Novel Approach to Effective and Uniform Inactivation of Gram-Positive Listeria monocytogenes and Gram-Negative Salmonella enterica by Photosensitization

The aim of this work is to compare the antimicrobial efficiency of aminolevulinic acid- -based photosensitization (ALA-photo) and chlorophyllin-based photosensitization (CHL -photo) against Gram-positive Listeria monocytogenes and Gram-negative Salmonella enterica, to assess some quantitative inactivation parameters of their survival curves and to propose a strategy for their effective inactivation. Cells were incubated with CHL (7.5·10^–5–7.5·10^–8 M) or ALA (7.5 mM) and then illuminated with visible light (λ=400 nm). The analysis of bacterial survival curves indicates that Salmonella is more resistant to CHL-photo than to ALA-photo. The shoulder of the inactivation curve was longer and the population reduction suddenness was smaller than that after ALA-photo. Listeria was more sensitive to CHL-photo than to ALA-photo. No shoulder of the survival curves of Listeria was detected after CHL-photo. Examination of bacterial survival curves enabled the determination of the optimal experimental set-up for combined ALA- and CHL-photo treatments. The obtained results indicate that both Salmonella and Listeria were inactivated to an undetectable level when combined treatment was applied. Thus, combined ALA- and CHL -photo treatment may be an effective tool to increase effective inactivation of both Gram(+) and Gram(–) food pathogens. Experimental results and the analysis of survival curves suggest innovative approach to combat Gram(+) and Gram(–) bacteria by photosensitization in effective and uniform way