Photodynamic inactivation of Salmonella enterica and Listeria monocytogenes inoculated onto stainless steel or polyurethane surfaces.

The photodynamic inactivation (PDI) uses molecules (photosensitizers) that absorb visible light (385-450 nm) energy, transfer it to adjacent molecular oxygen and thereby generating the biocidal singlet oxygen and other reactive oxygen species in situ. Efficacy of PDI was tested against Listeria monocytogenes and Salmonella enterica in three ways. Firstly, by adding the photosensitizer to bacterial suspensions. Secondly, bacteria were placed on inanimate surfaces and then sprayed with a photosensitizer suspension. Thirdly, bacteria were placed on coated inanimate surfaces, where the photosensitizer was permanently fixed in this coating (antimicrobial coating, AMC). Experiments were performed without and with soiling (albumin, sheep erythrocytes). In suspension, PDI reduced the number of viable Listeria monocytogenes and Salmonella enterica by more than 6 Log CFU/mL within seconds of light exposure. Photosensitizer spray suspension reduced the bacterial burden on surfaces with up to about 6 Log CFU/mL (5 s light exposure). PDI, even in the presence of high soiling, achieved a reduction of up to 5.1 ± 1.2 Log CFU/mL. The AMC showed a bacterial reduction that decreased from 5.1 to 0.7 Log CFU/mL with increasing soiling. Depending on the soiling and the respective bacteria, the spray suspension or AMC achieved a bacterial reduction on the running conveyor belt demonstrator ranging from 2.9 to 5.3 or 0.5 to 4.5 Log CFU/mL, respectively. PDI used visible light, phenalene-1-one and curcumin photosensitizers, and oxygen from ambient air to reduce the bioburden on typical surfaces in food processing. The AMC acts slower than the spray suspension but enables a permanent, self-sanitizing effect.

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Declarations of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.