The effect of emulsifier type and oil inclusion on stress-related gene expression of Salmonella typhimurium in oil-in-water emulsion

Salmonella has been associated with numerous outbreaks from contaminated food products, including emulsions. Emulsions are influenced by emulsifier type and oil presence, which can have varying degrees of stress or protection on bacteria. Although our previous research has shown that emulsifier solutions, rather than emulsions, provide a protective effect on Salmonella typhimurium after thermal treatment, the underlying mechanism remains unclear. This study selected S. typhimurium as the model microorganism and utilized the same emulsifiers (Tween 20, Tween 80, Triton X-100) to create emulsifier solutions and emulsions with the same oil fraction (60% (v/v)) to examine their effect on the expression of nine selected genes (rpoE, rpoH, otsB, proV, fadA, fabA, dnaK, ibpA, ompC) associated with stress response. Specifically, the study observed variations in gene expression under normal and thermal stress at 55°C. After 20-h incubation, Triton X-100 emulsion caused an upregulation of stress-related genes, rpoE, otsB, and fabA, suggesting stressful environment. After thermal treatment, S. typhimurium in Triton X-100 solution showed a longer 5-log reduction time with increased proV and decreased fabA and ompC expression, suggesting enhanced thermal protection compared to its emulsion. Conversely, Tween 80 solution increased fabA and ompC expression, indicating greater membrane fluidity and passive diffusion, potentially reducing thermal resistance. However, according to the upregulation of ibpA, this effect was likely mitigated by the overproduction of heat shock proteins. Notably, Triton X-100 environments exhibited the most significant gene expression changes after heat treatment, whereas Tween 80 without oil was the most inhospitable for bacterial survival. These findings inform bacterial responses under various conditions, aiding food safety strategies.

Assessment of trans-cinnamaldehyde and eugenol assisted heat treatment against Salmonella Typhimurium in low moisture food components

Increased thermal resistance of Salmonella at low water activity (a_w) is a significant food safety concern in low-moisture foods (LMFs). We evaluated whether trans-cinnamaldehyde (CA, 1000 ppm) and eugenol (EG, 1000 ppm), which can accelerate thermal inactivation of Salmonella Typhimurium in water, can show similar effect in bacteria adapted to low a_w in different LMF components. Although CA and EG significantly accelerated thermal inactivation (55 °C) of S. Typhimurium in whey protein (WP), corn starch (CS) and peanut oil (PO) at 0.9 a_w, such effect was not observed in bacteria adapted to lower a_w (0.4). The matrix effect on bacterial thermal resistance was observed at 0.9 a_w, which was ranked as WP > PO > CS. The effect of heat treatment with CA or EG on bacterial metabolic activity was also partially dependent on the food matrix. Bacteria adapted to lower a_w had lower membrane fluidity and unsaturated to saturated fatty acids ratio, suggesting that bacteria at low a_w can change its membrane composition to increase its rigidity, thus increasing resistance against the combined treatments. This study demonstrates the effect of a_w and food components on the antimicrobials-assisted heat treatment in LMF and provides an insight into the resistance mechanism.

The genetic response of Salmonella Typhimurium during trans-cinnamaldehyde assisted heat treatment and its correlation with bacterial resistance in different low moisture food components

Our previous study found that water activity (a_w)- and matrix-dependent bacterial resistance wasdeveloped in Salmonella Typhimurium during antimicrobial-assisted heat treatment in low moisture foods (LMFs) matrices. To better understand the molecular mechanism behind the observed bacterial resistance, gene expression analysis was conducted on S. Typhimurium adapted to different conditions with or without the trans-cinnamaldehyde (CA)-assisted heat treatment via quantitative polymerase chain reaction (qPCR). Expression profiles of nine stress-related genes were analyzed. The upregulation of rpoH and dnaK and downregulation of ompC were observed during bacterial adaptation in LMF matrices and the combined heat treatment, which likely contributed to the bacterial resistance during the combined treatment. Their expression profiles were partially consistent with the previously-observed effect of a_w or matrix on bacterial resistance. The upregulation of rpoE, otsB, proV, and fadA was also observed during adaptation in LMF matrices and might contribute to desiccation resistance, but likely did not contribute to bacterial resistance during the combined heat treatment. The observed upregulation of fabA and downregulation of ibpA could not be directly linked to bacterial resistance to either desiccation or the combined heat treatment. The results may assist the development of more efficient processing methods against S. Typhimurium in LMFs.

