Variation in resistance of natural isolates of Escherichia coli O157 to high hydrostatic pressure, mild heat, and other stresses

A hurdle strategy based on the combination of non-thermal treatments to control diarrheagenic E. coli in cheese

This study aimed to assess the efficacy of a multi-hurdle process combining mild High Hydrostatic Pressure (HHP) treatments and Thyme Oil (TO) edible films as a non-thermal method to combat pathogenic E. coli (aEPEC and STEC) in raw cow's-milk cheese stored at 7 °C and packaged under modified atmosphere. Changes in headspace atmosphere of cheese packs and treatment effects on Lactic Acid Bacteria (LAB) counts and diarrheagenic E. coli strains (aEPEC and STEC) were evaluated over a 28 d storage period. The results demonstrated that the combined treatment exhibited the most significant antimicrobial effect against both strains compared to individual treatments, achieving reductions of 4.30 and 4.80 log cfu/g after 28 d of storage for aEPEC and STEC, respectively. Notably, the synergistic effect of the combination treatment resulted in the complete inactivation of intact cells for STEC and nearly completed inactivation for aEPEC by the end of the storage period. These findings suggest that the combination of HHP with selected hurdles could effectively enhance microbial inactivation capacity, offering promising alternatives for improving cheese safety without affecting the starter microbiota.

Foodborne pathogen inactivation in fruit juices utilizing commercial scale high-pressure processing: Effects of acidulants and pH

The effects of juice pH, type of acidulant, and post-treatment refrigeration on the high-pressure processing (HPP) inactivation of Escherichia coli O157:H7, Salmonella enterica, and Listeria monocytogenes in acid beverages were evaluated. Inoculated apple, orange, and grape juices (at their original pH and adjusted to pH 4.00, 4.50, and 5.00) were treated at 550 MPa for 1 min at 5 °C. In addition, inoculated model solutions acidified to a pH of 5.00 with acetic, citric, malic, and tartaric acids were treated at 400 MPa for 1 min at 5 °C. The effect of refrigerated storage for 24 h after treatment on pathogen inactivation in both experiments was also assessed. A greater than 5-log reduction of the three pathogens inoculated was achieved in all juices immediately after HPP at the juices' original pH, and of L. monocytogenes under all experimental conditions. Refrigerated storage for 24 h after HPP treatment improved the inactivation of E. coli O157:H7, to >5-log reduction, at pH 4.00 in apple juice and of Salmonella in the three juices at pH 4.00. The type of acidulant did not significantly (p > 0.01) affect E. coli or Salmonella inactivation in acidified model solutions but a greater than 5-log reduction after HPP was only achieved for L. monocytogenes when acetic acid was used. The effectiveness of HPP for pathogen inactivation depended largely on product pH and the target pathogen of concern.

Effects of UV/H2O2 Degradation on the Physicochemical and Antibacterial Properties of Fucoidan

The applications of fucoidan in the food industry were limited due to its high molecular weight and low solubility. Moderate degradation was required to depolymerize fucoidan. A few studies have reported that fucoidan has potential antibacterial activity, but its antibacterial mechanism needs further investigation. In this study, the degraded fucoidans were obtained after ultraviolet/hydrogen peroxide treatment (UV/H2O2) at different times. Their physicochemical properties and antibacterial activities against Staphylococcus aureus and Escherichia coli were investigated. The results showed that the average molecular weights of degraded fucoidans were significantly decreased (up to 22.04 times). They were mainly composed of fucose, galactose, and some glucuronic acid. Fucoidan degraded for 90 min (DFuc-90) showed the strongest antibacterial activities against Staphylococcus aureus and Escherichia coli, with inhibition zones of 27.70 + 0.84 mm and 9.25 + 0.61 mm, respectively. The minimum inhibitory concentrations (MIC) were 8 mg/mL and 4 mg/mL, respectively. DFuc-90 could inhibit the bacteria by damaging the cell wall, accumulating intracellular reactive oxygen species, reducing adenosine triphosphate synthesis, and inhibiting bacterial metabolic activity. Therefore, UV/H2O2 treatment could effectively degrade fucoidan and enhance its antibacterial activity.

Barotolerance of acid-adapted and cold-adapted bacterial isolates of E. coli O157:H7, Salmonella spp., and L. monocytogenes in an acidic buffer model

The fruit and vegetable juice industry has shown a growing trend in minimally processed juices. A frequent technology used in the production of functional juices is cold pressure, which refers to the application of high pressure processing (HPP) at low temperatures to inactivate foodborne pathogens. HPP juice manufacturers are required to demonstrate a 5-log reduction of the pertinent microorganism to comply with FDA Juice HACCP. However, there is no consensus on validation study approaches for bacterial strain selection or their preparation. Individual bacterial strains were grown using three different growth conditions: neutral, cold-adapted, and acid-adapted. Approximately 6.0 - 7.0 log CFU/mL of the matrix-adapted bacterial strains were inoculated individually into buffered peptone water (BPW) at pH 3.50 ± 0.10 (HCl adjusted) and treated at sublethal pressures of 500 MPa for E. coli O157:H7 and 200 MPa for Salmonella spp. and L. monocytogenes (180 s, 4°C). Analyses were conducted at 0, 24 and 48 h (4°C storage) post-HPP on non-selective media. E. coli O157:H7 exhibited greater barotolerance than Salmonella spp. and L. monocytogenes. In neutral growth conditions, E. coli O157:H7 strain TW14359 demonstrated the greatest resistance (2.94 ± 0.64 log reduction) and E. coli O157:H7 strain SEA13B88 was significantly more sensitive (P <0.05). Salmonella isolates, neutral and acid-adapted, expressed similar barotolerance to one another. Cold-adapted S. Cubana and S. Montevideo showed greater resistance compared to other cold-adapted strains. Acid-adapted L. monocytogenes strain MAD328 had <1.00 ± 0.23 log reduction while acid-adapted L. monocytogenes strains CDC and Scott A were significantly more sensitive (P <0.05) with reductions of 2.13 ± 0.48 and 3.43 ± 0.50 log CFU/mL, respectively. These results suggested, under the conditions tested, bacterial strain and preparation methods influence HPP efficacy and should be considered when conducting validation studies.

