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

The Use of Ultraviolet Irradiation to Improve the Efficacy of Acids That Are Generally Recognized as Safe for Disinfecting Fresh Produce in the Ready-to-Eat Stage

Fresh-cut produce is usually produced under standardized disinfection processes, which are unavailable at the ready-to-eat stage. Currently, chemical sanitizers are used for washing, but their disinfection efficacy is limited. In this study, UV-C (1.03 kJ/m2) was combined with organic acids that are generally recognized as safe (GRAS), including citric, malic, acetic, and lactic acids (LAs), to wash lettuce and cherry tomatoes that are contaminated with Escherichia coli O157:H7 and Salmonella Typhimurium. The results showed that LA was the most effective treatment among the single treatments, with a pathogen reduction and cross-contamination incidence of 2.0-2.3 log CFU/g and 28-35%, respectively. After combining with UV-C, the disinfection efficacy and cross-contamination prevention capacity of the four GRAS acids significantly improved. Among the combination treatments, the highest pathogen reduction (2.5-2.7 log CFU/g) and the lowest cross-contamination incidence (11-15%) were achieved by LA-UV. The analyses of ascorbic acid, chlorophyll, lycopene, antioxidant capacity, and ΔE indicated that neither the single nor combination treatments negatively affected the quality properties. These results provide a potential hurdle technology for fresh produce safety improvement at the ready-to-eat stage.

Seasonal Diet Composition of Goitered Gazelle (Gazella subgutturosa) in an Arid and Semi-Arid Region of Western China

Global climate change, habitat fragmentation, and human interference have resulted in a significant, ongoing decline in the population of goitered gazelles. Effective conservation strategies require an understanding of resource requirements of threatened species, such as dietary needs. Therefore, we aimed to elucidate the food composition and seasonal dietary changes of goitered gazelles through microhistological analyses of fresh feces. Fabaceae (11.5%), Gramineae (9.4%), Chenopodiaceae (20.2%), Asteraceae (10.1%), and Rosaceae (19.5%) formed the primary dietary components of goitered gazelle. Additionally, Krascheninnikovia arborescens (13.4%) and Prunus sibirica (16.3%) were identified as the key forage plants. Forbs (50.4%) were the predominant plants for grazing throughout the year, particularly in the spring (72.9%). The proportion of trees in the diet was highest in the autumn (36.7%) and comparatively lower in other seasons. Furthermore, the proportions of shrubs (22.0%) and graminoids (14.8%) both reached their peaks in the winter. Our findings indicate that goitered gazelles strategically forage seasonally to cope with resource bottlenecks, enhancing their adaptability to arid and semi-arid habitats. Our study provides essential ecological information for the conservation of goitered gazelles and emphasizes the importance of dietary studies of species of ecological significance in environmentally sensitive areas.

A multi-omics analysis strategy reveals the molecular mechanism of the inhibition of Escherichia coli O157:H7 by anthocyanins from Aronia melanocarpa and its application

Water pollution causes the propagation of pathogenic microorganisms, which poses a serious threat to human life. Escherichia coli O157:H7, as a representative organism that can directly exhibit molecular response to stress, was selected as the indicator bacteria for the study. Tandem mass tag (TMT) quantitative proteomics and non-targeted metabolomics were used to study the response of Escherichia coli O157:H7 to Aronia melanocarpa anthocyanin (AMA) treatment. The results showed that 628 proteins and 1338 metabolites changed significantly after treatment with AMAs. According to bioinformatics analysis, integrated proteomics and metabolomics analysis differentially expressed proteins (DEPs) and metabolites participate in pyruvate metabolism, glycolysis/gluconeogenesis, alanine, aspartate and glutamate metabolism and the pentose phosphate pathway. This study preliminarily proposed the inhibition mechanism of AMAs on Escherichia coli O157:H7 from the perspective of multi-omics, providing a theoretical basis for the application of natural preservatives in fresh cut vegetables.