Salmonella enterica Outbreaks Linked to the Consumption of Tahini and Tahini-Based Products

Salmonella is a leading cause of bacterial foodborne illness in the world. Although typically associated with foods of animal origin, low-moisture foods, such as tahini, are quickly gaining recognition as an important vehicle of Salmonella exposure. This review offers the Canadian perspective on the issue of Salmonella in tahini and tahini-based products. A summary of several recent food product recalls and foodborne outbreaks related to the presence of Salmonella in tahini and tahini-based products such as halva are presented. The properties of the food vehicles, their production practices, and potential routes of contamination are discussed. Particular focus is placed on the ecology of Salmonella in the tahini production continuum, including its survival characteristics and response to intervention technologies.

Efficacy of Acidified Oils against Salmonella in Low-Moisture Environments

When processing low-moisture, high-fat foods such as peanut butter and nuts, water-based sanitization is unsuitable due to the immiscible nature of water and fats. Dry sanitization mainly uses flammable compounds such as isopropanol, requiring equipment cooling before application. The use of oils to deliver antimicrobials against foodborne pathogens enables the use of elevated temperatures, thus eliminating processing downtimes associated with dry sanitization. This study delivered organic acids and medium-chain fatty acids (100, 250, and 500 mM) in peanut oil against Salmonella enterica serovar Enteritidis desiccated at 75% relative humidity (RH). Acetic acid in peanut oil (AO) at 45°C was the most effective food-grade acid, causing a 4.4-log reduction in S. Enteritidis at 500 mM. AO caused cellular injury and was effective against a variety of S. Enteritidis strains. Confocal microscopy demonstrated that cells treated with 50 mM and 250 mM AO had significant membrane damage and reduced cellular respiration compared to untreated controls. Treatment efficacy increased with the increase in acid concentration, treatment duration, and treatment temperature from 20 to 45°C. Transmission electron microscopy after treatment with 100 and 250 mM AO revealed membrane ruffling and leakage in cell membranes, especially at 45°C. Reduction of the RH to 33% during desiccation of S. Enteritidis caused a decrease in AO efficacy compared to that at 75% RH, while at a higher RH of 90%, there was an increase in the efficacy of AO. Acidified oils can serve as robust, cost-effective replacements for dry-sanitation methods and improve safety of low moisture foods. IMPORTANCE Currently, dry sanitization products used during food processing often contain flammable compounds which require processing to stop and equipment to cool before application. This leads to processing downtimes and consequently, economic losses. This challenge is compounded by exposure to dryness which frequently renders Salmonella resistant to heat and different antimicrobials. Thus, the development of heat-tolerant oil-based antimicrobial compounds is a novel approach for sanitizing in low-moisture (dry) environments such as those found in peanut butter, tree nuts, and chocolate manufacturing. This study shows that acidified oils, especially acetic acid in peanut oil at elevated temperatures (45°C), was highly effective against desiccated Salmonella. Acidified oils have the potential to replace dry sanitizers, increasing the frequency of sanitization, leading to an improvement in food safety.

Salmonella spp. in low water activity food: Occurrence, survival mechanisms, and thermoresistance

The occurrence of disease outbreaks involving low-water-activity (a_w ) foods has gained increased prominence due in part to the fact that reducing free water in these foods is normally a measure that controls the growth and multiplication of pathogenic microorganisms. Salmonella, one of the main bacteria involved in these outbreaks, represents a major public health problem worldwide and in Brazil, which highlights the importance of good manufacturing and handling practices for food quality. The virulence of this pathogen, associated with its high ability to persist in the environment, makes Salmonella one of the main challenges for the food industry. The objectives of this article are to present the general characteristics, virulence, thermoresistance, control, and relevance of Salmonella in foodborne diseases, and describe the so-called low-water-activity foods and the salmonellosis outbreaks involving them.