Structure-Based Design of Dual Bactericidal and Bacteria-Releasing Nanosurfaces

Here, we report synergistic nanostructured surfaces combining bactericidal and bacteria-releasing properties. A polystyrene-block-poly(methyl methacrylate) (PS-block-PMMA) diblock copolymer is used to fabricate vertically oriented cylindrical PS structures ("PS nanopillars") on silicon substrates. The results demonstrate that the PS nanopillars (with a height of about 10 nm, size of about 50 nm, and spacing of about 70 nm) exhibit highly effective bactericidal and bacteria-releasing properties ("dual properties") against Escherichia coli for at least 36 h of immersion in an E. coli solution. Interestingly, the PS nanopillars coated with a thin layer (≈3 nm thick) of titanium oxide (TiO₂) ("TiO₂ nanopillars") show much improved dual properties against E. coli (a Gram-negative bacterium) compared to the PS nanopillars. Moreover, the dual properties emerge against Listeria monocytogenes (a Gram-positive bacterium). To understand the mechanisms underlying the multifaceted property of the nanopillars, coarse-grained molecular dynamics (MD) simulations of a lipid bilayer (as a simplified model for E. coli) in contact with a substrate containing hexagonally packed hydrophilic nanopillars were performed. The MD results demonstrate that when the bacterium-substrate interaction is strong, the lipid heads adsorb onto the nanopillar surfaces, conforming the shape of a lipid bilayer to the structure/curvature of nanopillars and generating high stress concentrations within the membrane (i.e., the driving force for rupture) at the edge of the nanopillars. Membrane rupture begins with the formation of pores between nanopillars (i.e., bactericidal activity) and ultimately leads to the membrane withdrawal from the nanopillar surface (i.e., bacteria-releasing activity). In the case of Gram-positive bacteria, the adhesion area to the pillar surface is limited due to the inherent stiffness of the bacteria, creating higher stress concentrations within a bacterial cell wall. The present study provides insight into the mechanism underlying the "adhesion-mediated" multifaceted property of nanosurfaces, which is crucial for the development of next-generation antibacterial surface coatings for relevant medical applications.

Recent Progress in the Synergistic Bactericidal Effect of High Pressure and Temperature Processing in Fruits and Vegetables and Related Kinetics

BACKGROUND

Traditional thermal processing is a widely used method to ensure food safety. However, thermal processing leads to a significant decline in food quality, especially in the case of fruits and vegetables. To overcome this drawback, researchers are extensively exploring alternative non-thermal High-Pressure Processing (HPP) technology to ensure microbial safety and retaining the sensory and nutritional quality of food. However, HPP is unable to inactivate the spores of some pathogenic bacteria; thus, HPP in conjunction with moderate- and low-temperature is employed for inactivating the spores of harmful microorganisms. Scope and approach: In this paper, the inactivation effect of high-pressure and high-pressure thermal processing (HPTP) on harmful microorganisms in different food systems, along with the bactericidal kinetics model followed by HPP in certain food samples, have been reviewed. In addition, the effects of different factors such as microorganism species and growth stage, process parameters and pressurization mode, and food composition on microbial inactivation under the combined high-pressure and moderate/low-temperature treatment were discussed.

KEY FINDINGS AND CONCLUSIONS

The establishment of a reliable bactericidal kinetic model and accurate prediction of microbial inactivation will be helpful for industrial design, development, and optimization of safe HPP and HPTP treatment conditions.

Aspects of high hydrostatic pressure food processing: Perspectives on technology and food safety

The last two decades saw a steady increase of high hydrostatic pressure (HHP) used for treatment of foods. Although the science of biomaterials exposed to high pressure started more than a century ago, there still seem to be a number of unanswered questions regarding safety of foods processed using HHP. This review gives an overview on historical development and fundamental aspects of HHP, as well as on potential risks associated with HHP food applications based on available literature. Beside the combination of pressure and temperature, as major factors impacting inactivation of vegetative bacterial cells, bacterial endospores, viruses, and parasites, factors, such as food matrix, water content, presence of dissolved substances, and pH value, also have significant influence on their inactivation by pressure. As a result, pressure treatment of foods should be considered for specific food groups and in accordance with their specific chemical and physical properties. The pressure necessary for inactivation of viruses is in many instances slightly lower than that for vegetative bacterial cells; however, data for food relevant human virus types are missing due to the lack of methods for determining their infectivity. Parasites can be inactivated by comparatively lower pressure than vegetative bacterial cells. The degrees to which chemical reactions progress under pressure treatments are different to those of conventional thermal processes, for example, HHP leads to lower amounts of acrylamide and furan. Additionally, the formation of new unknown or unexpected substances has not yet been observed. To date, no safety-relevant chemical changes have been described for foods treated by HHP. Based on existing sensitization to non-HHP-treated food, the allergenic potential of HHP-treated food is more likely to be equivalent to untreated food. Initial findings on changes in packaging materials under HHP have not yet been adequately supported by scientific data.

Assessing the pressure resistance of Escherichia coli O157:H7, Listeria monocytogenes and Salmonella enterica to high pressure processing (HPP) in citric acid model solutions for process validation

