Abstract
Listeria monocytogenes is an opportunistic intracellular pathogen commonly associated with serious infections and multiple food-borne outbreaks. In this study, we investigated the influence of atmospheric cold plasma (80 kV, 50 Hz) on L. monocytogenes (EGD-e) and its knockout mutants of sigB, rsbR, prfA, gadD, and lmo0799 genes at different treatment time intervals. Further, to ascertain if sub-lethal environmental stress conditions could influence L. monocytogenes survival and growth responses, atmospheric cold plasma (ACP) resistance was evaluated for the cultures exposed to cold (4°C) or acid (pH 4) stress for 1 h. The results demonstrate that both wild-type and knockout mutants were similarly affected after 1 min exposure to ACP (p > 0.05), with a difference in response noted only after 3 min of treatment. While all L. monocytogenes strains exposed to acid/cold stress were hypersensitive to ACP treatment and were significantly reduced or inactivated within 1 min of treatment (p < 0.05). The results indicate sigB and prfA are important for general stress resistance and biofilm, respectively, loss of these two genes significantly reduced bacterial resistance to ACP treatment. In addition, exposure to sub-lethal 1min ACP increased the gene expression of stress associated genes. SigB showed the highest gene expression, increasing by 15.60 fold, followed by gadD2 (7.19) and lmo0799 (8.6) after 1 min exposure. Overall, an increase in gene expression was seen in all stress associated genes analyzed both at 1 min treatment; while long treatment time reduced the gene expression and some cases down-regulated prfA and gadD3 gene expression. By comparing the response of mutants under ACP exposure to key processing parameters, the experimental results presented here provide a baseline for understanding the bacterial genetic response and resistance to cold plasma stress and offers promising insights for optimizing ACP applications.
Highlights
Listeria monocytogenes 1/2a serotypes are persistent strains, which are commonly associated with multiple food-borne outbreaks
In an attempt to evaluate treatment parameters influencing intracellular reactive oxygen species (ROS) generation, L. monocytogenes strains were exposed to cold plasma treatment for 1–5 min
The effect of different durations of atmospheric cold plasma (ACP) treatment was analyzed for L. monocytogenes EGD-e at the gene transcription level
Summary
Listeria monocytogenes 1/2a serotypes are persistent strains, which are commonly associated with multiple food-borne outbreaks. In 2015, the EU reported 2,206 numbers of confirmed cases of Listeriosis in humans and approximately 964 hospitalizations (European Food Safety Authority [EFSA], 2017). The main vehicle for transmission of L. monocytogenes to humans is through consumption of contaminated food products. Because of the theoretical risks and actual outbreaks associated with L. monocytogenes, it is important to understand how they respond and or survive on fresh produce in response to novel decontamination processes. Many of the foodborne pathogens including L. monocytogenes possess adaptive responses to physiological environmental stresses such as acid, heat, salt, alkali and oxidative, which are likely exposure conditions in the food processing environment (Bucur et al, 2018)
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