Enteritidis PT4 Research Articles

Salmonella control for dried apple cubes

Pathogen contamination in low-moisture foods prepared by drying is a food safety concern. Two thermal treatments were explored in this research as pathogen control measures for apple drying operations: high humidity air heating (HHAH) and ascorbic acid (AA) treatments. Their effectivenesses on Salmonella inactivation were assessed. Apple cubes (6.4 mm) inoculated with a three-strain Salmonella cocktail (S. Enteritidis PT 30, S. Montevideo, and S. Agona 447967) were subjected to HHAH and AA treatments, and then enumerated for the survival of Salmonella. In parallel, uninoculated apple cubes treated with HHAH or AA were dried by hot air at 90 °C for 70 min. The moisture content and color values of the dried apple cubes were measured. The results revealed that HHAH treatment at 70 °C for 16 min, at 90 °C for 9 min, and AA treatment using 3.4% ascorbic solutions at 65 °C for 90 s achieved 6.9 ± 0.4, 6.2 ± 0.2, and 7.9 ± 0.6 log CFU/g reduction, respectively. The moisture content of dried apple cubes was not significantly different, highlighting the potential of these methods for enhancing microbial safety of dried apples. The HHAH treatment resulted in an increased browning effect on dried apple cubes, while the AA treatment reduced the browning of dried apple cubes. This study offers valuable insight into preventive controls for pathogen management in food drying operations.

Effect of ohmic heating application on Salmonella Enteritidis in liquid whole egg

Liquid eggs have high nutrient levels and are prone to spoilage. Although, pasteurization can reduce the risk of pathogenic microorganisms in liquid eggs, applied high temperatures can damage the egg’s basic components and structure. Therefore, recent research is investigating new technologies that can inactivate pathogens in the shortest time and low heat application without adversely affecting food quality. One of these technologies is ohmic heating, which heats up food quickly by applying an electric field. This study aimed to investigate the inactivation level of Salmonella Enteritidis in liquid eggs by different voltage gradients of ohmic heating. Inoculated liquid egg samples with S. Enteritidis PT4 (NCTC 13349) were exposed to ohmic heating at 5, 10, and 20 V/cm for 5 minutes. The results showed that ohmic heating at 20 V/cm reduced S. Enteritidis counts by about 4 logs (65.6 % reduction) in 4 minutes without coagulation, while ohmic heating at 5 and 10 V/cm had no significant impact. In conclusion, the effectiveness of ohmic heating for inactivating pathogens in liquid eggs depends on the electric field intensity and the duration of treatment. Therefore, the best ohmic heating conditions should be chosen carefully to ensure food safety and quality.

Survival and thermal resistance of Salmonella in chocolate products with different water activities

Contamination of Salmonella in chocolate products has caused worldwide outbreaks and recalls. There is a lack of information on the impact of water activity (aw) on the stability of Salmonella in chocolate products during storage and thermal treatments. In this research, the survival and thermal resistance of a Salmonella cocktail (S. Enteritidis PT30, S. Tennessee K4643, S. Typhimurium S544) was examined in different chocolate products (dark chocolate, white chocolate, milk chocolate) at two aw levels (0.25, 0.50) over 12 months at 22 °C. A reduction of 4.19 log10 CFU/gof Salmonella was obtained in dark chocolate after 12 months (aw = 0.50, at 22 °C); less reductions were observed in white and milk chocolates. In all three products, more reductions were observed ataw = 0.50 than at aw = 0.25 over the 12-months storage. When treated at 80 °C, the D-values (time required to cause 1 log reduction) of the Salmonella cocktail in the chocolate samples with initial aw of 0.25 were 35.7, 25.2 and 11.6 min in dark, white and milk chocolate, respectively, before the storage. The D80°C -values of Salmonella cocktail in the samples with initial aw of 0.50 were 6.45, 7.46, and 3.98 min in dark, white and milk chocolate, respectively. After 12 months of storage at 22 °C, the D80°C-value of Salmonella cocktail decreased to 9.43 min (p < 0.05) in milk chocolate but remained 22.7 min in white chocolate with an aw of 0.25 at 22 °C. The data suggests that Salmonella can survive in chocolate products for up to 12 months, and its thermal resistance remained relatively stable. Thus, Salmonella is resistant to desiccation in chocolates, particularly in milk and white chocolates, and its thermal resistance remains during one-year storage, which could pose a potential threat for future outbreaks.