Despite the commercial success of high pressure processing (HPP) in the juice industry, some regulatory agencies still require process validation. However, there is a lack of consensus on various aspects regarding validation protocols, including the selection of representative strains to be used in challenge tests. This study characterized the variable response of Escherichia coli O157:H7 (34 strains), Listeria monocytogenes (44 strains) and Salmonella enterica (45 strains) to HPP, and identified potential candidates to use in process validation. Stationary phase cells were submitted to 500 MPa for 1 min at 10 °C in model solutions consisting of tryptic soy broth + 0.6% yeast extract (TSBYE) adjusted to pH 4.5 and 6.0 with citric acid. At pH 6.0, pressure resistance widely varied between species and within strains of the same species. E. coli O157:H7 and L. monocytogenes were the most pressure resistant and showed high variability at strain level, as the total count range given by minimum and maximum counts spread between 2.0 and 6.5 log₁₀ CFU/ml. S. enterica was the least resistant pathogen with more than 82% of the isolates displaying non-detectable counts after HPP. Recovery through storage at 12 °C was also variable for all pathogens, but eventually most strains recovered with median counts on day 14 between 8.3 and 8.9 log₁₀ CFU/ml. For pH 4.5 solutions, 26 E. coli O157:H7 strains displayed survivors after HPP but did not adapt, registering non-detectable counts in the next sampling dates. None of the L. monocytogenes and S. enterica strains survived HPP or incubation at pH 4.5 (<2.0 log₁₀ CFU/ml), suggesting that citric acid at 4.16 g/l is a safe barrier for pathogen control under moderate HPP conditions. Principal component and cluster analyses served to propose strain cocktails for each species based on their pressure resistant and adaptation phenotypes. Additionally, S. enterica was identified as less pressure resistant and less prone to recover following HPP than E. coli O157:H7 and L. monocytogenes, so its relevance in process validation for juices should be questioned. Future work will validate the proposed strain cocktails on real food systems.

Influence of growth temperature on thermal tolerance of leading foodborne pathogens

Accurate prediction of the thermal destruction rate of foodborne pathogens is important for food processors to ensure proper food safety. When bacteria are subjected to thermal stress during storage, sublethal stresses and/or thermal acclimation may lead to differences in their subsequent tolerance to thermal treatment. The aim of the current study was to evaluate the thermal tolerance of Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica, and Staphylococcus aureus that are incubated during overnight growth in tryptic soy broth at four temperatures (15, 25, 35, and 45°C). Following incubation, the bacteria were subjected to thermal treatments at 55, 60, and 65°C. At the end of each treatment time, bacterial survival was quantified and further calculated for the thermal death decimal reduction time (D-value) and thermal destruction temperature (z-value) using a linear model for thermal treatment time (min) vs. microbial population (Log CFU/ml) and thermal treatment temperature (°C) vs. D-value, respectively, for each bacterium. Among the four bacterial species, E. coli generally had longer D-values and lower z-values than did other bacteria. Increasing patterns of D- and z-values in Listeria were obtained with the increment of incubation temperatures from 15 to 45°C. The z-values of Staphylococcus (6.19°C), Salmonella (6.73°C), Listeria (7.10°C), and Listeria (7.26°C) were the highest at 15, 25, 35, and 45°C, respectively. Although further research is needed to validate the findings on food matrix, findings in this study clearly affirm that adaptation of bacteria to certain stresses may reduce the effectiveness of preservation hurdles applied during later stages of food processing and storage.

High hydrostatic pressure inactivation of microorganisms: A probabilistic model for target log-reductions

A probabilistic model based on logistic regression was developed for a target log reduction of microorganisms inactivated by high hydrostatic pressure. Published inactivation data of Salmonella Typhimurium in broth for 4 and 5 log reductions, and Escherichia coli in buffer and carrot juice for 5 log reduction were used. The probabilities of achieving 4 or 5 log reductions for S. Typhimurium in broth and 5 log reduction for E. coli in buffer and carrot juice could be calculated at different pressure, temperature and time levels. The fitted interfaces of achieving/not achieving the target log reduction were consistent with the experimental data. Although the reliability of the predictions of the developed models could be questioned due to strain variation and different food matrix, a validation study has demonstrated that the developed models could be used to predict the target log reduction of these microorganisms at different pressure, temperature and time levels. This study has indicated that the probabilistic modeling for target log reductions can be useful tool for HHP inactivation of microorganisms, but further studies could be performed with several other factors such as pH and water activity of the food, concentration of certain additives as well as initial number of bacteria present in the food.

The Polyextremophilic Bacterium Clostridium paradoxum Attains Piezophilic Traits by Modulating Its Energy Metabolism and Cell Membrane Composition

In polyextremophiles, i.e., microorganisms growing preferentially under multiple extremes, synergistic effects may allow growth when application of the same extremes alone would not. High hydrostatic pressure (HP) is rarely considered in studies of polyextremophiles, and its role in potentially enhancing tolerance to other extremes remains unclear. Here, we investigated the HP-temperature response in Clostridium paradoxum, a haloalkaliphilic moderately thermophilic endospore-forming bacterium, in the range of 50 to 70°C and 0.1 to 30 MPa. At ambient pressure, growth limits were extended from the previously reported 63°C to 70°C, defining C. paradoxum as an actual thermophile. Concomitant application of high HP and temperature compared to standard conditions (i.e., ambient pressure and 50°C) remarkably enhanced growth, with an optimum growth rate observed at 22 MPa and 60°C. HP distinctively defined C. paradoxum physiology, as at 22 MPa biomass, production increased by 75% and the release of fermentation products per cell decreased by >50% compared to ambient pressure. This metabolic modulation was apparently linked to an energy-preserving mechanism triggered by HP, involving a shift toward pyruvate as the preferred energy and carbon source. High HPs decreased cell damage, as determined by Syto9 and propidium iodide staining, despite no organic solute being accumulated intracellularly. A distinct reduction in carbon chain length of phospholipid fatty acids (PLFAs) and an increase in the amount of branched-chain PLFAs occurred at high HP. Our results describe a multifaceted, cause-and-effect relationship between HP and cell metabolism, stressing the importance of applying HP to define the boundaries for life under polyextreme conditions.IMPORTANCE Hydrostatic pressure (HP) is a fundamental parameter influencing biochemical reactions and cell physiology; however, it is less frequently applied than other factors, such as pH, temperature, and salinity, when studying polyextremophilic microorganisms. In particular, how HP affects microbial tolerance to other and multiple extremes remains unclear. Here, we show that under polyextreme conditions of high pH and temperature, Clostridium paradoxum demonstrates a moderately piezophilic nature as cultures grow to highest cell densities and most efficiently at a specific combination of temperature and HP. Our results highlight the importance of considering HP when exploring microbial physiology under extreme conditions and thus have implications for defining the limits for microbial life in nature and for optimizing industrial bioprocesses occurring under multiple extremes.

