Effects of high pressure on survival and metabolic activity of Lactobacillus plantarum TMW1.460

Proteomic analysis by two-dimensional differential gel electrophoresis (2D DIGE) of a high-pressure effect in Bacillus cereus

High hydrostatic pressure (HHP) is a new method used to reduce or eliminate microorganisms that are present in food. Proteins are known to be the most important target of high pressure in living organisms. The main goal of this investigation was focused on the changes that occur on the proteins of Bacillus cereus under HHP stress conditions. The two-dimensional differential gel electrophoresis (2D DIGE) technique allows for a simultaneous resolution of thousands of proteins based on fluorescent prelabeling of the samples with spectrally resolvable fluorescent CyDyes. The results of proteomics profiling show an average of 1300 spots being detected. The analysis revealed 75 spot proteins whose abundance is modified after the application of high pressure, of which 66 were decreased after the HHP treatment. Among them, flagellin was the protein that changed the most. The differential expression of some proteins after HHP treatment at 700 MPa may suggest a reduction of virulence and protective response against oxidative stress in flagellated Bacillus .

Comparison of inactivation pathways of thermal or high pressure inactivated Lactobacillus rhamnosus ATCC 53103 by flow cytometry analysis

The effect of thermal and pressure treatments on Lactobacillus rhamnosus ATCC 53103 was evaluated by flow cytometric analysis in conjunction to standard cultivation techniques. A double staining technique with fluorochromes carboxyfluorescein diacetate (cFDA) and propidium iodide (PI) revealed that depending on temperature regime used heat-killed cells had different fluorescence behaviors. Cells killed at 60 degrees C were not stained at all whereas heat treatment at 75 degrees C resulted in a single population entirely labelled by PI. These findings indicated that thermal-induced cell death was achievable with or without membrane degradation. Hydrostatic pressures beyond 400 MPa inactivated L. rhamnosus ATCC 53103 in a different way. It was observed that the irreversible damage of the membrane-bound transport systems could be largely accounted for the cause of high pressure-induced cell death.

Relationship between sublethal injury and microbial inactivation by the combination of high hydrostatic pressure and citral or tert-butyl hydroquinone

The aim was to investigate (i) the occurrence of sublethal injury in Listeria monocytogenes, Escherichia coli, and Saccharomyces cerevisiae after high hydrostatic pressure (HHP) treatment as a function of the treatment medium pH and composition and (ii) the relationship between the occurrence of sublethal injury and the inactivating effect of a combination of HHP and two antimicrobial compounds, tert-butyl hydroquinone (TBHQ) and citral. The three microorganisms showed a high proportion of sublethally injured cells (up to 99.99% of the surviving population) after HHP. In E. coli and L. monocytogenes, the extent of inactivation and sublethal injury depended on the pH and the composition of the treatment medium, whereas in S. cerevisiae, inactivation and sublethal injury were independent of medium pH or composition under the conditions tested. TBHQ alone was not lethal to E. coli or L. monocytogenes but acted synergistically with HHP and 24-h refrigeration, resulting in a viability decrease of >5 log(10) cycles of both organisms. The antimicrobial effect of citral depended on the microorganism and the treatment medium pH. Acting alone for 24 h under refrigeration, 1,000 ppm of citral caused a reduction of 5 log(10) cycles of E. coli at pH 7.0 and almost 3 log(10) cycles of L. monocytogenes at pH 4.0. The combination of citral and HHP also showed a synergistic effect. Our results have confirmed that the detection of sublethal injury after HHP may contribute to the identification of those treatment conditions under which HHP may act synergistically with other preserving processes.

Investigation of the immunomodulatory effects of Lactobacillus casei and Bifidobacterium lactis on Helicobacter pylori infection

BACKGROUND

Lactobacillus and Bifidobacterium species have shown beneficial effects in the treatment of Helicobacter pylori infection; however, the mechanisms behind such effects are not fully understood. In this study, we have investigated the immunomodulatory effects of probiotics in a mouse model of H. pylori infection.

MATERIALS AND METHODS

H. pylori-infected C57BL/6 mice were treated with L. casei L26, B. lactis B94, or no probiotics for 5 weeks, respectively. Mice not infected with H. pylori were included as normal controls. Gastric histology, protein levels of interleukin (IL)-1beta, IL-10, IL-12/23p40, and H. pylori colonization density in the gastric tissues, as well as H. pylori-specific antibodies were examined.

