An effective combined treatment using malic acid and ozone inhibits Shigella spp. on sprouts

Antimicrobial and Antibiofilm Efficacy and Mechanism of Oregano Essential Oil Against Shigella flexneri

The aim of this study was to assess the antimicrobial activity of oregano essential oil (OEO) against Shigella flexneri and eradication efficacy of OEO on biofilm. The results showed that the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of OEO against S. flexneri were 0.02% (v/v) and 0.04% (v/v), respectively. OEO effectively killed S. flexneri in Luria-Bertani (LB) broth and contaminated minced pork (the initial population of S. flexneri was about 7.0 log CFU/mL or 7.2 log CFU/g), and after treatment with OEO at 2 MIC in LB broth or at 15 MIC in minced pork, the population of S. flexneri decreased to an undetectable level after 2 or 9 h, respectively. OEO increased intracellular reactive oxygen species concentration, destroyed cell membrane, changed cell morphology, decreased intracellular ATP concentration, caused cell membrane depolarization, and destroyed proteins or inhibited proteins synthesis of S. flexneri. In addition, OEO effectively eradicated the biofilm of S. flexneri by effectively inactivating S. flexneri in mature biofilm, destroying the three-dimensional structure, and reducing exopolysaccharide biomass of S. flexneri. In conclusion, OEO exerts its antimicrobial action effectively and also has a valid scavenging effect on the biofilm of S. flexneri. These findings suggest that OEO has the potential to be used as a natural antibacterial and antibiofilm material in the control of S. flexneri in meat product supply chain, thereby preventing meat-associated infections.

Antibacterial Activity and Possible Mechanism of Litsea cubeba Essential Oil Against Shigella sonnei and Its Application in Lettuce

Shigella sonnei, the causative agents of bacillary dysentery, remains a significant threat to public health. Litsea cubeba essential oil (LC-EO), one of the natural essential oils, exhibited promising biological activities. In this study, the antibacterial effects and possible mechanisms of LC-EO on S. sonnei and its application in lettuce medium were investigated. The minimum inhibitory concentration (MIC) of LC-EO against S. sonnei ATCC 25931 and CMCC 51592 was 4 and 6 μL/mL, respectively. The LC-EO could inhibit the growth of S. sonnei, and decreased S. sonnei to undetectable levels with 4 μL/mL for 1 h in Luria-Bertani broth. The antibacterial mechanism indicated that after the treatment of LC-EO, the production of reactive oxygen species and the activity of superoxide dismutase were significantly elevated in S. sonnei cells, and eventually led to the lipid oxidation product, the malondialdehyde content that significantly increased. Moreover, LC-EO at 2 MIC could destroy 96.51% of bacterial cell membrane integrity, and made S. sonnei cells to appear wrinkled with a rough surface, so that the intracellular adenosine triphosphate leakage was about 0.352-0.030 μmol/L. Finally, the results of application evaluation indicated that the addition of LC-EO at 4 μL/mL in lettuce leaves and 6 μL/mL in lettuce juice could decrease the number of S. sonnei to undetectable levels without remarkable influence on the lettuce leaf sensory quality. In summary, LC-EO exerted strong antibacterial activity and has the potential to control S. sonnei in food industry.

Antibacterial Mechanism of Eugenol Against Shigella sonnei and Its Antibacterial Application in Lettuce Juice

Shigella sonnei is a species of Shigella, and the infection rate of S. sonnei is increasing year by year. Eugenol is an active ingredient in clove essential oil and is a generally recognized as safe (GRAS)-certified food ingredient. The mechanism of inhibition of S. sonnei by eugenol has been investigated in this study. The minimum inhibitory concentration of eugenol against both S. sonnei ATCC 25931 and S. sonnei CMCC 51592 was 0.5 mg/mL and minimum bactericidal concentration (MBC) for both strains was 0.8 mg/mL. The inhibition effect of eugenol against S. sonnei was due to increased levels of reactive oxygen species in cells, changed cell membrane permeability, and induced cell membrane dysfunction, for instance, cell membrane hyperpolarization and intracellular ATP concentration drops. The results of confocal laser scanning microscope and field emission scanning electron microscopy showed that eugenol leads to decreased cell membrane integrity, resulting in changed cell morphology. Moreover, eugenol inactivated S. sonnei in Luria-Bertani (LB) broth and lettuce juice. These results indicated that eugenol could inactivate S. sonnei and has the potential to control S. sonnei in the food industry.