Cellular events involved in E. coli cells inactivation by several agents for food preservation: A comparative study

Traditional and novel technologies for food preservation are being investigated to obtain safer products and fulfil consumer demands for less processed foods. These technologies inactivate microorganisms present in foods through their action on different cellular targets, but the final cause of cell loss of viability often remains not well characterized. The main objective of this work was to study and compare cellular events that could play a role on E. coli inactivation upon exposure to treatments with technologies of different nature. E. coli cells were exposed to heat, high hydrostatic pressure (HHP), pulsed electric fields (PEF) and acid treatments, and the occurrence of several alterations, including presence of sublethal injury, membrane permeabilization, increased levels of reactive oxygen species (ROS), DNA damage and protein damage were studied. Results reflected differences among the relevance of the several cellular events depending on the agent applied. Sublethally injured cells appeared after all the treatments. Cells consistently recovered in a higher percentage in non-selective medium, particularly in minimal medium, as compared to selective medium; however this effect was less relevant in PEF-treated cells. Increased levels of ROS were detected inside cells after all the treatments, although their order of appearance and relationship with membrane permeabilization varied depending on the technology. A high degree of membrane permeabilization was observed in PEF treated cells, DNA damage appeared as an important target in acid treatment, and protein damage, in HHP treated cells. Results obtained help to understand the mode of action of food preservation technologies on bacterial cells.

Population-Wide Survey of Salmonella enterica Response to High-Pressure Processing Reveals a Diversity of Responses and Tolerance Mechanisms

High-pressure processing is a nonthermal method of food preservation that uses pressure to inactivate microorganisms. To ensure the effective validation of process parameters, it is important that the design of challenge protocols consider the potential for resistance in a particular species. Herein, the responses of 99 diverse Salmonella enterica strains to high pressure are reported. Members of this population belonged to 24 serovars and were isolated from various Canadian sources over a period of 26 years. When cells were exposed to 600 MPa for 3 min, the average reduction in cell numbers for this population was 5.6 log₁₀ CFU/ml, with a range of 0.9 log₁₀ CFU/ml to 6 log₁₀ CFU/ml. Eleven strains, from 5 serovars, with variable levels of pressure resistance were selected for further study. The membrane characteristics (propidium iodide uptake during and after pressure treatment, sensitivity to membrane-active agents, and membrane fatty acid composition) and responses to stressors (heat, nutrient deprivation, desiccation, and acid) for this panel suggested potential roles for the cell membrane and the RpoS regulon in mediating pressure resistance in S. enterica The data indicate heterogeneous and multifactorial responses to high pressure that cannot be predicted for individual S. enterica strains.IMPORTANCE The responses of foodborne pathogens to increasingly popular minimal food decontamination methods are not understood and therefore are difficult to predict. This report shows that the responses of Salmonella enterica strains to high-pressure processing are diverse. The magnitude of inactivation does not depend on how closely related the strains are or where they were isolated. Moreover, strains that are resistant to high pressure do not behave similarly to other stresses, suggesting that more than one mechanism might be responsible for resistance to high pressure and the mechanisms used may vary from one strain to another.

Insights on Osmotic Tolerance Mechanisms in Escherichia coli Gained from an rpoC Mutation

An 84 bp in-frame duplication (K370_A396dup) within the rpoC subunit of RNA polymerase was found in two independent mutants selected during an adaptive laboratory evolution experiment under osmotic stress in Escherichia coli, suggesting that this mutation confers improved osmotic tolerance. To determine the role this mutation in rpoC plays in osmotic tolerance, we reconstructed the mutation in BW25113, and found it to confer improved tolerance to hyperosmotic stress. Metabolite analysis, exogenous supplementation assays, and cell membrane damage analysis suggest that the mechanism of improved osmotic tolerance by this rpoC mutation may be related to the higher production of acetic acid and amino acids such as proline, and increased membrane integrity in the presence of NaCl stress in exponential phase cells. Transcriptional analysis led to the findings that the overexpression of methionine related genes metK and mmuP improves osmotic tolerance in BW25113. Furthermore, deletion of a stress related gene bolA was found to confer enhanced osmotic tolerance in BW25113 and MG1655. These findings expand our current understanding of osmotic tolerance in E. coli, and have the potential to expand the utilization of high saline feedstocks and water sources in microbial fermentation.

Profiling of Campylobacter jejuni Proteome in Exponential and Stationary Phase of Growth

Campylobacter jejuni has been reported as a major cause of bacterial food-borne enteritides in developed countries during the last decade. Despite its fastidious growth requirements, including low level of oxygen and high level of CO2, this pathogen is able to persist in the environment without permanent loss of its viability and virulence. As C. jejuni is not able to multiply outside a host, the cells spend significant amount of time in stationary phase of growth. The entry into the stationary phase is often correlated to resistance to various stresses in bacteria. The switching between exponential and stationary phases is frequently mediated by the regulator sigma S (RpoS). However, this factor is absent in C. jejuni and molecular mechanisms responsible for transition of cells to the stationary phase remain elusive. In this work, proteomic profiles of cells from exponential and stationary phases were compared using 2-D electrophoresis (2DE) fingerprinting combined with mass spectrometry analysis and qRT-PCR. The identified proteins, whose expression differed between the two phases, are mostly involved in protein biosynthesis, carbon metabolism, stress response and motility. Altered expression was observed also in the pleiotropic regulator CosR that was over-expressed during stationary phase. A shift between transcript and protein level evolution of CosR throughout the growth of C. jejuni was observed using qRT-PCR and (2DE). From these data, we hypothesized that CosR could undergo a negative autoregulation in stationary phase. A consensus sequence resulting from promoter sequence alignment of genes potentially regulated by CosR, including its own upstream region, among C. jejuni strains is proposed. To verify experimentally the potential autoregulation of CosR at the DNA level, electrophoretic mobility shift assay was performed with DNA fragments of CosR promoter region and rCosR. Different migration pattern of the promoter fragments indicates the binding capacity of CosR, suggesting its auto-regulation potential.