Cyclopropane Fatty Acids are Important for Salmonella enterica serovar Typhimurium Virulence

A variety of eubacteria, plants and protozoa can modify membrane lipids by cyclopropanation, which is reported to modulate membrane permeability and fluidity. The ability to cyclopropanate membrane lipids has been associated with resistance to oxidative stress in Mycobacterium tuberculosis, organic solvent stress in Escherichia coli, and acid stress in E. coli and Salmonella. In bacteria, the cfa gene encoding cyclopropane fatty acid (CFA) synthase is induced during the stationary phase of growth. In the present study we constructed a cfa mutant of Salmonella enterica serovar Typhimurium 14028s (S. Typhimurium) and determined the contribution of CFA-modified lipids to stress resistance and virulence in mice. Cyclopropane fatty acid content was quantified in wild-type and cfa mutant S. Typhimurium. CFA levels in a cfa mutant were greatly reduced compared to wild-type, indicating that CFA synthase is the major enzyme responsible for cyclopropane modification of lipids in Salmonella. S. Typhimurium cfa mutants were more sensitive to extreme acid pH, the protonophore CCCP, and hydrogen peroxide, compared to wild-type. In addition, cfa mutants exhibited reduced viability in murine macrophages and could be rescued by addition of the NADPH phagocyte oxidase inhibitor diphenyleneiodonium (DPI) chloride. S. Typhimurium lacking cfa was also attenuated for virulence in mice. These observations indicate that CFA modification of lipids makes an important contribution to Salmonella virulence.

Induction of a Viable but Nonculturable State, Thermal and Sanitizer Tolerance, and Gene Expression Correlation with Desiccation-Adapted Biofilm and Planktonic Salmonella in Powdered Infant Formula

ABSTRACT

This study was conducted to investigate the effects of the physiological state, desiccation adaptation, and storage of powdered infant formula on Salmonella cell survival and expression of desiccation stress-related genes. Powdered infant formula was inoculated with Salmonella Typhimurium in the biofilm state on beads and in the planktonic state on nitrocellulose filters and stored at 25°C for up to 270 days. Both 5-cyano-2,3-ditoyl tetrazolium chloride flow cytometry and xylose lysine deoxycholate agar plate counts revealed that biofilm-forming Salmonella cells tended to enter the viable but nonculturable (VBNC) state (P < 0.05). The population reduction of all desiccation-adapted Salmonella Typhimurium decreased significantly in both physiological states after exposure to mild heat (60°C) compared with nonadapted control cells (P < 0.05). Salmonella cells were cross-protected from heat in both physiological states, but cross-protection against hydrogen peroxide was induced for only planktonic Salmonella cells. The reverse transcription quantitative PCR results revealed that the rpoS gene in biofilm Salmonella cells on all desiccation adaption days and in planktonic Salmonella cells on day 7 of dry storage was significantly upregulated (P < 0.05). The rpoE, grpE, and invA genes in Salmonella cells in both physiological states were significantly down-regulated (P < 0.05). Physiological state and storage time might affect expression of these genes. Prior exposure to adverse conditions, including low water activity, and the physiological state impacted Salmonella survival, and its ability to enter the VBNC state and gene expression.

HIGHLIGHTS

Assessment of butylparaben (4-hydroxybenzoic acid butyl ester)-assisted heat treatment against Escherichia coli O157:H7 and Salmonella enterica serotype Typhimurium in meat and bone meal