The Effect of Different Organic Acids and Their Combination on the Cell Barrier and Biofilm of Escherichia coli

Organic acids are natural antimicrobial compounds commonly used in the food industry. In this study, acetic, lactic, butyric, citric, and malic acid at minimum inhibitory concentrations and their combinations at optimal inhibition concentrations were used to treat E. coli, and the effects on the cell barrier and biofilm of E. coli were evaluated. Acetic acid showed the highest membrane-damaging effect, while citric acid and malic acid could specifically damage the cell wall of E. coli, leading to alkaline phosphatase leakage. The RT-qPCR results showed that organic acids upregulated the membrane-protein-related genes of E. coli, and the combination of organic acids had a wider range of effects than single organic acid treatment. Moreover, organic acids inhibited the formation of E. coli biofilm and cellular activity within the biofilm. This study showed that the combination of organic acids plays a synergistic inhibitory role mainly through multiple destructive effects on the cell barrier and exhibited synergistic anti-biofilm effects. The three-three combination of acetic, lactic acid, and a third organic acid (butyric, citric, or malic) can play a better synergistic antibacterial effect than the two-pair combination of acetic and lactic acid. These findings have implications for the usage, development, and optimization of organic acid combinations.

Microbial Evaluation of Ozone Water Combined with Ultrasound Cleaning on Crayfish (Procambarus clarkii)

The effects of ozone water (OW) and ultrasound cleaning (UL) on microbial community diversity of crayfish were studied through microbial viable count and 16S rRNA gene sequencing. The results showed that compared with the control (CK), the ozone water combined with ultrasound cleaning (OCU) showed a significant reduction (p < 0.05) in total viable count (TVC), psychrophilic viable count (PVC), mesophilic viable count (MVC), Pseudomonas, hydrogen sulfide-producing bacteria (HSPB), molds and yeasts. Concretely, the TVC of the CK, OW, UL and OCU were 5.09, 4.55, 4.32 and 4.06 log CFU/g, respectively. The dominant bacterium in untreated crayfish was Chryseobacterium, and its relative abundance was reduced by combined treatment. Color measurement and sensory evaluation suggested that a satisfactory sensory experience could be obtained on the crayfish applied with OCU. In brief, OCU could be used as a cleaning strategy to control the microbial quality of crayfish and have no influence on its quality.

Use of aqueous ozone rinsing to improve the disinfection efficacy and shorten the processing time of ultrasound-assisted washing of fresh produce

In minimal processing industry, chlorine is widely used in the disinfection process and ultrasound (US) increases the disinfection efficacy of chlorine and reduces the cross-contamination incidence during washing. Tap water (TW), which has no disinfection effect, is generally used to rinse off sanitizer residues on the surface of disinfected fresh-cut vegetables. In this study, aqueous ozone (AO), a low-cost and residue-free sanitizer, was used to replace TW rinsing in combination with US (28 kHz)-chlorine (free chlorine [FC] at 10 ppm, a concentration recommended for industrial use) for the disinfection of fresh-cut lettuce as a model. US-chlorine (40 s) + 2.0 ppm AO (60 s) treatment resulted in browning spots on lettuce surface at the end of storage. In contrast, US-chlorine (40 s) + 1.0 ppm AO (60 s) did not lead to deterioration of the sensory quality (sensory crispness, color, and flavor) and a change in total color difference, and the activities of browning-related enzymes were significantly lower. Moreover, US-chlorine (40 s) + 1.0 ppm of AO (60 s) treatment led to significantly lower counts of Escherichia coli O157:H7, Salmonella Typhimurium, aerobic mesophilic (AMC), and molds and yeasts (M&Y) on days 0-7 than US-chlorine (60 s) + TW (60 s) and single 1.0 ppm AO (120 s) treatments, suggesting that AO provided an additional disinfection effect over TW, while reducing the overall processing time by 20 s. Cell membrane permeability analysis (alkaline phosphatase, protein, nucleotide, and adenosine triphosphate leakage) showed that the combination with 1.0 ppm AO caused more severe cell membrane damage in E. coli O157:H7 and S. Typhimurium, explaining the higher disinfection efficacy. 16S rRNA sequencing revealed that following US-chlorine (40 s) + 1.0 ppm of AO (60 s) treatment, Massilia and Acinetobacter had higher relative abundances (RAs) on day 7 than after US-chlorine (60 s) + TW (60 s) treatment, whereas the RAs of Escherichia-Shigella was significantly lower, indicating that the former treatment has a superior capacity in maintaining a stable microbial composition. This explains from an ecological point of view why US-chlorine (40 s) + 1.0 ppm of AO (60 s) led to the lowest AMC and M&Y counts during storage. The study results provide evidence that AO has potential as an alternative to TW rinsing to increase the disinfection efficacy of US-chlorine.