Thermal and Starvation Stress Response of Escherichia coli O157:H7 Isolates Selected from Agricultural Environments

Pathogens exposed to agricultural production environments are subject to multiple stresses that may alter their survival under subsequent stress conditions. The objective of this study was to examine heat and starvation stress response of Escherichia coli O157:H7 strains isolated from agricultural matrices. Seven E. coli O157:H7 isolates from different agricultural matrices-soil, compost, irrigation water, and sheep manure-were selected, and two ATCC strains were used as controls. The E. coli O157:H7 isolates were exposed to heat stress (56°C in 0.1% peptone water for up to 1 h) and starvation (in phosphate-buffered saline at 37°C for 15 days), and their survival was examined. GInaFiT freeware tool was used to perform regression analyses of the surviving populations. The Weibull model was identified as the most appropriate model for response of the isolates to heat stress, whereas the biphasic survival curves during starvation were fitted using the double Weibull model, indicating the adaptation to starvation or a resistant subpopulation. The inactivation time during heating to achieve the first decimal reduction time (δ) calculated with the Weibull parameters was the highest (45 min) for a compost isolate (Comp60A) and the lowest (28 min) for ATCC strain 43895. Two of the nine isolates (ATCC 43895 and a manure isolate) had β < 1, indicating that surviving populations adapted to heat stress, and six strains demonstrated downward concavity (β > 1), indicating decreasing heat resistance over time. The ATCC strains displayed the longest δ₂ (>1,250 h) in response to starvation stress, compared with from 328 to 812 h for the environmental strains. The considerable variation in inactivation kinetics of E. coli O157:H7 highlights the importance of evaluating response to stress conditions among individual strains of a specific pathogen. Environmental isolates did not exhibit more robust response to stress conditions in this study compared with ATCC strains.

Note: Ultra High Hydrostatic Pressure Inactivation of Escherichia Coli in Milk, and Orange and Peach Juices

Inactivation of Escherichia coli by high hydrostatic pressure (UHHP) was determined in broth, milk and orange and peach juices inoculated with the bacteria. HHP ranged from 200 to 700 MPa at 25 C and different treatment times. No cell growth occurred in broth after 60, 25, 15, 10 and 7 min at 300, 400, 500, 600 and 700 MPa, respectively. Reduction of aerobic bacteria in milk and peach juice were 3.08 and 6.07 log units after 15 min at 400 MPa, respectively, while all bacterial cells were inactivated in orange juice. Sterilisation of raw milk contaminated with E. coli occurred at 600 MPa for 30 min, while peach and orange juices needed 12 and 10 min, respectively. The injury of cells in broth at 300 MPa ranged from 8.8 to 100% depending on magnitude of pressure and treated time. In general, inactivation of aerobic bacteria and E. coli was enhanced significantly (P<0.01) by increasing the pressure.

Development of High Hydrostatic Pressure Applied in Pathogen Inactivation for Plasma

High hydrostatic pressure has been used to inactivate pathogens in foods for decades. There is a great potential to adapt this technology to inactivate pathogens in plasma and derivatives. To better evaluate the potential of this method, pathogen inoculated plasma samples were pressurized under different pressure application modes and temperatures. The inactivation efficacy of pathogens and activities of plasma proteins were monitored after treatment. The CFUs of E.coli was examined as the indicator of the inactivation efficiency. The factor V and VIII were chosen as the indicator of the plasma function. Preliminary experiments identified optimized treatment conditions: 200-250MPa, with 5×1 minute multi-pulsed high pressure at near 0°C (ice-water bath). Under this conditions, the inactivation efficacy of EMCV was >8.5log. The CFUs of E. coli were reduced by 7.5log, B. cereus were 8log. However, PPV and S. aureus cannot be inactivated efficiently. The activities of factor II, VII, IX, X, XI, XII, fibrinogen, IgG, IgM stayed over 95% compared to untreated. Factor V and VIII activity was maintained at 46–63% and 77–82%, respectively.

Quantifying Variability in Growth and Thermal Inactivation Kinetics of Lactobacillus plantarum

The presence and growth of spoilage organisms in food might affect the shelf life. In this study, the effects of experimental, reproduction, and strain variabilities were quantified with respect to growth and thermal inactivation using 20 Lactobacillus plantarum strains. Also, the effect of growth history on thermal resistance was quantified. The strain variability in μmax was similar (P > 0.05) to reproduction variability as a function of pH, aw, and temperature, while being around half of the reproduction variability (P < 0.05) as a function of undissociated lactic acid concentration [HLa]. The cardinal growth parameters were estimated for the L. plantarum strains, and the pHmin was between 3.2 and 3.5, the aw,min was between 0.936 and 0.953, the [HLamax], at pH 4.5, was between 29 and 38 mM, and the Tmin was between 3.4 and 8.3°C. The average D values ranged from 0.80 min to 19 min at 55°C, 0.22 to 3.9 min at 58°C, 3.1 to 45 s at 60°C, and 1.8 to 19 s at 63°C. In contrast to growth, the strain variability in thermal resistance was on average six times higher than the reproduction variability and more than ten times higher than the experimental variability. The strain variability was also 1.8 times higher (P < 0.05) than the effect of growth history. The combined effects of strain variability and growth history on D value explained all of the variability as found in the literature, although with bias. Based on an illustrative milk-processing chain, strain variability caused ∼2-log10 differences in growth between the most and least robust strains and >10-log10 differences after thermal treatment.

IMPORTANCE

Accurate control and realistic prediction of shelf life is complicated by the natural diversity among microbial strains, and limited information on microbiological variability is available for spoilage microorganisms. Therefore, the objectives of the present study were to quantify strain variability, reproduction (biological) variability, and experimental variability with respect to the growth and thermal inactivation kinetics of Lactobacillus plantarum and to quantify the variability in thermal resistance attributed to growth history. The quantitative knowledge obtained on experimental, reproduction, and strain variabilities can be used to improve experimental designs and to adequately select strains for challenge growth and inactivation tests. Moreover, the integration of strain variability in prediction of microbial growth and inactivation kinetics will result in more realistic predictions of L. plantarum dynamics along the food production chain.