RESULTS

In mice receiving L. casei L26 and B. lactis B94, gastric neutrophil infiltration and IL-1beta were significantly decreased and IL-10 was significantly increased as compared with mice receiving no probiotics. In mice receiving B. lactis B94, IL-12/23p40 was significantly increased and H. pylori IgG was significantly reduced as compared with mice receiving no probiotics. No significant difference of H. pylori colonization was observed among the three groups of mice.

CONCLUSION

The reduced level of IL-1beta and neutrophil infiltration observed in mice infected with H. pylori following treatment with L. casei L26 and B. lactis B94 resulted from a modulation of immune response rather than a decrease of H. pylori colonization. Furthermore, B. lactis B94 has the intrinsic ability to promote a Th1 immune response through an increase in IL-12/IL-23.

Damage in Escherichia coli cells treated with a combination of high hydrostatic pressure and subzero temperature

The relationship between membrane permeability, changes in ultrastructure, and inactivation in Escherichia coli strain K-12TG1 cells subjected to high hydrostatic pressure treatment at room and subzero temperatures was studied. Propidium iodide staining performed before and after pressure treatment made it possible to distinguish between reversible and irreversible pressure-mediated cell membrane permeabilization. Changes in cell ultrastructure were studied using transmission electron microscopy (TEM), which showed noticeable condensation of nucleoids and aggregation of cytosolic proteins in cells fixed after decompression. A novel technique used to mix fixation reagents with the cell suspension in situ under high hydrostatic pressure (HHP) and subzero-temperature conditions made it possible to show the partial reversibility of pressure-induced nucleoid condensation. However, based on visual examination of TEM micrographs, protein aggregation did not seem to be reversible. Reversible cell membrane permeabilization was noticeable, particularly for HHP treatments at subzero temperature. A correlation between membrane permeabilization and cell inactivation was established, suggesting different mechanisms at room and subzero temperatures. We propose that the inactivation of E. coli cells under combined HHP and subzero temperature occurs mainly during their transiently permeabilized state, whereas HHP inactivation at room temperature is related to a balance of transient and permanent permeabilization. The correlation between TEM results and cell inactivation was not absolute. Further work is required to elucidate the effects of pressure-induced damage on nucleoids and proteins during cell inactivation.

Evaluation of the response of Lactobacillus rhamnosus VTT E-97800 to sucrose-induced osmotic stress

Environmental osmotic changes are one of the stresses live probiotics may encounter either in their natural habitats or as a result of usage in food formulations and processing. Response to osmotic stress, induced by sucrose, of the probiotic strain Lactobacillus rhamnosus VTT E-97800 (E800) was investigated. The fluorescence-based approach used, by combined staining with caboxyfluorescein (cFDA) and propidium iodide (PI) could give insights on the osmotic-induced changes of microbial esterase activity and membrane integrity; also the extrusion of intracellular accumulated carboxyfluorescein (cF) upon energizing with glucose. Comparison of the flowcytometric viability assessment with the conventional culture techniques revealed that sucrose-stressed cells had a slight loss of culturability (logN/N(0) approximately -0.3) at 1.2 and 1.5M sucrose concentration though they could perform an enzymatic conversion of cFDA into cF. The presence of such metabolically active bacteria in food might be critical as they may excrete toxic or food spoilage metabolites. Moreover, the perturbation of cF extrusion activities became a limiting factor for reproductive capacities. There was no change in the cell morphology. These results proved the ability of the strain of study to tolerate sucrose, even at extreme concentrations and these must be taken into consideration for its usage in the formulation/processing of sugar-based foods, e.g. jams, candies, etc.

Optochin resistance in Streptococcus pneumoniae induced by frozen storage in glycerol

Susceptibility to optochin is frequently the only test used to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci isolated from clinical specimens. The current study shows that storage of S. pneumoniae isolates in tryptic soy broth containing 15% glycerol at -70 degrees C can lead to optochin resistance. This optochin resistance was sometimes reversible by growing the bacteria in broth. Optochin-susceptible and optochin-resistant variants of individual S. pneumoniae isolates have similar pulsed-field gel electrophoresis pattern. However, optochin-resistant S. pneumoniae isolates exhibit differences in ultrastructure compared with optochin-susceptible variants. This study demonstrates that the frozen storages of S. pneumoniae in glycerol may affect the optochin phenotype. Thus, this characteristic should not be the only one used for identification of S. pneumoniae after frozen storage of isolates.