Performance and transcriptome analysis of Salmonella enterica serovar Enteritidis PT 30 under persistent desiccation stress: Cultured by lawn and broth methods

Lawn-harvest method uses a solid medium (e.g., tryptic soy agar, TSA) to produce bacterial lawns and is widely accepted for the culture of microorganisms in microbial studies of low-moisture foods (LMFs, foods with water activity less than 0.85). It produces desiccation-tolerant cells with higher D-values in LMFs; however, little is known about the molecular mechanisms underlying bacterial resistance. Salmonella enterica Enteritidis PT 30 (S. Enteritidis), the most pertinent pathogen in LMFs, was cultured in TSA and tryptic soy broth (TSB). Cells were harvested and inoculated on filter papers to assess their performance under a relative humidity of 32 ± 2%. Transcriptome analysis of cultured cells during long-term desiccation (24, 72, and 168 h) was conducted in TruSeq PE Cluster Kit (Illumina) by paired-end methods. Lawn-cultured S. Enteritidis cells have stronger survivability (only decreased by 0.78 ± 0.12 log after 130 d of storage) and heat tolerance (higher D/β value) than those from the broth method. More desiccation genes of lawn-cultured cells were significantly upregulated from growth to long-term desiccation. Differentially expressed genes were the most enriched in the ribosome and sulfur metabolism pathways in the lawn- and broth-cultured groups. This study tracked the transcriptomic differences between two cultured groups in response to long-term desiccation stress and revealed some molecular mechanisms underlying their different suitability in microbial studies of LMFs.

Increased heat tolerance and transcriptome analysis of Salmonella enterica Enteritidis PT 30 heat-shocked at 42 ℃

In this study, we compared the heat tolerance parameter (D65℃) values of Salmonella enterica serovar Enteritidis PT 30 (S. Enteritidis ) heat adapted at different degrees (at 42 ℃ for 20–180 min) and cultivated using two methods. The treated group with the highest D65℃ value (LP-42 ℃-60 min) and the untreated groups (Control-TSB and Control-TSA) were subjected to transcriptome analysis. Heat-adaptation increased the D65℃ values of S. Enteritidis by 24.5–60.8%. The D65℃ values of the LP-42 ℃-60 min group (1.85 ± 0.13 min, 7.7% higher) was comparable to that of the Control-TSA. A total of 483 up- and 443 downregulated genes of S. enteritidis were identified in the LP-42 ℃-60 min group (log2fold change > 1, adjusted p-value < 0.05). Among these genes, 5 co-expressed and 15 differentially expressed genes in the LP-42 ℃-60 min and Control-TSA grops possibly contributed to the high D65℃ values of S. Enteritidis . The Rpo regulon was involved in the heat adaptation of S. Enteritidis , as evidenced by the significant upregulation of rpoS, rpoN, and rpoE. KEGG enrichment pathways, such as biosynthesis of secondary metabolites, tricarboxylic acid, and ribosomes were identified and mapped to reveal the molecular mechanisms of S. enteritidis during heat adaptation. This study quantified the enhanced heat tolerance of S. Enteritidis heat adapted at different degrees of heat-adaptation. The results of this study may serve as a basis for elucidating the molecular mechanisms underlying the enhanced heat tolerance at the transcriptome level.

Determination of time dependent antibacterial activities of curcumin, carvacrol and styrax liquidus on Salmonella Enteritidis

Salmonella Enteritidis is amongst the most common causes of foodborne salmonellosis. Multi-drug resistant Salmonella strains has been associated with treatment failures. Plant-derived phytochemicals may be an alternative to antibiotics in combating these bacteria. The purpose of this study is to investigate the antibacterial activity of curcumin, carvacrol and styrax liquidus on S. Enteritidis and S. Enteritidis PT4. Minimum inhibitory concentration (MIC) values of these substances were detected at 1.5, 3, 7.5 and 24 h by broth microdilution method to evaluate their time-dependent antibacterial activities. The findings of the present study showed that MIC values of carvacrol, curcumin and styrax liquids for both S. Enteritidis and S. Enteritidis PT4 were 125.0 μg/mL, 132.5 μg/mL, 31.3 mg/mL for 24 h, respectively. Also, a time-dependent change was observed in the MIC values of curcumin. Carvacrol, curcumin and styrax liquidus can be used to provide antimicrobial effect on S. Enteritidis and S. Enteritidis PT4 in food applications, taking into account the MIC values and contact times.