Inactivation of Salmonella and Shiga toxin-producing Escherichia coli (STEC) from the surface of alfalfa seeds and sprouts by combined antimicrobial treatments using ozone and electrolyzed water

Individual chemical and non-chemical treatments have failed to disinfect alfalfa seeds and sprouts from pathogens thoroughly. This study investigated the disinfection of alfalfa seeds and sprouts using a procedure combining ozone with acidic (pH 3.0) electrolyzed water (AEW). Inoculated alfalfa seeds with a cocktail of 3 strains Salmonella and 3 strains of STEC were treated sequentially with aqueous ozone followed by AEW. Treatment started by immersing the samples into ozonated water (5 mg/L ozone) for 15 or 20 min with persistent oxygen feeding pressurized with 10 psi. The samples then were immersed in 1 L of AEW for 15 min. Salmonella and STEC were significantly (P < 0.05) reduced by 3.6 and 2.9 log CFU/g on seeds respectively, and by 3.1 and 3.0 log CFU/g reduction on sprouts. Significant differences (P < 0.05) were found in the magnitude of the log reduction between Salmonella and STEC on seeds and between seeds and sprouts. Using combined treatments showed no significant changes in the quality, including shelf life, weight, and color in sprouts as compared to controls. The findings suggest that the combination of ozone and AEW is effective in inactivation of Salmonella and STEC on alfalfa seeds and sprouts with no adverse effects on sprouts quality.

In Situ Generation of Chlorine Dioxide for Decontamination of Salmonella, Listeria monocytogenes, and Pathogenic Escherichia coli on Cantaloupes, Mung Beans, and Alfalfa Seeds

ABSTRACT

In situ generation of chlorine dioxide to reduce microbial populations on produce surfaces has been shown to be effective on produce models. This study examined the treatment for decontamination of bacterial pathogens on whole cantaloupes and sprout seeds. Whole cantaloupes, mung beans, and alfalfa seeds were inoculated with Salmonella, Listeria monocytogenes, and Shiga toxin-producing Escherichia coli, sprayed with or dipped in 0.4 to 1.6% sodium chlorite (NaClO2) solutions, dried, and treated with 6 mM hydrochloric acid (HCl; sequential treatment). Controls were samples treated with NaClO2 or HCl (individual treatment). The pathogen populations on samples before and after treatments were enumerated to determine the reductions of pathogen populations by the treatments. The methods of applying NaClO2 and HCl (dipping for 30 min or spraying 0.2 g on cantaloupe rind [2 by 2 cm]), NaClO2 concentrations of 0.4 to 1.6% for cantaloupes, and treatment times of 5, 15, and 30 min for sprout seeds were evaluated to identify treatment parameters. For cantaloupes treated with spraying with 1.6% NaClO2, the sequential treatment caused significantly (P < 0.05) higher reductions (6.2 to 7.7 log CFU/cm2) than the combined reductions (3.2 to 5.2 log CFU/cm2) by the individual treatments. For cantaloupes treated by dipping in 1.6% NaClO2 and by spraying with 0.4 and 0.8% NaClO2, the reductions caused by the sequential treatment were not significantly (P > 0.05) different from those by the individual treatments. For mung beans, sequential 15- and 30-min treatments caused significantly (P < 0.05) higher reductions of 4.3 to 5.0 and 4.7 to 6.7 log CFU/g, respectively, than the individual treatments. The sequential 15-min treatment also caused high reductions of 5.1 to 7.3 log CFU/g on alfalfa seeds. The treatments did not bleach the color of cantaloupes and did not affect the germination rates of mung beans and alfalfa seeds. This study identified 1.6% NaClO2 and 6 mM HCl for sequential spraying treatment for cantaloupes and for sequential dipping (15-min) treatment for mung beans and alfalfa seeds that may be used for decontamination of whole cantaloupes and sprout seeds.