Use of Acetic Acid to Partially Replace Lactic Acid for Decontamination against Escherichia coli O157:H7 in Fresh Produce and Mechanism of Action

Escherichia coli O157:H7 is frequently detected in ready-to-eat produce and causes serious food-borne diseases. The decontamination efficacy of lactic acid (LA) is clearly established. In this study, LA was mixed with acetic acid (AA) to reduce costs while achieving consistent or better inhibitory effects. Time-kill curves and inoculation experiments using fresh-cut spinach and arugula indicated that 0.8%LA+0.2%AA shows similar antibacterial effects to those of 1%LA. To determine whether 1%LA and 0.8%LA+0.2%AA exert antibacterial effects by similar mechanisms, proteomics analysis was used. The proteins related to macromolecule localization, cellular localization, and protein unfolding were uniquely altered after the treatment with 1%LA, and the proteins related to taxis, response to stress, catabolic process, and the regulation of molecular function were uniquely altered after the treatment with 0.8%LA+0.2%AA. Based on these findings, combined with the results of a network clustering analysis, we speculate that cell membrane damage is greater in response to LA than to 0.8%LA+0.2%AA. This prediction was supported by cell membrane permeability experiments (analyses of protein, nucleotide, ATP, and alkaline phosphatase leakage), which showed that LA causes greater membrane damage than 0.8%LA+0.2%AA. These results provide a theoretical basis for the application of an acid mixture to replace LA for produce decontamination.

Combination of ozone and ultrasonic-assisted aerosolization sanitizer as a sanitizing process to disinfect fresh-cut lettuce

Reduction of sanitizer dosage and development of non-immersion disinfection methods have become major focuses of research. Here, we examined the disinfection efficacy of combining gaseous ozone (4 and 8 ppm) with aerosolized oxidizing sanitizer [sodium hypochlorite (SH, 100 and 200 ppm)] and aerosolized organic acid [acetic acid (AA, 1% and 2%) and lactic acid (LA, 1% and 2%)]. Notably, 1% AA and 4 ppm gaseous ozone were ineffective for disinfecting Salmonella Typhimurium, and treatment with 1% AA + 8 ppm ozone caused browning of lettuce leaves and stimulated increases in aerobic mesophilic count (AMC), aerobic psychrotrophic count (APC), S. Typhimurium, and Escherichia coli O157:H7. Treatment with 2% LA + 8 ppm ozone resulted in the lowest S. Typhimurium, E. coli O157:H7, Listeria monocytogenes, AMC, APC, and molds and yeasts during storage (0-7 days at 4 °C). Quality analysis indicates that LA + 8 ppm ozone and SH + 8 ppm ozone did not negatively affect L*, a*, b*, polyphenolic content, weight loss, and sensory properties; however, the levels of two individual phenolic compounds (3,4-dihydroxybenzoic acid and vanillin), responsible for phenylpropanoid synthesis, were significantly increased after treatment with 2% LA + 8 ppm ozone. These findings provided insights into the use of LA combined with gaseous ozone for application in disinfecting fresh produce.

Occurrence, diversity and temperature-dependent growth kinetics of Aeromonas spp. in lettuce

Bagged, pre-cut and prewashed lettuce products are marketed as ready to eat. This concept poses a food safety concern, due to lack of efficient hurdles to eliminate possible microbial contaminants from the fresh produce and/or the processing itself. Aeromonas spp. are potential foodborne pathogens that are frequently isolated from lettuce. High counts of, e.g., A. hydrophila have been found in retail ready-to-eat (RTE) vegetable salads. The aim of this study was to assess the general microbiological quality, the occurrence and diversity of potential human pathogenic mesophilic Aeromonas spp. of retail RTE lettuce products. Additionally, temperature-dependent growth kinetic parameters of Aerobic Plate Counts (APC) and Aeromonas spp. in one selected RTE lettuce product, rocket lettuce, were quantified by performing storage experiments at 4 °C, 8 °C and 12 °C. The Aeromonas isolates were further characterized regarding pathogenic traits and phylogenetic relationship. The overall hygienic quality of the lettuce products was unsatisfactory, as 60% of the products had an APC level higher than 7.0 log CFU/g. Presumptive Aeromonas spp. were detected in 52% of the samples, levels ranging from approximately 2.0-6.0 log CFU/g. Significantly lower counts of APC and Aeromonas spp. were found in uncut and unwashed products. Presumptive Aeromonas spp. were able to proliferate in rocket lettuce stored at 4 °C (μmax = 0.39 ± 0.06/d and μmax = 0.43 ± 0.05/d for lettuce from producers A and B, respectively), and μmax was approximately 2× higher at 8 °C and 3× higher at 12 °C. Eighty-four percent of the collected isolates were identified as A. media, based on partial gyrB sequencing. Additionally A. salmonicida and A. bestiarum were detected. The pathogenic potential in this material was high, most of the isolates harbored at least one of the toxin genes, act, ast, alt.