An impaired metabolic response to hydrostatic pressure explains Alcanivorax borkumensis recorded distribution in the deep marine water column

Alcanivorax borkumensis is an ubiquitous model organism for hydrocarbonoclastic bacteria, which dominates polluted surface waters. Its negligible presence in oil-contaminated deep waters (as observed during the Deepwater Horizon accident) raises the hypothesis that it may lack adaptive mechanisms to hydrostatic pressure (HP). The type strain SK2 was tested under 0.1, 5 and 10 MPa (corresponding to surface water, 500 and 1000 m depth, respectively). While 5 MPa essentially inactivated SK2, further increase to 10 MPa triggered some resistance mechanism, as indicated by higher total and intact cell numbers. Under 10 MPa, SK2 upregulated the synthetic pathway of the osmolyte ectoine, whose concentration increased from 0.45 to 4.71 fmoles cell⁻¹. Central biosynthetic pathways such as cell replication, glyoxylate and Krebs cycles, amino acids metabolism and fatty acids biosynthesis, but not β-oxidation, were upregulated or unaffected at 10 MPa, although total cell number was remarkably lower with respect to 0.1 MPa. Concomitantly, expression of more than 50% of SK2 genes was downregulated, including genes related to ATP generation, respiration and protein translation. Thus, A. borkumensis lacks proper adaptation to HP but activates resistance mechanisms. These consist in poorly efficient biosynthetic rather than energy-yielding degradation-related pathways, and suggest that HP does represent a major driver for its distribution at deep-sea.

Osmotic Stress Confers Enhanced Cell Integrity to Hydrostatic Pressure but Impairs Growth in Alcanivorax borkumensis SK2

Alcanivorax is a hydrocarbonoclastic genus dominating oil spills worldwide. While its presence has been detected in oil-polluted seawaters, marine sediment and salt marshes under ambient pressure, its presence in deep-sea oil-contaminated environments is negligible. Recent laboratory studies highlighted the piezosensitive nature of some Alcanivorax species, whose growth yields are highly impacted by mild hydrostatic pressures (HPs). In the present study, osmotic stress was used as a tool to increase HP resistance in the type strain Alcanivorax borkumensis SK2. Control cultures grown under standard conditions of salinity and osmotic pressure with respect to seawater (35.6 ppt or 1136 mOsm kg^-1, respectively) were compared with cultures subjected to hypo- and hyperosmosis (330 and 1720 mOsm kg^-1, or 18 and 62 ppt in salinity, equivalent to brackish and brine waters, respectively), under atmospheric or increased HP (0.1 and 10 MPa). Osmotic stress had a remarkably positive impact on cell metabolic activity in terms of CO₂ production (thus, oil bioremediation) and O₂ respiration under hyperosmosis, as acclimation to high salinity enhanced cell activity under 10 MPa by a factor of 10. Both osmotic shocks significantly enhanced cell protection by reducing membrane damage under HP, with cell integrities close to 100% under hyposmosis. The latter was likely due to intracellular water-reclamation as no trace of the piezolyte ectoine was found, contrary to hyperosmosis. Notably, ectoine production was equivalent at 0.1 MPa in hyperosmosis-acclimated cells and at 10 MPa under isosmotic conditions. While stimulating cell metabolism and enhancing cell integrity, osmotic stress had always a negative impact on culture growth and performance. No net growth was observed during 4-days incubation tests, and CO₂:O₂ ratios and pH values indicated that culture performance in terms of hydrocarbon degradation was lowered by the effects of osmotic stress alone or combined with increased HP. These findings confirm the piezosensitive nature of A. borkumensis, which lacks proper resistance mechanisms to improve its metabolic efficiency under increased HP, thus explaining its limited role in oil-polluted deep-sea environments.

Diversity of Survival Patterns among Escherichia coli O157:H7 Genotypes Subjected to Food-Related Stress Conditions

The purpose of this study was to evaluate the resistance patterns to food-related stresses of Shiga toxin producing Escherichia coli O157:H7 strains belonging to specific genotypes. A total of 33 E. coli O157:H7 strains were exposed to seven different stress conditions acting as potential selective pressures affecting the transmission of E. coli O157:H7 to humans through the food chain. These stress conditions included cold, oxidative, osmotic, acid, heat, freeze-thaw, and starvation stresses. The genotypes used for comparison included lineage-specific polymorphism, Shiga-toxin-encoding bacteriophage insertion sites, clade type, tir (A255T) polymorphism, Shiga toxin 2 subtype, and antiterminator Q gene allele. Bacterial resistance to different stressors was calculated by determining D-values (times required for inactivation of 90% of the bacterial population), which were then subjected to univariate and multivariate analyses. In addition, a relative stress resistance value, integrating resistance values to all tested stressors, was calculated for each bacterial strain and allowed for a ranking-type classification of E. coli O157:H7 strains according to their environmental robustness. Lineage I/II strains were found to be significantly more resistant to acid, cold, and starvation stress than lineage II strains. Similarly, tir (255T) and clade 8 encoding strains were significantly more resistant to acid, heat, cold, and starvation stress than tir (255A) and non-clade 8 strains. Principal component analysis, which allows grouping of strains with similar stress survival characteristics, separated strains of lineage I and I/II from strains of lineage II, which in general showed reduced survival abilities. Results obtained suggest that lineage I/II, tir (255T), and clade 8 strains, which have been previously reported to be more frequently associated with human disease cases, have greater multiple stress resistance than strains of other genotypes. The results from this study provide a better insight into how selective pressures encountered through the food chain may play a role in the epidemiology of STEC O157:H7 through controlling the transmission of highly adapted strains to humans.

