Responses of Escherichia coli and Listeria monocytogenes to ozone treatment on non-host tomato: Efficacy of intervention and evidence of induced acclimation

Authors: SHU, XIAOMEI - Texas Tech University; SINGH, MANAVI - Texas Tech University; KARAMPUDI, N - Texas Tech University; BRIDGES, DAVID - Volunteer; KITAZUMI, AI - Texas Tech University; Wu, Vivian; DE LOS REYES, BENILDO - Texas Tech University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2021
Publication Date: 10/28/2021
Citation: Shu, X., Singh, M., Karampudi, N., Bridges, D.F., Kitazumi, A., Wu, V.C., De los Reyes, B.G. 2021. Responses of Escherichia coli and Listeria monocytogenes to ozone treatment on non-host tomato: Efficacy of intervention and evidence of induced acclimation. PLoS ONE. 16(10). Article e0256324. https://doi.org/10.1371/journal.pone.0256324.
DOI: https://doi.org/10.1371/journal.pone.0256324

Interpretive Summary: The continued rise in cases of illness caused by treatment-resistant bacteria has forced post-harvest processors and other industries to reevaluate the effectiveness of their utilized antimicrobial treatments. Specifically, any treatment that does not sufficiently reduce levels of bacteria creates a scenario where treatment-resistance bacteria can develop. The use of ozone (O3) as an antimicrobial treatment has been well established in food-processing industries. However, if ozone treatments do not sufficiently eliminate bacterial threats, treatment-resistant strains of bacterial pathogens can develop and create a food safety risk. Therefore, the objective of this study was to monitor the responses of two bacterial pathogens associated with foodborne illness, Escherichia coli O157:H7 and Listeria monocytogenes, to better understand if sub-lethal ozone treatments can generate treatment-resistant bacteria and the real-time adaptations that bacteria employ to survive treatment. Our results demonstrate that the two bacteria utilize different genetic mechanisms to survive treatment, indicating that ozone treatment affects each species in distinctly different ways. Additionally, different genes were being expressed as the exposure time to the gas increased, which indicates adaptation during treatment. The results of this study demonstrate that sub-lethal ozone treatments could eventually lead to treatment-resistance bacteria and that bacterial response to treatment can vary depending on the species of bacteria.

Technical Abstract: Due to the rise of foodborne illness outbreaks caused by the consumption of raw fruits and vegetables, the development of effective antimicrobial strategies in post-harvest environments is needed. In this study, we evaluated the dose × time effects on the antimicrobial action of ozone (O3) gas using two foodborne pathogens, the Gram-negative Escherichia coli O157:H7 and Gram-positive Listeria monocytogenes. The study was performed in a non-host tomato environment by correlating the dose × time aspects of xenobiosis with bacterial survival and responses at the level of gene expression. In E. coli, exposure to low (1 µg) and moderate (2 µg) doses of O3 caused only partial killing. In comparison, the high-dose (3 µg) caused significant reductions in bacterial survival in a time-dependent manner. However, in L. monocytogenes, exposure to the 2 µg dose of O3 caused a significant reduction in bacterial survival even with short-duration exposure. The observed discrepancies in responses to O3 xenobiosis between E. coli and L. monocytogenes are likely due to differences in membrane and cytoplasmic structure and components. Transcriptome profiling by RNA-Seq showed that the primary defenses in E. coli were attenuated after exposure to 1 µg of O3 while (2 µg) dose responses were characterized by massive upregulation of pathogenesis and stress-related genes, indicating the activation of defense responses in the bacteria. Comparatively, after exposure to 3 µg of O3, more genes were downregulated during the first hour; with a large number of genes significantly upregulated after 2 and 3 h, suggesting that the O3 exposure led to potential adaptation. However, in L. monocytogenes, all three doses of O3 caused massive downregulation of genes at all time points, showing very different defense mechanisms in response to O3 treatment. This study provides important knowledge on the possible selection of target molecules for eliminating bacterial contamination on fresh produce without overlooking the potential risks of adaptation.