Pressure effects on the abiotic polymerization of glycine

Polymerization experiments were performed using dry glycine under various pressures of 5-100 MPa at 150 degrees C for 1-32 days. The series of experiments was carried out under the assumption that the pore space of deep sediments was adequate for dehydration polymerization of pre-biotic molecules. The products show various colors ranging from dark brown to light yellow, depending on the pressure. Visible and infrared spectroscopy reveal that the coloring is the result of formation of melanoidins at lower pressures. High-performance liquid chromatography and mass spectrometry analyses of the products show that: (1) glycine in all the experimental runs oligomerizes from 2-mer to 10-mer; (2) the yields are dependent on pressure up to 25 MPa and decrease slightly thereafter; and (3) polymerization progressed for the first 8 days, while the amounts of oligomers remained constant for longer-duration runs of up to 32 days. These results suggest that pressure inhibits the decomposition of amino acids and encourages polymerization in the absence of a catalyst. Our results further imply that abiotic polymerization could have occurred during diagenesis in deep sediments rather than in oceans.

Damage of cell envelope of Lactobacillus helveticus during vacuum drying

AIMS

The aim of this study was to gain insight into the inactivation mechanisms of Lactobacillus helveticus during vacuum drying.

METHODS AND RESULTS

Early stationary phase cells of L. helveticus were dried in a vacuum drier. Viability, cell integrity and metabolic activity of cells were assessed over time by plate counts on de Man Rogosa and Sharpe broth agar medium and cytological methods employing fluorescent reagents and nucleic acid stains. The cell envelope damage was visualized by atomic force microscopy (AFM). Fourier transform infrared spectroscopy (FT-IR) was used to indirectly observe changes in cell components during drying. Viability, metabolic activity and cell integrity decreased during vacuum drying, and different inactivation curves, characterized by the loss of ability to resume growth, and cell injuries were found. AFM images showed cracks on the surface of dried cells. Main changes in FT-IR spectra were attributed to the damage in cell envelope.

CONCLUSION

The cell envelope was the main site of damage in L. helveticus during vacuum drying.

SIGNIFICANCE AND IMPACT OF THE STUDY

Inactivation mechanisms of L. helveticus during vacuum drying were partly elucidated. This information is useful for the improvement of the viability of vacuum-dried starter cultures.

Characterization of a highly hop-resistant Lactobacillus brevis strain lacking hop transport

Resistance to hops is a prerequisite for lactic acid bacteria to spoil beer. In this study we analyzed mechanisms of hop resistance of Lactobacillus brevis at the metabolism, membrane physiology, and cell wall composition levels. The beer-spoiling organism L. brevis TMW 1.465 was adapted to high concentrations of hop compounds and compared to a nonadapted strain. Upon adaptation to hops the metabolism changed to minimize ethanol stress. Fructose was used predominantly as a carbon source by the nonadapted strain but served as an electron acceptor upon adaptation to hops, with concomitant formation of acetate instead of ethanol. Furthermore, hop adaptation resulted in higher levels of lipoteichoic acids (LTA) incorporated into the cell wall and altered composition and fluidity of the cytoplasmic membrane. The putative transport protein HitA and enzymes of the arginine deiminase pathway were overexpressed upon hop adaptation. HorA was not expressed, and the transport of hop compounds from the membrane to the extracellular space did not account for increased resistance to hops upon adaptation. Accordingly, hop resistance is a multifactorial dynamic property, which can develop during adaptation. During hop adaptation, arginine catabolism contributes to energy and generation of the proton motive force until a small fraction of the population has established structural improvements. This acquired hop resistance is energy independent and involves an altered cell wall composition. LTA shields the organism from accompanying stresses and provides a reservoir of divalent cations, which are otherwise scarce as a result of their complexation by hop acids. Some of the mechanisms involved in hop resistance overlap with mechanisms of pH resistance and ethanol tolerance and as a result enable beer spoilage by L. brevis.