The influence of temperature and water activity on thermal resistance of Salmonella in milk chocolate

Salmonella contamination of chocolate-derived products has caused several outbreaks and recalls in recent years. Earlier research found that reducing moisture content or water activity of low-moisture foods sharply enhances the resistance of Salmonella during thermal treatments. However, there is a lack of data that correlates the relationship between temperature, water activity (aw), and thermal resistance of Salmonella in milk chocolate. In this study, milk chocolate was inoculated with Salmonella Enteritidis PT 30 and conditioned to aw of 0.23, 0.33, and 0.43 at room temperature (21 ± 2 °C). The chocolate samples were heated at 70, 75, and 80 °C to obtain the D-values (time to inactivate 90% of the test microorganisms at a given temperature) of S. Enteritidis PT 30. The change of aw of milk chocolates at elevated temperatures (up to 80 °C) was also investigated. The results showed that the D-value of S. Enteritidis PT 30 decreased exponentially with the increase of aw or temperature. The maximum D-value was 47.4 ± 3.7 min obtained at 70 °C and aw of 0.23. The minimum D-value was 5.2 ± 1.0 min at 80 °C and aw of 0.43. The z-values were found as the followings: zaw,70°C = 0.42, zaw,75°C = 0.36, and zaw,80°C = 0.46 °C; zT, 0.23 = 18.8 ± 2.5, zT, 0.33 = 20.6 ± 4.1, zT, 0.43 = 18.1 ± 0.5 °C. Overall, the results from this research may provide useful information to help the industry control the risk of Salmonella contamination and improve microbial safety in chocolate production.

Survival kinetics, membrane integrity and metabolic activity of Salmonella enterica in conventionally and osmotically dehydrated coconut flakes

Many outbreaks involving Salmonella enterica in dehydrated coconut have been reported. Little is known about the survival of S. enterica in dehydrated coconut flakes at common retail or domestic storage conditions. This study evaluated the behavior of a S. enterica cocktail (S. Enteritidis PT4, S. Typhimurium PT4, S. Bredeney, S. Muenster and S. Agona) in conventionally and osmotically dehydrated coconut flakes under four storage regimes: 25 °C for 120 days, 25 °C for 30 days then 7 °C for 90 days, 7 °C for 30 days then 25 °C for 90 days, and 7 °C for 120 days. S. enterica membrane integrity (using with propidium iodide and bis-1,3-dibutylbarbutyric acid) and metabolic activity (using 5-cyano-2,3-ditolyl tetrazolium chloride) were assessed by flow cytometry analysis after dehydration and storage at 7 °C or 25 °C for 120 days. Lower S. enterica inactivation rates (kmax 0.02 to 0.04 1/days) were observed in conventionally dehydrated coconut flakes compared to osmotically dehydrated coconut flakes (kmax 0.16 to 0.20 1/days). Changes in storage temperature did not affect the behavior of S. enterica in conventionally or osmotically dehydrated coconut flakes. Results show that S. enterica inactivation in conventionally dehydrated coconut flakes could be described by log-linear with tail models. S. enterica inactivation in osmotically dehydrated coconut flakes could be described by log-linear with shoulder and tail models. Subpopulations of S. enterica cells with damaged membranes and without metabolic activity were larger in conventionally (32.1% and 90.9%, respectively) than osmotically dehydrated coconut (18.5% and 82.2%, respectively) flakes at the beginning of the storage. Subpopulations of S. enterica cells with damaged membrane decreased by 9.4–14.4%, while cells with membrane potential and intact membrane increased by 23.7 and 24.2% in conventionally dehydrated coconut flakes after 120 days of storage at 7 °C or 25 °C, respectively. Subpopulations of S. enterica with damaged membranes did not change significantly in osmotically dehydrated coconut flakes. Our findings suggest that S. enterica populations decline during storage occurs due in part to membrane integrity losses. These data can contribute to the development of risk management strategies for S. enterica in dehydrated coconut flakes.