HIGHLIGHTS

Effect of Ozone Treatment on Flavonoid Accumulation of Satsuma Mandarin (Citrus unshiu Marc.) during Ambient Storage

This study aimed to compare the flavonoid accumulation between ozone-treated and untreated Satsuma mandarin (Citrusunshiu Marc.) fruits. The fruits exposed to gaseous ozone were found to have higher antioxidant activities and content of flavonoid during the storage period by ultra-high performance liquid chromatography (UPLC). To reveal the molecular regulation of flavonoid accumulation by ozone, chalcone synthase (CHS), chalcone isomerase (CHI), β-1,3-glucanase (GLU), chitinase (CHT), phenylalanine ammonia-lyase (PAL), and peroxidase (POD) were identified and their expression was examined by quantitative real-time polymerase chain reaction (q-PCR). These results support the promising application of ozone treatment as a safe food preservation technique for controlling postharvest disease and extending shelf-life of harvested Satsuma mandarin.

Reduction of Escherichia coli O157:H7, Listeria monocytogenes, and Naturally Present Microbe Counts on Lettuce using an Acid Mixture of Acetic and Lactic Acid

Lactic acid (LA) and acetic acid (AA) are independently used to disinfect fresh leaf vegetables. LA has a higher efficacy but costs more than AA. Herein, we compared the disinfection efficacy of LA, AA, and their mixture on lettuce to determine whether the cheaper acid mixture shows similar or more efficacy than LA. Quality analysis indicated that the acid mixture and individual acids did not cause additional loss of instrument color and polyphenolic content compared with that of the control; however, visible defects were observed at AA concentrations exceeding 0.8%. Analysis of Escherichia coli O157:H7, Listeria monocytogenes, and naturally present microbes (aerobic mesophilic and psychrotrophic bacteria, coliforms, molds, and yeasts) showed that the acid mixture led to the highest reduction in microbial count during storage. 16S rRNA sequencing was further employed to understand the effects of the acid mixture and individual acids on lettuce microbial ecology. During storage, the acid mixture and individual acids significantly decreased the abundance of Massilia spp. and Alkanindiges spp. but there was a marked increase in Escherichia-Shigella abundance (LA: 0.003-58.82%; AA: 0.01-55.34%; acid mixture: undetected to 50.71%; control: 0.007-33.09%), indicating that acid disinfection altered the microbial ecology to stimulate Escherichia-Shigella growth. These results enhance our understanding of the relationship between lettuce disinfection and ecological changes.

Reduction of Escherichia coli O157:H7 and naturally present microbes on fresh-cut lettuce using lactic acid and aqueous ozone

Lactic acid (LA) is an effective sanitizer for disinfection of fresh produce. Tap water is generally used to wash disinfected fresh produce because sanitizer residues negatively affect the quality and organoleptic properties of the produce. However, tap water is ineffective for secondary disinfection compared with sanitizers. Thus, we propose a disinfection method using LA plus aqueous ozone (AO), an oxidizing sanitizer that does not lead to secondary residue. We compared the proposed method of 1% LA (90 s) plus 1 mg L-1 AO (30 s) or 2 mg L-1 AO (30 s) with the traditional method of 100 ppm chlorine (120 s) or 1% LA (120 s) plus tap water (30 s) and 2 mg L-1 AO (150 s). Microbial analysis showed that LA plus AO led to the greatest reductions in microbes (Escherichia coli O157:H7, aerobic mesophilic counts, aerobic psychrophilic counts, moulds, and yeasts) during storage (0-5 days at 5 °C). Quality analysis (colour, sensory qualities, electrolyte leakage, polyphenolic content, and weight loss) showed that LA + AO did not cause additional quality loss compared with tap water treatment. These results indicate that the hurdle technology proposed (LA plus AO) has a good potential for use in fresh produce disinfection.