Experimental and numerical study of heterogeneous pressure-temperature-induced lethal and sublethal injury ofLactococcus Lactis in a medium scale high-pressure autoclave

The present contribution is dedicated to experimental and theoretical assessment of microbiological process heterogeneities of the high-pressure (HP) inactivation of Lactococcus lactis ssp. cremoris MG 1363. The inactivation kinetics are determined in dependence of pressure, process time, temperature and absence or presence of co-solutes in the buffer system namely 4 M sodium chloride and 1.5 M sucrose. The kinetic analysis is carried out in a 0.1-L autoclave in order to minimise thermal and convective effects. Upon these data, a deterministic inactivation model is formulated with the logistic equation. Its independent variables represent the counts of viable cells (viable but injured) and of the stress-resistant cells (viable and not injured). This model is then coupled to a thermo-fluiddynamical simulation method, high-pressure computer fluid dynamics technique (HP-CFD), which yields spatiotemporal temperature and flow fields occurring during the HP application inside any considered autoclave. Besides the thermo-fluiddynamic quantities, the coupled model predicts also the spatiotemporal distribution of both viable (VC) and stress-resistant cell counts (SRC). In order to assess the process non-uniformity of the microbial inactivation in a 3.3-L autoclave experimentally, microbial samples are placed at two distinct locations and are exposed to various process conditions. It can be shown with both, experimental and theoretical models that thermal heterogeneities induce process non-uniformities of more than one decimal power in the counts of the viable cells at the end of the treatment.

High pressure-sensitive gene expression in Lactobacillus sanfranciscensis

Lactobacillus sanfranciscensis is a Gram-positive lactic acid bacterium used in food biotechnology. It is necessary to investigate many aspects of a model organism to elucidate mechanisms of stress response, to facilitate preparation, application and performance in food fermentation, to understand mechanisms of inactivation, and to identify novel tools for high pressure biotechnology. To investigate the mechanisms of the complex bacterial response to high pressure we have analyzed changes in the proteome and transcriptome by 2-D electrophoresis, and by microarrays and real time PCR, respectively. More than 16 proteins were found to be differentially expressed upon high pressure stress and were compared to those sensitive to other stresses. Except for one apparently high pressure-specific stress protein, no pressure-specific stress proteins were found, and the proteome response to pressure was found to differ from that induced by other stresses. Selected pressure-sensitive proteins were partially sequenced and their genes were identified by reverse genetics. In a transcriptome analysis of a redundancy cleared shot gun library, about 7% of the genes investigated were found to be affected. Most of them appeared to be up-regulated 2- to 4-fold and these results were confirmed by real time PCR. Gene induction was shown for some genes up-regulated at the proteome level (clpL/groEL/rbsK), while the response of others to high hydrostatic pressure at the transcriptome level seemed to differ from that observed at the proteome level. The up-regulation of selected genes supports the view that the cell tries to compensate for pressure-induced impairment of translation and membrane transport.

Pressure inactivation of Bacillus endospores

The inactivation of bacterial endospores by hydrostatic pressure requires the combined application of heat and pressure. We have determined the resistance of spores of 14 food isolates and 5 laboratory strains of Bacillus subtilis, B. amyloliquefaciens, and B. licheniformis to treatments with pressure and temperature (200 to 800 MPa and 60 to 80 degrees C) in mashed carrots. A large variation in the pressure resistance of spores was observed, and their reduction by treatments with 800 MPa and 70 degrees C for 4 min ranged from more than 6 log units to no reduction. The sporulation conditions further influenced their pressure resistance. The loss of dipicolinic acid (DPA) from spores that varied in their pressure resistance was determined, and spore sublethal injury was assessed by determination of the detection times for individual spores. Treatment of spores with pressure and temperature resulted in DPA-free, phase-bright spores. These spores were sensitive to moderate heat and exhibited strongly increased detection times as judged by the time required for single spores to grow to visible turbidity of the growth medium. The role of DPA in heat and pressure resistance was further substantiated by the use of the DPA-deficient mutant strain B. subtilis CIP 76.26. Taken together, these results indicate that inactivation of spores by combined pressure and temperature processing is achieved by a two-stage mechanism that does not involve germination. At a pressure between 600 and 800 MPa and a temperature greater than 60 degrees C, DPA is released predominantly by a physicochemical rather than a physiological process, and the DPA-free spores are inactivated by moderate heat independent of the pressure level. Relevant target organisms for pressure and temperature treatment of foods are proposed, namely, strains of B. amyloliquefaciens, which form highly pressure-resistant spores.

Environmental stress responses in Lactobacillus: a review

Environmental stress responses in Lactobacillus, which have been investigated mainly by proteomics approaches, are reviewed. The physiological and molecular mechanisms of responses to heat, cold, acid, osmotic, oxygen, high pressure and starvation stresses are described. Specific examples of the repercussions of these effects in food processing are given. Molecular mechanisms of stress responses in lactobacilli and other bacteria are compared.