Survivability of Salmonella and Enterococcus faecium in chili, cinnamon and black pepper powders during storage and isothermal treatments

Outbreaks and recalls associated with foods containing spices suggest a need for risk assessment of Salmonella in spices. In this study, the survivability of Salmonella Enteritidis PT 30, Salmonella cocktail (S. Enteritidis PT 30, S. Tennessee K4643 and S. Agona 447967), and Enterococcus faecium NRRL B-2354 in chili, cinnamon and black pepper at water activities (aw) 0.3 and 0.5 were evaluated during one-year storage at 21 °C. The thermal resistance of Salmonella cocktail in spices was also evaluated at 70 °C before and after storage. At aw 0.5, 4-month storage caused 5 log reduction of Salmonella cocktail in chili, while 8 months led to the same level of reduction in cinnamon. But only 3 log reduction were observed in black pepper over one year. Storage at aw 0.3 caused less reduction in Salmonella cocktail during the same storage periods. Less than 2 log reduction of E. faecium were observed over the one year storage at both aw levels, except for in chili stored at 0.5 aw. The D70°C-values for Salmonella cocktail in chili, cinnamon and black pepper of aw 0.3 before storage were 15.4, 20.8 and 36.6 min, respectively. 21–50% drops in the D70°C-value were obtained after two-month of storage, mostly in chili and least in black pepper. The high D70°C-value in black pepper persisted over one-year storage. Based on these results, chili powder showed the highest antimicrobial effect, followed by cinnamon and black pepper powders during storage and isothermal treatments.

N-acyl-homoserine lactone produced by Rahnella inusitata isolated from the gut of Galleria mellonella influences Salmonella phenotypes.

The most studied mechanism of quorum sensing in Gram-negative bacteria is mediated by autoinducer 1 (AI-1), namely, acyl-homoserine lactone (AHL). This system allows communication among different bacterial species and regulates the expression of virulence genes in many pathogens. Although AHL-producing bacteria have been detected in the intestines of humans and other animals, no report was found about AHL-producing bacteria in the insect gut and the possible effects of these autoinducers on enteropathogenic bacteria. Therefore, this study aimed to identify AHL-producing bacteria in the gut of larvae of Galleria mellonella and to evaluate the influence of this quorum sensing signal on the regulation of adhesion and motility phenotypes in the intestinal pathogen Salmonella. Sequencing of the 16S rRNA gene, 16S rRNA gene-based phylogenetic analyses, and phenotypic characterization of gut isolates was performed. The profile of AHLs produced by the isolates was determined using thin-layer chromatography (TLC) and revealed with the biosensor strain Chromobacterium violaceum CV026. Sequencing, phylogenetic analyses and phenotypic characterization of gut isolates showed that the three AHL-producing strains belong to the species Rahnella inusitata, named GM34, GM56, and GM60. The TLC showed that R. inusitata produces a six-carbon AHL. In the presence of cell-free extract of R. inusitata containing AHL and under anaerobic conditions, Salmonella enterica increased the adhesion to stainless steel coupons and presented swarming motility. Extracts from the culture medium of R. inusitata isolates containing AHL increased the adhesion on stainless steel coupons and swarming motility of Salmonella enterica serovar Enteritidis PT4 under anaerobic conditions. The results suggest the possibility of communication between members of the G. mellonella intestinal microbiota with pathogens such as Salmonella.