Tomato type and post-treatment water rinse affect efficacy of acid washes against Salmonella enterica inoculated on stem scars of tomatoes and product quality

A study was conducted to evaluate the effects of post-treatment rinsing with water on the inactivation efficacy of acid treatments against Salmonella inoculated onto stem scar areas of two types of tomatoes. In addition, impact on fruit quality was investigated during 21 days post-treatment storage at 10 °C. A four-strain cocktail of Salmonella enterica (S. Montevideo, S. Newport, S. Saintpaul, and S. Typhimurium) was inoculated onto stem scar areas of grape and large round tomatoes. The inoculated fruits were then treated for 2 min with the following solutions: water, 2% lactic acid +2% acetic acid +2% levulinic acid, 1.7% lactic acid +1.7% acetic acid +1.7% levulinic acid, and 3% lactic acid +3% acetic acid. After treatments, half of the fruits were rinsed with water while another half were not rinsed. Non-inoculated grape tomatoes for quality analysis were treated with the same solutions with and without subsequent water rinse. Results demonstrated that the acid combinations reduced populations of Salmonella enterica on the stem scar area of grape tomatoes by 1.52-1.90 log CFU/fruit, compared with the non-treated control while water wash and rinse removed the bacterium by only 0.23-0.30 log CFU/fruit. On the stem scar of large round tomatoes, the same acid treatments achieved 3.54 log CFU/fruit reduction of the pathogen. The varying response to the acid washes between grape and large round tomatoes seems to be related to the differences in surface characteristics of stem scar areas observed with SEM. Rinsing with water after acid combination treatments did not significantly affect the efficacy of the treatments in either grape or large round tomatoes. Acidic off-odor was detected on fruits treated with acid combination without water rinse 1 day after treatment while water rinse eliminated the off-odor. The acid treatments with and without water rinse did not consistently affect appearance, color, firmness, or lycopene or ascorbic acid contents of tomatoes during 21-days storage at 10 °C. Considering the similarity in antimicrobial efficacy between the fruits with and without water rinse following acid treatments, and the elimination of acidic odor by water rinse, fruits should be rinsed with water after acid treatments. Overall, our results demonstrated that the acids were more effective in inactivating Salmonella on large round tomatoes than on grape tomatoes, and water rinses following acid treatments eliminated the acidic odor without affecting the efficacy of the acids against Salmonella.

Effects of malic acid or/and grapefruit seed extract for the inactivation of common food pathogens on fresh-cut lettuce

This study investigated the antimicrobial activity of malic acid (MA), grapefruit seed extract (GSE), and combined (MA+GSE) treatment against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on fresh-cut lettuce. The antimicrobial effects of 1% MA and 0.5% GSE alone and in combination (1% MA+0.5% GSE) were tested on artificially inoculated lettuce during storage at 5°C for 14 days. The maximum reductions of E. coli O157:H7, S. Typhimurium, and L. monocytogenes were 4.96, 4.80, and 3.95 log CFU/g observed with MA+GSE during storage for 14 days, respectively. MA+GSE showed the greatest reduction against in E. coli O157:H7 and L. monocytogenes. These results indicate that the combined treatment was more effective than MA and GSE alone treatment. Therefore, it suggests that MA + GSE could be used as an effective intervention method for improving microbiological safety of fresh-cut lettuce.

Current intervention strategies for the microbial safety of sprouts

Sprouts have gained popularity worldwide due to their nutritional values and health benefits. The fact that their consumption has been associated with numerous outbreaks of foodborne illness threatens the $250 million market that this industry has established in the United States. Therefore, sprout manufacturers have utilized the U.S. Food and Drug Administration recommended application of 20,000 ppm of calcium hypochlorite solution to seeds before germination as a preventative method. Concentrations of up to 200 ppm of chlorine wash are also commonly used on sprouts. However, chlorine-based treatment achieves on average only 1- to 3-log reductions in bacteria and is associated with negative health and environmental issues. The search for alternative strategies has been widespread, involving chemical, biological, physical, and hurdle processes that can achieve up to 7-log reductions in bacteria in some cases. The compilation here of the current scientific data related to these techniques is used to compare their efficacy for ensuring the microbial safety of sprouts and their practicality for commercial producers. Of specific importance for alternative seed and sprout treatments is maintaining the industry-accepted germination rate of 95% and the sensorial attributes of the final product. This review provides an evaluation of suggested decontamination technologies for seeds and sprouts before, during, and after germination and concludes that thermal inactivation of seeds and irradiation of sprouts are the most practical stand-alone microbial safety interventions for sprout production.