Protective effect of sucrose and sodium chloride for Lactococcus lactis during sublethal and lethal high-pressure treatments

The bactericidal effect of hydrostatic pressure is reduced when bacteria are suspended in media with high osmolarity. To elucidate mechanisms responsible for the baroprotective effect of ionic and nonionic solutes, Lactococcus lactis was treated with pressures ranging from 200 to 600 MPa in a low-osmolarity buffer or with buffer containing 0.5 M sucrose or 4 M NaCl. Pressure-treated cells were characterized in order to determine viability, the transmembrane difference in pH (DeltapH), and multiple-drug-resistance (MDR) transport activity. Furthermore, pressure effects on the intracellular pH and the fluidity of the membrane were determined during pressure treatment. In the presence of external sucrose and NaCl, high intracellular levels of sucrose and lactose, respectively, were accumulated by L. lactis; 4 M NaCl and, to a lesser extent, 0.5 M sucrose provided protection against pressure-induced cell death. The transmembrane DeltapH was reversibly dissipated during pressure treatment in any buffer system. Sucrose but not NaCl prevented the irreversible inactivation of enzymes involved in pH homeostasis and MDR transport activity. In the presence 0.5 M sucrose or 4 M NaCl, the fluidity of the cytoplasmic membrane was maintained even at low temperatures and high pressure. These results indicate that disaccharides protect microorganisms against pressure-induced inactivation of vital cellular components. The protective effect of ionic solutes relies on the intracellular accumulation of compatible solutes as a response to the osmotic stress. Thus, ionic solutes provide only asymmetric protection, and baroprotection with ionic solutes requires higher concentrations of the osmolytes than of disaccharides.

Barotolerance is inducible by preincubation under hydrostatic pressure, cold-, osmotic- and acid-stress conditions in Lactobacillus sanfranciscensis DSM 20451T

AIMS

This study addresses the inducibility of barotolerance by preincubation of Lactobacillus sanfranciscensis DSM 20451T under various sublethal stress conditions.

METHODS AND RESULTS

Stress conditions which reduce the growth rate of L. sanfranciscensis DSM 20451T to 10% of its maximum were determined. These conditions were met at 43, 12.5 degrees C, a pH value of 3.7, 1.9% NaCl, or 80 MPa respectively. In contrast to heat preincubation, other prestresses, including salt, cold and pressure led to an increase of barotolerance by hydrostatic pressure of 300 MPa for 30 min. Stationary-phase cells also showed an increased barotolerance. Sublethal pressure leads to enhanced heat tolerance.

CONCLUSIONS

Stress response to salt, low temperature and acidic pH as well as starvation overlap with that one to high pressure by inducing barotolerance.

SIGNIFICANCE AND IMPACT OF THE STUDY

Inactivation of bacteria by high pressure treatment is influenced by their history which modulates barotolerance. Mechanisms of barotolerance appear different from heat shock defence.

Injury recovery of foodborne pathogens in high hydrostatic pressure treated milk during storage

Bacteria are expected to be injured or killed by high hydrostatic pressure (HHP). This depends on pressure levels, species and strain of the microorganism and subsequent storage. Injured bacteria may be repaired which could affect the microbiological quality of foodstuffs with an important safety consideration especially in low acid food products. In this study two Gram-positive (Listeria monocytogenes CA and Staphylococcus aureus 485) and two Gram-negative (Escherichia coli O157:H7 933 and Salmonella enteritidis FDA) relatively pressure resistant strains of foodborne pathogens were pressurized at 350, 450 and 550 MPa in milk (pH 6.65) and stored at 4, 22 and 30 degrees C. The results of shelf life studies indicated two types of injury, I1 and I2, for all the pathogens studied. It is obvious that I2 type injury is a major injury and after its repair (I2 to I1), the cells can form colonies on non-selective but not on selective agar. The formation of colonies on both selective and non-selective agar occurs only after full recovery of injury (I1 to AC). The results presented in this study show that even if injured cells are not detected immediately after HHP treatment, I2 type injury could be potentially present in the food system. Therefore, it is imperative that shelf life studies must be conducted over a period of time for potential repair of I2 type injury either to detectable injury (I1) or to active cells (AC) to ascertain microbiological safety of low acid food products.