Inactivation efficacy of 405 nm light emitting diodes (LEDs) on Salmonella Enteritidis at different illumination temperatures

Salmonella Enteritidis is the major cause of foodborne salmonellosis affecting human health. The light emitting diodes (LEDs) is a novel approach to inactivate of the foodborne pathogens. The aim of this study was to determine the antibacterial effect of 405 nm LEDs illumination on S. Enteritidis and S. Enteritidis PT4. The irradiance of the 405 nm LEDs was 27.7 mW/cm2. Bacterial cultures suspended in tryptic soy broth were illuminated by 10-watt LEDs at a distance of 4.5 cm for 24 hours at 4 °C, 25 °C and 37 °C. Approximately 7-log reduction in colony forming unit (CFU) counts of both S. Enteritidis and S. Enteritidis PT4 at each temperature were observed following exposure after 7.5 hours to the LEDs, concluding that temperature did not affect the inactivation of the bacteria. The decimal reduction times (D-values) for the serotypes ranged from 55.78 to 67.88 min at 4, 25 and 37 °C after 405 nm LEDs illumination. No significant difference in D-values was observed among both the serotypes and temperatures, except for S. Enteritidis which had lower D-value at 4 °C. The LEDs technology has shown antibacterial efficacy and can be implemented in the food processing for reducing S. Enteritidis.

Thermal inactivation of Salmonella Enteritidis PT30 in ground cinnamon as influenced by water activity and temperature

Reported outbreaks and recalls related to spices, including an on-going recall of cinnamon involved apple chips, reveal a need to understand thermal inactivation of Salmonella in spices. Recent studies have documented quantitative relationships between water activity (aw) and thermal resistance of Salmonella in a wide range of low-moisture foods. Such quantitative data are useful in developing effective thermal treatments. However, the influence of aw on thermal inactivation of Salmonella in spices has not been systematically studied. Cinnamon is known for its antimicrobial effect on pathogenic bacteria. We hypothesized that the synergetic effect of heat and the natural antimicrobial compounds in cinnamon would reduce the intensity of thermal treatments for cinnamon compared to that for other low-moisture foods. This study investigated the thermal resistance of Salmonella Enteritidis PT 30 in ground cinnamon at three inactivation temperatures (70, 75 and 80 °C). The log10D-values of S. Enteritidis PT 30 in ground cinnamon decreased linearly with increasing aw and treatment temperature. By comparing the log10D-values obtained in ground cinnamon with the reported log10D80°C-values of S. Enteritidis PT 30 in other low-moisture foods, we found that the thermal treatments at 70 °C for S. Enteritidis PT 30 in cinnamon powder was roughly equivalent to the treatments at 80 °C for the same bacterial strain in other low-moisture foods, such as wheat flour and egg powders. Thus, milder thermal treatments can be used for the control of Salmonella in cinnamon powder, and perhaps other spices or herbs that contain antimicrobial compounds, for better retention of product quality.

Moisture Content of Bacterial Cells Determines Thermal Resistance of Salmonella enterica Serotype Enteritidis PT 30

Salmonella spp. are resilient bacterial pathogens in low-moisture foods. There has been a general lack of understanding of critical factors contributing to the enhanced thermal tolerance of Salmonella spp. in dry environments. In this study, we hypothesized that the moisture content (XW ) of bacterial cells is a critical intrinsic factor influencing the resistance of Salmonella spp. to thermal inactivation. We selected Salmonella enterica serotype Enteritidis PT 30 to test this hypothesis. We first produced viable freeze-dried S. Enteritidis PT 30, conditioned the bacterial cells to different XW s (7.7, 9.2, 12.4, and 15.7 g water/100 g dry solids), and determined the thermal inactivation kinetics of those cells at 80°C. The results show that the D-value (the time required to achieve a 1-log reduction) decreased exponentially with increasing XW We further measured the water activities (aw) of the freeze-dried S. Enteritidis PT 30 as influenced by temperatures between 20 and 80°C. By using those data, we estimated the XW of S. Enteritidis PT 30 from the published papers that related the D-values of the same bacterial strain at 80°C with the aw of five different food and silicon dioxide matrices. We discovered that the logarithmic D-values of S. Enteritidis PT 30 in all those matrices also decreased linearly with increasing XW of the bacterial cells. The findings suggest that the amount of moisture in S. Enteritidis PT 30 is a determining factor of its ability to resist thermal inactivation. Our results may help future research into fundamental mechanisms for thermal inactivation of bacterial pathogens in dry environments.IMPORTANCE This study established a logarithmic relationship between the thermal death time (D-value) of S. Enteritidis PT 30 and the moisture content (XW ) of the bacterial cells by conducting thermal inactivation tests on freeze-dried S Enteritidis PT 30. We further verified this relationship using literature data for S. Enteritidis PT 30 in five low-moisture matrices. The findings suggest that the XW of S. Enteritidis PT 30, which is rapidly adjusted by microenvironmental aw, or relative humidity, during heat treatments, is the key intrinsic factor determining the thermal resistance of the bacterium. The quantitative relationships reported in this study may help guide future designs of industrial thermal processes for the control of S. Enteritidis PT 30 or other Salmonella strains in low-moisture foods. Our findings highlight a need for further fundamental investigation into the role of water in protein denaturation and the accumulation of compatible solutes during thermal inactivation of bacterial pathogens in dry environments.