Heat-resistant foodborne pathogens have been a concern in low-moisture foods and ingredients (LMFs). Due to low thermal conductivity of low moisture materials, thermal treatment is not efficient and may cause nutritional loss. This study investigated the enhancement of thermal treatment of meat and bone meal (MBM) at low water activity (a_w ) by inclusion of butylparaben (BP) as a model antimicrobial compound. Stationary phase Escherichia coli O157:H7 (Shiga toxin-negative) or Salmonella enterica serotype Typhimurium was inoculated into MBM containing 0-2000 ppm BP and incubated at 55 or 60°C for up to 5 hr. A biphasic inactivation pattern was observed for both pathogens, indicating existence of potentially thermal resistant subpopulations. Addition of 1000 ppm BP to MBM (a_w  = 0.4) significantly lowered the D-value at 55°C for E. coli O157:H7 (2.6 ± 0.5 hr) compared to thermal treatment alone (5.1 ± 0.6 h) during the treatment after the first 1 hr (p < 0.05), indicating that addition of BP accelerated the inactivation of thermal-resistant subpopulation of E. coli O157:H7 in MBM. Interestingly, similar enhancement in thermal inactivation upon addition of BP was not observed in either the sensitive or resistant subpopulation of S. Typhimurium at a_w of 0.4 or 0.7, which is likely caused by the higher thermal resistance developed by S. Typhimurium within a low a_w environment (a_w  < 0.85). These results suggest that addition of certain antimicrobial compounds can improve the thermal processing efficiency in LMFs, while their efficiency against different pathogens may vary. PRACTICAL APPLICATION: Addition of appropriate food-grade compounds may help to improve thermal treatment efficiency in low moisture foods with varied efficiency against different pathogens. This approach has the potential to reduce the required heat treatment intensity while minimizing food safety risk.

Modification of membrane properties and fatty acids biosynthesis-related genes in Escherichia coli and Staphylococcus aureus: Implications for the antibacterial mechanism of naringenin

In this work, modifications of cell membrane fluidity, fatty acid composition and fatty acid biosynthesis-associated genes of Escherichia coli ATCC 25922 (E. coli) and Staphylococcus aureus ATCC 6538 (S. aureus), during growth in the presence of naringenin (NAR), one of the natural antibacterial components in citrus plants, was investigated. Compared to E. coli, the growth of S. aureus was significantly inhibited by NAR in low concentrations. Combination of gas chromatography-mass spectrometry with fluorescence polarization analysis revealed that E. coli and S. aureus cells increased membrane fluidity by altering the composition of membrane fatty acids after exposure to NAR. For example, E. coli cells produced more unsaturated fatty acids (from 18.5% to 43.3%) at the expense of both cyclopropane and saturated fatty acids after growth in the concentrations of NAR from 0 to 2.20mM. For S. aureus grown with NAR at 0 to 1.47mM, the relative proportions of anteiso-branched chain fatty acids increased from 37.2% to 54.4%, whereas iso-branched and straight chain fatty acids decreased from 30.0% and 33.1% to 21.6% and 23.7%, respectively. Real time q-PCR analysis showed that NAR at higher concentrations induced a significant down-regulation of fatty acid biosynthesis-associated genes in the bacteria, with the exception of an increased expression of fabA gene. The minimum inhibitory concentration (MIC) of NAR against these two bacteria was determined, and both of bacteria underwent morphological changes after exposure to 1.0 and 2.0 MIC.

Thermal Resistance and Gene Expression of both Desiccation-Adapted and Rehydrated Salmonella enterica Serovar Typhimurium Cells in Aged Broiler Litter