Effects of high-pressure processing on Listeria monocytogenes, spoilage microflora and multiple compound quality indices in chilled cold-smoked salmon

AIMS

To evaluate the effect of high-pressure processing (HPP) on Listeria monocytogenes, microbial and chemical changes and shelf-life in chilled cold-smoked salmon (CSS).

METHODS AND RESULTS

First, challenge tests with L. monocytogenes were carried out using HPP of the product at 0.1 (control), 150, 200 and 250 MPa. Secondly, storage trials with the naturally contaminated product and HPP at 0.1 (control) and 200 MPa were realized. Shelf-life, microbial changes and chemical changes were determined and existing predictive models and multiple compound quality indices evaluated. HPP with 250 MPa did not inactivate L. monocytogenes but significant lag phases of 17 and 10 days were observed at ca 5 and 10 degrees C, respectively. HPP with 200 MPa had a marked effect on both colour and texture of CSS.

CONCLUSIONS

High-pressure processing was unable to prevent growth of L. monocytogenes or spoilage of chilled CSS. Existing mathematical models allowed growth rates of L. monocytogenes and shelf-life of samples without high-pressure treatments to be predicted.

SIGNIFICANCE AND IMPACT OF THE STUDY

High-pressure processing seems more appropriate for new types of salmon products than for a classical product like CSS where consumers expect specific quality attributes.

Use of redox potential modification by gas improves microbial quality, color retention, and ascorbic acid stability of pasteurized orange juice

The aim of this paper was to study the effect of both redox potential (Eh) and pasteurization of orange juice on stability of color and ascorbic acid, and growth recovery of microorganisms during storage at 15 degrees C for 7 weeks. Three conditions of Eh, +360 mV (ungassed), +240 mV (gassed with N2), and -180 mV (gassed with N2-H2) were applied to orange juice. Both thermal destruction and recovery of sublethally heat-injured cells of Lactobacillus plantarum and Saccharomyces cerevisiae were investigated. While oxidizing conditions were the most effective for thermal destruction of L. plantarum and S. cerevisiae, reducing conditions decreased recovery of heated cells of S. cerevisiae. In addition, gassing the juice with N2 or N2-H2 increased color retention and ascorbic acid stability. The present study demonstrated that juice must be reduced just after the heat treatment in order, firstly, to maximize microbial destruction during pasteurization, and secondly, to prevent the development of microorganisms and stabilize color and ascorbic acid during storage.

Effects of pulsed electric fields on inactivation and metabolic activity of Lactobacillus plantarum in model beer

AIMS

Inactivation and sublethal injury of Lactobacillus plantarum at different pulsed electric field (PEF) strengths and total energy inputs were investigated to differentiate reversible and irreversible impacts on cell functionality.

METHODS AND RESULTS

Lactobacillus plantarum was treated with PEF in model beer (MB) to determine critical values of field strength and energy input for cell inactivation. Below critical values, metabolic activity and membrane integrity were initially reduced without loss of viability. Above critical values, however, irreversible cell damage occurred. Presence of nisin or hop extract, during PEF treatment, resulted in an additional reduction of cell viability by 1;5 log cycles. Also, addition of the hop extract resulted in an additional two log cycles of sublethal injury. Partial reversibility of membrane damage was observed using propidium iodide (PI) uptake and staining. Inoculated MB containing hops was stored after PEF to evaluate the efficacy of such treatment for beer preservation.

CONCLUSION

Cells were inactivated only above critical values of 13 kV x cm(-1) and 64 kJ x kg(-1); below these values cell damage was reversible. Storage experiments revealed that surviving cells were killed after 15 h storage in MB containing hops.

SIGNIFICANCE AND IMPACT OF THE STUDY

Both reversible and irreversible cell damage due to PEF treatment was detected, depending on specific treatment conditions. The combination of PEF and hop addition is a promising nonthermal method of preservation for beer.