Influence of low water activity on the thermal resistance of Salmonella Enteritidis PT30 and Enterococcus faecium as its surrogate in egg powders.

Egg powders are increasingly popular ingredients, due to their functionality and compactness, in industrial food production and preparation at homes. However, there is a lack of studies that evaluate the thermal resistance of Salmonella Enteritidis PT30 and its potential surrogate Enterococcus faecium NRRL B-2354 in egg powders. This study examined the log-linear relationship between the thermal resistance of Salmonella Enteritidis (D-value) and the water activity (aw) of egg powders. The changes of aw in the egg powders with temperature were measured using a Vapor Sorption Analyzer and a high-temperature cell. The D80 ℃-value of S. Enteritidis PT30 and E. faecium inoculated in the egg powders preconditioned to three aw levels (0.3, 0.45, and 0.6) at 20 ℃ were determined using aluminum thermal death test cells. The aw values increased (P < 0.05) in all three egg powders when the temperature of the samples was raised from room temperature to 80 ℃. The D80 ℃-values ranged from 5.3 ± 0.1 to 25.9 ± 0.2 min for S. Enteritidis while 10.4 ± 0.4 to 43.8 ± 0.4 for E. faecium in samples of the three different aw levels. S. Enteritidis PT30 showed a log-linear relationship between D80 ℃-values and aw80 ℃ for the egg powders. This study contributes to our understanding of the impact of aw on the development of thermal treatments for low-moisture foods.

Thermal resistance of Salmonella in low-moisture high-sugar products

Recent foodborne outbreaks in low-moisture foods were caused mostly by Salmonella. The main common carbohydrates in low-moisture high-sugar products are D-glucose and D-fructose. There is a lack of information on the thermal resistance of Salmonella in pure sugar and high-sugar products with low water activity (aw) levels. The objective of this study was to study the thermal resistance of Salmonella Enteritidis PT30 in sugar products. Two pure sugars (glucose and fructose) and honey powder were selected for this research. The initial aw and moisture content for glucose, fructose, and honey powder are 0.30, 0.30, 0.40 and 0.02, 0.15, and 0.50% db., respectively. The D-values of Salmonella in different sugar products are determined at low aw levels (0.18, 0.30, 0.40, and 0.50 measured at 80 °C). The results showed that the D80 °C-values of S. Enteritidis PT30 decreased with increasing aw of all samples. At aw80 °C 0.18, S. Enteritidis PT30 in glucose powder has higher D-value than those in fructose and honey powder whereas, at aw80 °C 0.30, 0.40, and 0.50, S. Enteritidis PT30 in fructose powder has higher D-values than glucose and honey powder at 80 °C. Zaw values of S. Enteritidis PT30 in fructose and honey powder are comparable but was lower in glucose because of the phase change. The results of this study provide insights on understanding the thermal resistance of Salmonella in sugars and guidance on thermal process design to improve the safety of low-moisture high-sugar products.