The objective of this study was to investigate the thermal resistance and gene expression of both desiccation-adapted and rehydrated Salmonella enterica serovar Typhimurium cells in aged broiler litter. S Typhimurium was desiccation adapted in aged broiler litter with a 20% moisture content (water activity [a_w], 0.81) for 1, 2, 3, 12, or 24 h at room temperature and then rehydrated for 3 h. As analyzed by quantitative real-time reverse transcriptase PCR (qRT-PCR), the rpoS, proV, dnaK, and grpE genes were upregulated (P < 0.05) under desiccation stress and could be induced after 1 h but in less than 2 h. Following rehydration, fold changes in the levels of these four genes became significantly lower (P < 0.05). The desiccation-adapted ΔrpoS mutant was less heat resistant at 75°C than was the desiccation-adapted wild type (P < 0.05), whereas there were no differences in heat resistance between desiccation-adapted mutants in two nonregulated genes (otsA and PagfD) and the desiccation-adapted wild type (P > 0.05). Survival characteristics of the desiccation-adapted ΔPagfD (rdar [red, dry, and rough] morphotype) and ΔagfD (saw [smooth and white] morphotype) mutants were similar (P > 0.05). Trehalose synthesis in the desiccation-adapted wild type was not induced compared to a nonadapted control (P > 0.05). Our results demonstrated the importance of the rpoS, proV, dnaK, and grpE genes in the desiccation survival of S Typhimurium. By using an ΔrpoS mutant, we found that the rpoS gene was involved in the cross-protection of desiccation-adapted S Typhimurium against high temperatures, while trehalose synthesis or rdar morphology did not play a significant role in this phenomenon. In summary, S Typhimurium could respond rapidly to low-a_w conditions in aged broiler litter while developing cross-protection against high temperatures, but this process could be reversed upon rehydration.IMPORTANCE Physical heat treatment is effective in eliminating human pathogens from poultry litter used as biological soil amendments. However, prior to physical heat treatment, some populations of microorganisms may be adapted to the stressful conditions in poultry litter during composting or stockpiling, which may cross-protect them against subsequent high temperatures. Our previous study demonstrated that desiccation-adapted S. enterica cells in aged broiler litter exhibited enhanced thermal resistance. However, there is limited research on the underlying mechanisms of the extended survival of pathogens under desiccation conditions in animal wastes and cross-tolerance to subsequent heat treatment. Moreover, no information is available about the thermal resistance of desiccation-adapted microorganisms in response to rehydration. Therefore, in the present study, we investigated the gene expression and thermal resistance of both desiccation-adapted and rehydrated S Typhimurium in aged broiler litter. This work will guide future research efforts to control human pathogens in animal wastes used as biological soil amendments.

Impact of treatments for recycled broiler litter on the viability and infectivity of microorganisms

The microbiological risk of recycled litter depends on the efficacy of the management system applied to inactivate residual microorganisms and preserve the health of the successive broiler flock. This study aimed to assess the viability and infectivity of the Newcastle Disease Virus (NDV), Infectious Bursal Disease Virus (IBDV) and Salmonella Heidelberg in recycled litter exposed to different treatments. The litter was contaminated with microorganisms and submitted to the treatments (T): T1: shallow fermentation; T2: quicklime (calcium oxide); T3: shallow fermentation followed by addition of quicklime; T4: no treatment. Sentinel chicks housed on the treated litter showed that T1 and T3 inactivated residual IBDV. Analysis of the litter subjected to T1 also showed reduced levels of total enterobacteria. T2 was not able to reduce the microorganisms assessed and its association with T1 (T3) failed to enhance the effect of the treatment. NDV did not survive in the broiler litter, regardless of the treatment applied, and it was also not detected in the sentinel chicks. S. Heidelberg remained viable in the litter submitted to all studied treatments, being isolated from the sentinel chicks of all the experimental groups. The antimicrobial activity of T1 and T3 was associated to higher ammonia contents in the broiler litter. The results indicate that the shallow fermentation treatment is efficient for controlling residual IBDV and total enterobacteria in the recycled litter.

Effect of Pulsed Electric Field on Membrane Lipids and Oxidative Injury of Salmonella typhimurium

Salmonella typhimurium cells were subjected to pulsed electric field (PEF) treatment at 25 kV/cm for 0–4 ms to investigate the effect of PEF on the cytoplasmic membrane lipids and oxidative injury of cells. Results indicated that PEF treatment induced a decrease of membrane fluidity of Salmonella typhimurium (S. typhimuriumi), possibly due to the alterations of fatty acid biosynthesis-associated gene expressions (down-regulation of cfa and fabA gene expressions and the up-regulation of fabD gene expression), which, in turn, modified the composition of membrane lipid (decrease in the content ratio of unsaturated fatty acids to saturated fatty acids). In addition, oxidative injury induced by PEF treatment was associated with an increase in the content of malondialdehyde. The up-regulation of cytochrome bo oxidase gene expressions (cyoA, cyoB, and cyoC) indicated that membrane damage was induced by PEF treatment, which was related to the repairing mechanism of alleviating the oxidative injury caused by PEF treatment. Based on these results, we achieved better understanding of microbial injury induced by PEF, suggesting that micro-organisms tend to decrease membrane fluidity in response to PEF treatment and, thus, a greater membrane fluidity might improve the efficiency of PEF treatment to inactivate micro-organisms.