Disinfection of Ready-to-Eat Lettuce Using Polyhexamethylene Guanidine Hydrochloride

As a novel and safe sanitizer, polyhexamethylene guanidine hydrochloride (PHMG) has been used to inhibit the spoilage of agricultural products caused by fungi. However, little is known about its antibacterial effects on vegetables. In this study, we evaluated the disinfection efficacy of PHMG on ready-to-eat lettuce. PHMG (150-200 mg/L) treatment for 5 min was optimal for lettuce disinfection. Compared to several household sanitizers (vinegar: 1% acetic acid; kettle descaler: 1% citric acid; "84" disinfectant: 200 mg/L sodium hypochlorite), PHMG showed the greatest reductions in Escherichia coli O157:H7, Listeria monocytogenes, aerobic mesophilic counts, aerobic psychrotrophic counts and molds and yeasts. Quality analysis of color (as determined by L*, a* and b*) and determination of electrolyte leakage indicated that PHMG did not cause any additional quality loss as compared to other household sanitizers. These results provide a reference for the application of PHMG as a vegetable sanitizer at the ready-to-eat stage.

Combination of polyhexamethylene guanidine hydrochloride and potassium peroxymonosulfate to disinfect ready-to-eat lettuce

There is increasing demand for improved fresh produce disinfection technology during the ready-to-eat stage, especially in low-income developing countries. We previously reported that polyhexamethylene guanidine hydrochloride (PHMG) is an effective sanitizer using fresh-cut lettuce as a model. As a low-cost alternative, in the present study, we examined the disinfection efficacy of combining PHMG with the oxidizing sanitizer potassium peroxymonosulfate (PMS). PHMG (150 mg L⁻¹) reduced the counts of Escherichia coli O157:H7, non-O157 E. coli, Listeria monocytogenes, Salmonella typhimurium, and naturally present microbes on ready-to-eat lettuce. The disinfection efficacy of PMS was significantly lower than that of PHMG; however, the efficacy of their combination (100 mg L⁻¹ PHMG + 50 mg L⁻¹ PMS, 50 mg L⁻¹ PHMG + 100 mg L⁻¹ PMS, and 50 mg L⁻¹ PHMG + 50 mg L⁻¹ PMS) was equivalent to that of PHMG alone. Color and sensory analyses (crispness, color, flavour, and off-odor) indicated that the combination of PHMG and PMS will not lead to additional quality loss when compared with tap water treatment, and electrolyte leakage analysis showed no additional lettuce surface damage of the combination when compared with PHMG-only treatment. These results show that partial replacement of PHMG by PMS is a cost-reducing strategy, providing a theoretical foundation for its practical application.

Combination of polyhexamethylene guanidine hydrochloride and potassium peroxymonosulfate can achieve consistent disinfection effects as those obtained with polyhexamethylene guanidine hydrochloride but at a lower cost.

Elucidating Escherichia Coli O157:H7 Colonization and Internalization in Cucumbers Using an Inverted Fluorescence Microscope and Hyperspectral Microscopy

Contamination of fresh cucumbers (Cucumis sativus L.) with Escherichia coli O157:H7 can impact the health of consumers. Despite this, the pertinent mechanisms underlying E. coli O157:H7 colonization and internalization remain poorly documented. Herein we aimed to elucidate these mechanisms in cucumbers using an inverted fluorescence microscope and hyperspectral microscopy. We observed that E. coli O157:H7 primarily colonized around the stomata on cucumber epidermis without invading the internal tissues of intact cucumbers. Once the bacterial cells had infiltrated into the internal tissues, they colonized the cucumber placenta and vascular bundles (xylem vessels, in particular), and also migrated along the xylem vessels. Moreover, the movement rate of E. coli O157:H7 from the stalk to the flower bud was faster than that from the flower bud to the stalk. We then used hyperspectral microscope imaging to categorize the infiltrated and uninfiltrated areas with high accuracy using the spectral angle mapper (SAM) classification method, which confirmed the results obtained upon using the inverted fluorescence microscope. We believe that our results are pivotal for developing science-based food safety practices, interventions for controlling E. coli O157:H7 internalization, and new methods for detecting E. coli O157:H7-plant interactions.