Lactocin 160, a Bacteriocin Produced by Vaginal Lactobacillus rhamnosus, Targets Cytoplasmic Membranes of the Vaginal Pathogen, Gardnerella vaginalis

Bacterial vaginosis (BV) is a commonly occurring vaginal infection that is associated with a variety of serious risks related to the reproductive health of women. Conventional antibiotic treatment for this condition is frequently ineffective because the antibiotics tend to inhibit healthy vaginal microflora along with the pathogens. Lactocin 160, a bacteriocin produced by healthy vaginal lactobacilli, is a promising alternative to antibiotics; this compound specifically inhibits the BV-associated vaginal pathogens such as Gardnerella vaginalis and Prevotella bivia without affecting the healthy microflora. This study investigates the molecular mechanism of action for lactocin 160 and reveals that this compound targets the cytoplasmic membrane of G. vaginalis, causing the efflux of ATP molecules and dissipation of the proton motive force.

Survival and High-Hydrostatic Pressure Inactivation of Foodborne Pathogens in Salmorejo, a Traditional Ready-to-Eat Food

Salmorejo is a traditional tomato-based creamy product. Because salmorejo is not heat-processed, there is a risk of contamination with foodborne pathogens from raw materials. Even though bacterial growth in salmorejo is strongly inhibited because of its acidic pH (close to 3.9), the growth and survival of 3 foodborne pathogens in this food has not been studied before. In this study, 3 cocktails consisting of Escherichia coli O157, Salmonella enterica serovar Enteritidis, and Listeria monocytogenes strains were inoculated in freshly prepared salmorejo. The food was treated by high hydrostatic pressure (HHP) at 400, 500, or 600 MPa for 8 min, or left untreated, and stored at 4 °C for 30 d. Viable cell counts were determined on selective media and also by the triple-layer agar method in order to detect sublethally injured cells. In control samples, L. monocytogenes viable cells decreased by 2.4 log cycles at day 7 and were undetectable by day 15. S. enterica cells decreased by 0.5 or 2.4 log cycles at days 7 and 15 respectively, but still were detectable at day 30. E. coli O157 cells survived much better in salmorejo, decreasing only by 1.5 log cycles at day 30. Treatments at pressures of 400 MPa or higher reduced viable counts of L. monocytogenes and S. enterica to undetectable levels. HHP treatments significantly (P < 0.05) reduced E. coli counts by approximately 5.2 to 5.4 log cycles, but also yielded surviving cells that apparently were sublethally injured. Only samples treated at 600 MPA for 8 min were devoid of detectable E. coli cells during storage.

PRACTICAL APPLICATION

Salmorejo is a traditional, vitamin-rich food, usually produced on a small scale. HHP treatment at 600 MPa for 8 min can be an efficient nonthermal method for industrial-scale preparation of preservative-free salmorejo with improved safety against transmission of foodborne pathogens L. monocytogenes serotyes 4a and 4b, S. enterica serovar Enteritidis, and E. coli O157.

Mechanisms of pressure-mediated cell death and injury in Escherichia coli: from fundamentals to food applications

High hydrostatic pressure is commercially applied to extend the shelf life of foods, and to improve food safety. Current applications operate at ambient temperature and 600 MPa or less. However, bacteria that may resist this pressure level include the pathogens Staphylococcus aureus and strains of Escherichia coli, including shiga-toxin producing E. coli. The resistance of E. coli to pressure is variable between strains and highly dependent on the food matrix. The targeted design of processes for the safe elimination of E. coli thus necessitates deeper insights into mechanisms of interaction and matrix-strain interactions. Cellular targets of high pressure treatment in E. coli include the barrier properties of the outer membrane, the integrity of the cytoplasmic membrane as well as the activity of membrane-bound enzymes, and the integrity of ribosomes. The pressure-induced denaturation of membrane bound enzymes results in generation of reactive oxygen species and subsequent cell death caused by oxidative stress. Remarkably, pressure resistance at the single cell level relates to the disposition of misfolded proteins in inclusion bodies. While the pressure resistance E. coli can be manipulated by over-expression or deletion of (stress) proteins, the mechanisms of pressure resistance in wild type strains is multi-factorial and not fully understood. This review aims to provide an overview on mechanisms of pressure-mediated cell death in E. coli, and the use of this information for optimization of high pressure processing of foods.

Inactivation of Escherichia coli O157:H7 on Orange Fruit Surfaces and in Juice Using Photocatalysis and High Hydrostatic Pressure

Nonpasteurized orange juice is manufactured by squeezing juice from fruit without peel removal. Fruit surfaces may carry pathogenic microorganisms that can contaminate squeezed juice. Titanium dioxide-UVC photocatalysis (TUVP), a nonthermal technique capable of microbial inactivation via generation of hydroxyl radicals, was used to decontaminate orange surfaces. Levels of spot-inoculated Escherichia coli O157:H7 (initial level of 7.0 log CFU/cm(2)) on oranges (12 cm(2)) were reduced by 4.3 log CFU/ml when treated with TUVP (17.2 mW/cm(2)). Reductions of 1.5, 3.9, and 3.6 log CFU/ml were achieved using tap water, chlorine (200 ppm), and UVC alone (23.7 mW/cm(2)), respectively. E. coli O157:H7 in juice from TUVP (17.2 mW/cm(2))-treated oranges was reduced by 1.7 log CFU/ml. After orange juice was treated with high hydrostatic pressure (HHP) at 400 MPa for 1 min without any prior fruit surface disinfection, the level of E. coli O157:H7 was reduced by 2.4 log CFU/ml. However, the E. coli O157:H7 level in juice was reduced by 4.7 log CFU/ml (to lower than the detection limit) when TUVP treatment of oranges was followed by HHP treatment of juice, indicating a synergistic inactivation effect. The inactivation kinetics of E. coli O157:H7 on orange surfaces followed a biphasic model. HHP treatment did not affect the pH, °Brix, or color of juice. However, the ascorbic acid concentration and pectinmethylesterase activity were reduced by 35.1 and 34.7%, respectively.