Vigilance for Salmonella in Feedstuffs Available in Costa Rica: Prevalence, Serotyping and Tetracycline Resistance of Isolates Obtained from 2009 to 2014

Relevant epidemiological information is provided in this report for Salmonella based on data obtained from a Costa Rican surveillance program for animal feeds. In addition to prevalence, a description in terms of serotypes and tetracycline (TET) resistance of the isolates is included. A total of 1725 feed and feed ingredients samples were analyzed during 2009 and 2014, from which 110 Salmonella strains were recovered (76 from poultry, 23 from meat and bone meal [MBM], 3 from pet foods, and 8 from other feed). Retrieved isolates were serotyped and tested for minimum inhibitory concentration (MIC) against TET. Salmonella strains were found mainly from poultry feed (different growth stages, n = 76/110; 69.1%) and MBM (n = 23/109; 21.1%). The rest of the isolates were recovered from feather meal, pet food, fish meal (n = 3/110; 2.3% each) and swine feed (n = 1/110; 0.9%). From the different serotypes recovered (n = 21), the most common were Salmonella Give (n = 18; 13.8%) and Salmonella Rissen (n = 6; 4.6%) for MBM and Salmonella Havana (n = 14; 10.8%), Salmonella Rissen, Salmonella Soerenga, and Salmonella Schwarzengrund (n = 8; 6.2% each) in poultry feed. Recovered strains were regarded to be sensitive or have an intermediate resistance to TET as evidenced by their MIC50 and MIC90 concentrations of 4 and 8 μg/mL for MBM and poultry feed, respectively. Compound feed and MBM samples exhibited strains characterized by 86.8 and 88.9% of the isolates classified (according to CLSI, 2015 ) as sensitive, 7.7 and 3.7% as intermediate, and 5.5% (with >256 μg/mL as the highest concentration) and 7.4% (with 64 μg/mL as the highest concentration) as resistant to TET, respectively. Salmonella serovars Anatum and Havana exhibited the highest resistance profile >256 and 128 μg/mL, respectively. Hence, MBM and poultry feed seem to be a target of interest if Salmonella incidence is to be controlled. Serotypes recovered have in the past demonstrated pathogenic capability; therefore, hereafter a stricter surveillance program may be in order.

Antimicrobial Activity of Cinnamaldehyde, Carvacrol, and Lauric Arginate against Salmonella Tennessee in a Glycerol-Sucrose Model and Peanut Paste at Different Fat Concentrations

The objective of this study was to investigate the antimicrobial activities of carvacrol, cinnamaldehyde, and lauric arginate (LAE) against Salmonella in a low water activity (aw ) glycerol-sucrose model and in peanut paste with different fat concentrations. Salmonella Tennessee was inoculated into the model and the low fat (<5%) and high fat (50%) peanut paste adjusted to aw 1.0, 0.7, 0.5, and 0.3 and with or without cinnamaldehyde, carvacrol, or LAE. The survival of the bacterium over 3 or 5 days at 25°C was evaluated. Reduced aw alone decreased the viable population over time, with the highest reduction at the lowest aw. In the glycerol-sucrose model, all antimicrobial agents significantly reduced the population over time (P < 0.05) compared with the controls. LAE was more lethal than the essential oil components, reducing the population to undetectable levels by day 2 for all aw. Cinnamaldehyde was more effective than carvacrol at aw 0.5 and 0.3 (2.7- to 2.9-log versus 0.39- to 1.97-log reductions on day 3). In low-fat peanut paste, none of the antimicrobial agents inhibited growth of the pathogen at aw 1.0. However, inactivation was enhanced at reduced aw. Cinnamaldehyde and LAE both reduced the pathogen population to undetectable levels on day 5 at the highest concentration tested (ca. 10 times higher than that in the glycerol-sucrose model). Inactivation efficacy of all antimicrobial agents was greatly decreased but not eliminated in 50% fat peanut paste. Results suggest that the test antimicrobial agents were effective under low aw conditions, but significantly higher concentrations are needed for potential food applications, and fat concentration can negatively impact the efficacy of these antimicrobial agents.