Light-driven antimicrobial activities of vitamin K3 against Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella Enteritidis

Due to the lack of thermal inactivation steps, the fresh produce industry is actively looking for new, environmentally friendly and effective non-thermal control strategies to eliminate foodborne pathogens on produce surfaces and to prevent cross-contamination. Photodynamic inactivation of bacteria has emerged as a novel alternative approach to inactivate microorganisms on fresh produce. In this study, the photo-sensitizing efficacy of vitamin K3 (VK3) under UV-A and simulated sunlight and their antimicrobial efficacy against common foodborne pathogens in broth and on lemon surfaces were investigated. The minimal bactericidal concentration (MBC) of VK3 against Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella Enteritidis PT30 in darkness was 625.0, 156.3, and 625.0 μg/ml, respectively. When treatments were carried out under light, the MBC values of VK3 (VK3 + light) were reduced in strain, light source, and exposure time dependent manners. The enhanced antimicrobial effect can be attributed to the production of reactive oxygen species of VK3, including hydroxyl radical and singlet oxygen under light exposure. Based on scanning electron microscope examination, VK3 + light treatment damaged the cell integrity of L. monocytogenes and Salmonella; and altered the cell morphology of E. coli O157:H7 including filament formation and fewer fimbriae/flagella. When applied on artificially inoculated lemon surfaces, VK3 + light treatments at MBC levels immediately reduced the levels of L. monocytogenes and E. coli O157:H7 by more than 5.0 Log10 CFU/disk and S. Enteritidis PT30 by ~4.0 Log10 CFU/disk. Bacterial populations of all tested strains were reduced to below the limit of detection (1.0 Log10 CFU/disk) after one day of storage at 4 °C; the numbers of positive samples after enrichment further decreased as the storage time increased, demonstrating the continuous antimicrobial effects the VK3 generated on lemon surfaces. In summary, the results of this study demonstrated the promising antimicrobial effects of VK3 + light treatments and their great application potential for fresh produce industry.

Inactivation of Salmonella Enteritidis and Enterococcus faecium NRRL B-2354 in corn flour by radio frequency heating with subsequent freezing

This study aimed to evaluate the feasibility of using Enterococcus faecium NRRL B-2354 (E. faecium) as a surrogate of Salmonella enterica Enteritidis PT30 (S. Enteritidis PT30), validate radio frequency (RF) heating in pasteurizing corn flour, and study the effect of subsequent freezing treatment after RF heating in enhancing the microbial inactivation effect. Corn flour with water activity of 0.45 ± 0.05 at 25 °C was homogeneously inoculated with S. Enteritidis PT30 and E. faecium separately at 8.50 ± 0.23 log CFU/g. Thermal resistance parameters in corn flour were determined in a water bath at 75, 80, and 85 °C and RF heating was used to pasteurize 3.18 kg corn flour with subsequent freezing storage at −20 °C. Results showed that the thermal resistance of E. faecium was higher than S. Enteritidis PT30. Samples heated to 85 °C, held for 10 min after heating in a RF system, and then stored at −20 °C for 48 h, resulted in the reduction of S. Enteritidis PT30 by 6.59 ± 0.21 log and E. faecium by 4.79 ± 0.17 log. E. faecium could be used as its surrogate for validation studies in the packaged corn flour. Results also confirmed that RF heating combined with freezing storage treatment could significantly reduce the survival of both microorganisms in corn flour.

High temperature water activity as a key factor influencing survival of Salmonella Enteritidis PT30 in thermal processing

Salmonella in low-moisture foods has enhanced thermal tolerance and is difficult to control. The objective of this research was to study relationship between thermal tolerance of Salmonella Enteritidis PT30 and water activity (aw) of food matrices measured at elevated temperatures during thermal processing. Three different foods were selected for this study. They were wheat flour (WF), almond flour (AF) and whey protein (WP), representing carbohydrate-, fat-, and protein-rich food systems, respectively. Pre-equilibrated powders were inoculated independently with S. Enteritidis PT30 and conditioned to aw of 0.25, 0.45, 0.60, and 0.80 at room temperature (∼20 °C). Aluminum thermal death time test cells (TDT cells) and newly designed thermal aw cells (TAC, with controlled aw) were heated at 80 °C to determine D-values (the time needed to active 90% of target bacteria) of S. Enteritidis PT30 in the three powders. Water activities of powders in the TDT cells at 80 °C were calculated to be between 0.41 and 0.89, while in the TAC were controlled to 0.32, 0.50, 0.63, and 0.81, respectively. Results showed that D80°C-values of S. Enteritidis PT30 decreased exponentially with increasing aw of foods at the treatment temperature 80 °C regardless of the food matrices and the testing methods. Thus, it is critical to understand how aw of a food matrix changes with temperature when selecting appropriate treatment conditions for thermal control of Salmonella in low-moisture foods.

