CFU/cm(2)). Increasing the dose to 4,008 J/cm(2) from 405 nm (24 h), improved its efficacy only on SS and polyvinyl chloride. Biofilm micrographs displayed a decrease in biofilm biomass due to the removal of biofilm portions from the surface and a shift from live to dead cells suggesting damage to biofilm cell membranes. These results suggest that aBL is a potential intervention to treat Lm contamination on typical material surfaces used in food production. IMPORTANCE: Current cleaning and sanitation programs are often not capable of controlling pathogen biofilms on equipment surfaces, which transmit the bacteria to ready-to-eat foods. The presence of native plant microbiota and organic matter can protect pathogenic bacteria by reducing the efficacy of sanitizers as well as promoting biofilm formation. Post-operation washing and sanitizing of produce contact surfaces might not be adequate in eliminating the presence of pathogens and commensal bacteria. The use of a dynamic and harmless light technology during downtime and close of operation could serve as a useful tool in preventing biofilm formation and persistence. Antimicrobial blue light (aBL) technology has been explored for hospital disinfection with very promising results, but its application to control foodborne pathogens remains relatively limited. The use of aBL could be a complementary strategy to inactivate surfaces in restaurant or supermarket deli settings.