Effects of high hydrostatic pressure and temperature increase on Escherichia coli spp. and pectin methyl esterase inactivation in orange juice

The aim of this study was to evaluate the effect of high hydrostatic pressure treatment combined with moderate processing temperatures (25 ℃-50 ℃) on the inactivation of Escherichia coli O157: H7 (ATCC 700728), E. coli K12 (ATCC 23716), and pectin methyl esterase in orange juice, using pressures of 250 to 500 MPa with times ranging between 1 and 30 min. Loss of viability of E. coli O157:H7 increased significantly as pressure and treatment time increased, achieving a 6.5 log cycle reduction at 400 MPa for 3 min at 25 ℃ of treatment. With regard to the inactivation of pectin methyl esterase, the greatest reduction obtained was 90.05 ± 0.01% at 50 ℃ and 500 MPa of pressure for 15 min; therefore, the pectin methyl esterase enzyme was highly resistant to the treatments by high hydrostatic pressure. The results obtained in this study showed a synergistic effect between the high pressure and moderate temperatures in inactivating E. coli cells.

Pressure resistance of cold-shocked Escherichia coli O157:H7 in ground beef, beef gravy and peptone water

AIMS

(i) To study the effects of cold shock on Escherichia coli O157:H7 cells. (ii) To determine if cold-shocked E. coli O157:H7 cells at stationary and exponential phases are more pressure-resistant than their non-cold-shocked counterparts. (iii) To investigate the baro-protective role of growth media (0·1% peptone water, beef gravy and ground beef).

METHODS AND RESULTS

Quantitative estimates of lethality and sublethal injury were made using the differential plating method. There were no significant differences (P > 0·05) in the number of cells killed; cold-shocked or non-cold-shocked. Cells grown in ground beef (stationary and exponential phases) experienced lowest death compared with peptone water and beef gravy. Cold-shock treatment increased the sublethal injury to cells cultured in peptone water (stationary and exponential phases) and ground beef (exponential phase), but decreased the sublethal injury to cells in beef gravy (stationary phase).

CONCLUSIONS

Cold shock did not confer greater resistance to stationary or exponential phase cells pressurized in peptone water, beef gravy or ground beef. Ground beef had the greatest baro-protective effect.

SIGNIFICANCE AND IMPACT OF THE STUDY

Real food systems should be used in establishing food safety parameters for high-pressure treatments; micro-organisms are less resistant in model food systems, the use of which may underestimate the organisms' resistance.

Development and validation of a surrogate strain cocktail to evaluate bactericidal effects of pressure on verotoxigenic Escherichia coli

Many strains of verotoxigenic Escherichia coli (VTEC) are highly resistant to pressure. To facilitate future studies to improve the elimination of VTEC by pressure processing of food, this study developed and validated a cocktail of non-pathogenic strains of E. coli with equal or higher resistance to pressure when compared to pressure resistant strains of VTEC. Strains of E. coli obtained from a beef processing plant were screened for their resistance to heat and pressure. Treatments were carried out in LB broth. Cell counts of 3 out of 16 strains were reduced by 5-6 log (cfu/mL) after 30 min at 60 °C, and cell counts of 10 out of 16 strains were reduced by 5-6 log (cfu/mL) after 30 min at 40 °C and 400 MPa. All highly heat resistant strains were also pressure resistant but not all pressure resistant strains were also heat resistant. Pressure resistant and -sensitive strains of E. coli were treated in presence of 0 or 2% NaCl and at 3, 20, or 40 °C. The effect of these parameters on the lethality of pressure treatments was comparable for all strains. The addition of 2% NaCl did not increase pressure resistance. The bactericidal effect of treatments at 3 and 20 °C and 600 MPa was comparable but inactivation of E. coli was faster at 40 °C and 600 MPa. The resistance to treatment with 600 MPa at 20 °C of a cocktail of 5 non-pathogenic strains of E. coli was compared to a 5 strain cocktail of pressure resistant VTEC. Treatments were performed in ground beef containing 15% fat. Survival and sublethal injury of the two cocktails was comparable; cell counts of beef inoculated with either cocktail were reduced by about 4 log (cfu/mL) after 30 min of treatment. In conclusion, this study validated a cocktail of non-pathogenic strains of E. coli for use as surrogate organisms in studies on the elimination of E. coli by pressure.

Variation in heat and pressure resistance of verotoxigenic and nontoxigenic Escherichia coli

This study evaluated the heat and pressure resistance of 112 strains of Escherichia coli, including 102 strains of verotoxigenic E. coli (VTEC) representing 23 serotypes and four phylogenetic groups. In an initial screening, the heat and pressure resistance of 100 strains, including 94 VTEC strains, were tested in phosphate-buffered saline (PBS). Treatment at 60°C for 5 min reduced cell counts by 2.0 to 5.5 log CFU/ml; treatment at 600 MPa for 3 min at 25°C reduced the cell counts by 1.1 to 5.5 log CFU/ml. Heat or pressure resistance did not correlate to the phylogenetic group or the serotype. A smaller group of E. coli strains was evaluated for heat and pressure resistance in Luria-Bertani (LB) broth. Generally, the levels of heat resistance of E. coli strains in LB and PBS were similar; however, the levels of pressure resistance observed for treatments in LB broth or PBS were variable. The cell counts of pressure-resistant strains of VTEC were reduced by less than 1.5 log CFU/ml after treatment at 600 MPa for 3 min. E. coli strains were also treated with 600 MPa for 3 min in ground beef or inoculated into beef patties and grilled to 63 or 71°C. The cell counts of the VTEC E. coli O26:H11 strain 05-6544 were reduced by 2 log CFU/g by pressure treatment in ground beef. The cell counts of the heat-resistant E. coli strain AW1.7 were reduced by 1.4 and 3.4 log CFU/g in beef patties grilled to internal temperatures of 63 and 71°C, respectively. The cell counts of E. coli 05-6544 were reduced by less than 3 and 6 log CFU/g in beef patties grilled to internal temperatures of 63 and 71°C, respectively. To study whether the composition of the beef patties influenced heat resistance, E. coli strains AW1.7, AW1.7 Δ pHR1, MG1655, and LMM1030 were mixed into beef patties containing 15 or 35% fat and 0 or 2% NaCl, and the patties were grilled to an internal temperature of 63°C. The highest heat resistance of E. coli was observed in patties containing 15% fat and 2% NaCl.