N-dodecanoyl-homoserine lactone influences the levels of thiol and proteins related to oxidation-reduction process in Salmonella.

Quorum sensing is a cell-cell communication mechanism mediated by chemical signals that leads to differential gene expression in response to high population density. Salmonella is unable to synthesize the autoinducer-1 (AI-1), N-acyl homoserine lactone (AHL), but is able to recognize AHLs produced by other microorganisms through SdiA protein. This study aimed to evaluate the fatty acid and protein profiles of Salmonella enterica serovar Enteritidis PT4 578 throughout time of cultivation in the presence of AHL. The presence of N-dodecanoyl-homoserine lactone (C12-HSL) altered the fatty acid and protein profiles of Salmonella cultivated during 4, 6, 7, 12 and 36 h in anaerobic condition. The profiles of Salmonella Enteritidis at logarithmic phase of growth (4 h of cultivation), in the presence of C12-HSL, were similar to those of cells at late stationary phase (36 h). In addition, there was less variation in both protein and fatty acid profiles along growth, suggesting that this quorum sensing signal anticipated a stationary phase response. The presence of C12-HSL increased the abundance of thiol related proteins such as Tpx, Q7CR42, Q8ZP25, YfgD, AhpC, NfsB, YdhD and TrxA, as well as the levels of free cellular thiol after 6 h of cultivation, suggesting that these cells have greater potential to resist oxidative stress. Additionally, the LuxS protein which synthesizes the AI-2 signaling molecule was differentially abundant in the presence of C12-HSL. The NfsB protein had its abundance increased in the presence of C12-HSL at all evaluated times, which is a suggestion that the cells may be susceptible to the action of nitrofurans or that AHLs present some toxicity. Overall, the presence of C12-HSL altered important pathways related to oxidative stress and stationary phase response in Salmonella.

Exponentially Increased Thermal Resistance of Salmonella spp. and Enterococcus faecium at Reduced Water Activity.

Salmonella spp. exhibit prolonged survivability and high tolerance to heat in low-moisture foods. The reported thermal resistance parameters of Salmonella spp. in low-moisture foods appear to be unpredictable due to various unknown factors. We report here that temperature-dependent water activity (aw, treatment temperature) plays an important role in the sharply increased thermal resistance of Salmonella enterica serovar Enteritidis PT 30 and its potential surrogate Enterococcus faecium NRRL B-2354. In our study, silicon dioxide granules, as carriers, were separately inoculated with these two microorganisms and were heated at 80°C with controlled relative humidity between 18 and 72% (resulting in corresponding aw,80°C values for bacteria between 0.18 and 0.72) in custom-designed test cells. The inactivation kinetics of both microorganisms fitted a log-linear model (R2, 0.83 to 0.97). Reductions in the aw,80°C values of bacterial cells exponentially increased the D80°C (the time needed to achieve a 1-log reduction in a bacterial population at 80°C) values for S Enteritidis and E. faecium on silicon dioxide. The log-linear relationship between the D80°C values for each strain in silicon dioxide and its aw,80°C values was also verified for organic wheat flour. E. faecium showed consistently higher D80°C values than S Enteritidis over the aw,80°C range tested. The estimated zaw (the change in aw,80°C needed to change D80°C by 1 log) values of S Enteritidis and E. faecium were 0.31 and 0.28, respectively. This study provides insight into the interpretation of Salmonella thermal resistance that could guide the development and validation of thermal processing of low-moisture foods.IMPORTANCE In this paper, we established that the thermal resistance of the pathogen S Enteritidis and its surrogate Enterococcus faecium, as reflected by D values at 80°C, increases sharply with decreasing relative humidity in the environment. The log-linear relationship between the D80°C values of each strain in silicon dioxide and its aw,80°C values was also verified for organic wheat flour. The results provide new quantitative insight into the way in which the thermal resistance of microorganisms changes in low-moisture systems, and they should aid in the development of effective thermal treatment strategies for pathogen control in low-moisture foods.