Light-driven antimicrobial activities of vitamin K3 against Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella Enteritidis

Abstract

Due to the lack of thermal inactivation steps, the fresh produce industry is actively looking for new, environmentally friendly and effective non-thermal control strategies to eliminate foodborne pathogens on produce surfaces and to prevent cross-contamination. Photodynamic inactivation of bacteria has emerged as a novel alternative approach to inactivate microorganisms on fresh produce. In this study, the photo-sensitizing efficacy of vitamin K3 (VK3) under UV-A and simulated sunlight and their antimicrobial efficacy against common foodborne pathogens in broth and on lemon surfaces were investigated. The minimal bactericidal concentration (MBC) of VK3 against Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella Enteritidis PT30 in darkness was 625.0, 156.3, and 625.0 μg/ml, respectively. When treatments were carried out under light, the MBC values of VK3 (VK3 + light) were reduced in strain, light source, and exposure time dependent manners. The enhanced antimicrobial effect can be attributed to the production of reactive oxygen species of VK3, including hydroxyl radical and singlet oxygen under light exposure. Based on scanning electron microscope examination, VK3 + light treatment damaged the cell integrity of L. monocytogenes and Salmonella; and altered the cell morphology of E. coli O157:H7 including filament formation and fewer fimbriae/flagella. When applied on artificially inoculated lemon surfaces, VK3 + light treatments at MBC levels immediately reduced the levels of L. monocytogenes and E. coli O157:H7 by more than 5.0 Log10 CFU/disk and S. Enteritidis PT30 by ~4.0 Log10 CFU/disk. Bacterial populations of all tested strains were reduced to below the limit of detection (1.0 Log10 CFU/disk) after one day of storage at 4 °C; the numbers of positive samples after enrichment further decreased as the storage time increased, demonstrating the continuous antimicrobial effects the VK3 generated on lemon surfaces. In summary, the results of this study demonstrated the promising antimicrobial effects of VK3 + light treatments and their great application potential for fresh produce industry.