Synergistic inactivation of Escherichia coli O157:H7 by plasma-activated water and mild heat
Introduction
Plasma-activated water (PAW), also called plasma water or plasma activated liquids, is prepared by treating water with non-thermal plasma (Shen et al., 2016). In the last few years, PAW has received lots of attention for its great potential applications in microbial decontamination, food preservation, and agriculture (Thirumdas et al., 2018). As a novel disinfectant, PAW can efficiently inactivate a wide range of microorganisms, such as bacteria (Thirumdas et al., 2018), bacterial spores (Sun et al., 2012), yeasts and molds (Guo et al., 2017, Xu et al., 2016), biofilms (Hozák et al., 2018), and viruses (Guo et al., 2018). Previous studies have shown that PAW can improve the microbiological safety and extend the shelf life of fresh fruits (Guo et al., 2017, Ma et al., 2015), vegetables (Xiang et al., 2019b), and edible fungus (Xu et al., 2016) by inactivating foodborne pathogenic and spoilage microorganisms. A variety of reactive nitrogen species (e.g. NO2−,NO3−, and ONOO−) are produced in PAW during the plasma activation. Therefore, PAW can be used as an alternative source of nitrite in the manufacture of cured meat products (Jung et al., 2015, Yong et al., 2018). More recent work has revealed that PAW ice can effectively inhibit microbial growth and extend the storage time of fresh shrimps (Liao et al., 2018b). The mechanism of PAW-induced microbial inactivation has not been completely elucidated. At present, it is generally accepted that reactive chemical species (e.g. hydrogen peroxide, peroxynitrite, nitric oxide, nitrates, and nitrite ions), acidic pH, and higher oxidation reduction potential (ORP) play a crucial role in PAW-induced microbial inactivation (Naïtali et al., 2010, Thirumdas et al., 2018).
Hurdle technology, the combination of different preservation techniques at an optimum level, has been widely utilized in food preservation (Leistner, 2000). Several successful combinations of thermal and emerging non-thermal technologies have been reported, such as mild heat, acidic electrolysed water, organic acids, high hydrostatic pressure (HPP), cold plasma, ultrasound, and pulsed electric fields (Dikici et al., 2015, Khan et al., 2017, Liao et al., 2018a, Liu et al., 2017). Mild heat treatment, a short heat treatment at 40–60 °C, is one of the most commonly used processes for microbial inactivation of processed food products (Choi et al., 2019). Mild heat is often combined with other non-thermal technologies, e.g., ultraviolet light (Carrillo, Ferrario, & Guerrero, 2017), ultrasound (Li et al., 2019a, Li et al., 2019b), high pressure carbon dioxide (Liu, Hu, Zhao, & Song, 2012), chlorine dioxide (Park, Ahn, & Kang, 2018), acidic electrolyzed water (Liu et al., 2017, Tirawat et al., 2016), and high pressure (Uchida & Silva, 2017). The combinations of mild heat with other food preservation technologies not only allow the optimisation of microbial inactivation, but also effectively maintain the nutritional and sensory attributes of food products during storage (Khan et al., 2017). For instance, Carrillo et al. (2017) found that the ultraviolet light-mild heat combination synergistically induced the inactivation of Escherichia coli, Saccharomyces cerevisiae, and Pseudomonas fluorescens in freshly squeezed carrot-orange juice blend.
However, the combination of PAW with other food preservation technologies is not yet well investigated at present. Recently, Choi et al. (2019) reported that the sequential combination of washing with PAW followed by mild heating (60 °C) showed marked synergistic bactericidal effect on salted Chinese cabbage without deteriorating the quality of this food product, but the synergistic bactericidal mechanisms of PAW and mild heat were not well understood.
Based on this scenario, the aim of this study was to assess the antimicrobial effect of PAW combined with mild heat against E. coli O157:H7. In addition, the mechanisms of synergistic bacteria inactivation by PAW combined with mild heat were also elucidated by measuring the changes in cell-surface morphology and membrane permeability.
Section snippets
Materials
Tryptic soy broth (TSB), tryptic soy agar (TSA), and sodium chloride were acquired from Beijing AoBoXing Bio-Tech Co., Ltd (Beijing, China). Propidium iodide (PI), glutaraldehyde, osmium tetroxide (OsO4), and N-phenyl-1-napthylamine (NPN) were obtained from Aladdin Industrial Corporation (Shanghai, China). All chemicals used were of analytical reagent grade and were used without further purification.
Bacterial strains and growth conditions
E. coli O157:H7 (CICC 10907) used in this study was purchased from the China Center of
Thermal stability of the antimicrobial activity of PAW
In consistent with our previous findings (Xiang et al., 2019a), PAW exhibited remarkable antibacterial activity against E. coli O157:H7. As shown in Fig. 1, a 2.15-log reduction of E. coli O157:H7 was observed after the PAW treatment alone at 30 °C for 8 min. The heating at 40, 50, 60, 70, and 80 °C for 10 min resulted in slight reduction in the antimicrobial capacity of PAW against E. coli O157:H7, respectively. The bactericidal effect of PAW against E. coli cells decreased with increasing
Conclusions
In a whole, the combined treatment of PAW and mild heat demonstrated higher bactericidal activity against E. coli O157:H7 than the PAW and mild heat treatments alone. The combined treatment of PAW and mild heat synergistically disrupted the membrane integrity cells and resulted in leakage of intracellular components (e.g. proteins and nucleic acids). Consequently, the bacterial membrane may be one of the main targets for bacterial inactivation induced by the combined treatment of PAW and mild
Acknowledgements
This work was financially supported by the National Key R & D Program of China (No. 2018YFD0401204), the China Postdoctoral Science Foundation (No. 2018M632765), the Fundamental Research Funds for the Universities in Henna Province (No. 18KYYWF0404), the Foundation for University Young Key Teachers of Henan Province (No. 2017GGJS095), and the Graduate’s Scientific Research Foundation of Zhengzhou University of Light Industry (No. 2018001).
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These authors contributed equally to this work.