In situ determination of the intracellular pH of Lactococcus lactis and Lactobacillus plantarum during pressure treatment

Hydrostatic pressure may affect the intracellular pH of microorganisms by (i) enhancing the dissociation of weak organic acids and (ii) increasing the permeability of the cytoplasmic membrane and inactivation of enzymes required for pH homeostasis. The internal pHs of Lactococcus lactis and Lactobacillus plantarum during and after pressure treatment at 200 and 300 MPa and at pH values ranging from 4.0 to 6.5 were determined. Pressure treatment at 200 MPa for up to 20 min did not reduce the viability of either strain at pH 6.5. Pressure treatment at pH 6.5 and 300 MPa reduced viable cell counts of Lactococcus lactis and Lactobacillus plantarum by 5 log after 20 and 120 min, respectively. Pressure inactivation was faster at pH 5 or 4. At ambient pressure, both strains maintained a transmembrane pH gradient of 1 pH unit at neutral pH and about 2 pH units at pH 4.0. During pressure treatment at 200 and 300 MPa, the internal pH of L. lactis was decreased to the value of the extracellular pH during compression. The same result was observed during treatment of Lactobacillus plantarum at 300 MPa. Lactobacillus plantarum was unable to restore the internal pH after a compression-decompression cycle at 300 MPa and pH 6.5. Lactococcus lactis lost the ability to restore its internal pH after 20 and 4 min of pressure treatment at 200 and 300 MPa, respectively. As a consequence, pressure-mediated stress reactions and cell death may be considered secondary effects promoted by pH and other environmental conditions.

Effects of high pressure on the viability, morphology, lysis, and cell wall hydrolase activity of Lactococcus lactis subsp. cremoris

Viability, morphology, lysis, and cell wall hydrolase activity of Lactococcus lactis subsp. cremoris MG1363 and SK11 were determined after exposure to pressure. Both strains were completely inactivated at pressures of 400 to 800 MPa but unaffected at 100 and 200 MPa. At 300 MPa, the MG1363 and SK11 populations decreased by 7.3 and 2.5 log cycles, respectively. Transmission electron microscopy indicated that pressure caused intracellular and cell envelope damage. Pressure-treated MG1363 cell suspensions lysed more rapidly over time than did non-pressure-treated controls. Twenty-four hours after pressure treatment, the percent lysis ranged from 13.0 (0.1 MPa) to 43.3 (300 MPa). Analysis of the MG1363 supernatants by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) confirmed pressure-induced lysis. Pressure did not induce lysis or membrane permeability of SK11. Renaturing SDS-PAGE (zymogram analysis) revealed two hydrolytic bands from MG1363 cell extracts treated at all pressures (0.1 to 800 MPa). Measuring the reducing sugars released during enzymatic cell wall breakdown provided a quantitative, nondenaturing assay of cell wall hydrolase activity. Cells treated at 100 MPa released significantly more reducing sugar than other samples, including the non-pressure-treated control, indicating that pressure can activate cell wall hydrolase activity or increase cell wall accessibility to the enzyme. The cell suspensions treated at 200 and 300 MPa did not differ significantly from the control, whereas cells treated at pressures greater than 400 MPa displayed reduced cell wall hydrolase activity. These data suggest that high pressure can cause inactivation, physical damage, and lysis in L. lactis. Pressure-induced lysis is strain dependent and not solely dependent upon cell wall hydrolase activity.

Effects of pressure-induced membrane phase transitions on inactivation of HorA, an ATP-dependent multidrug resistance transporter, in Lactobacillus plantarum

The effects of pressure on cultures of Lactobacillus plantarum were characterized by determination of the viability and activity of HorA, an ATP-binding cassette multidrug resistance transporter. Changes in the membrane composition of L. plantarum induced by different growth temperatures were determined. Furthermore, the effect of the growth temperature of a culture on pressure inactivation at 200 MPa was determined. Cells were characterized by plate counts on selective and nonselective agar after pressure treatment, and HorA activity was measured by ethidium bromide efflux. Fourier transform-infrared spectroscopy and Laurdan fluorescence spectroscopy provided information about the thermodynamic phase state of the cytoplasmic membrane during pressure treatment. A pressure-temperature diagram for cell membranes was established. Cells grown at 37 degrees C and pressure treated at 15 degrees C lost >99% of HorA activity and viable cell counts within 36 and 120 min, respectively. The membranes of these cells were in the gel phase region at ambient pressure. In contrast, cells grown at 15 degrees C and pressure treated at 37 degrees C lost >99% of HorA activity and viable cell counts within 4 and 8 min, respectively. The membranes of these cells were in the liquid crystalline phase region at ambient pressure. The kinetic analysis of inactivation of L. plantarum provided further evidence that inactivation of HorA is a crucial step during pressure-induced cell death. Comparison of the biological findings and the membrane state during pressure treatment led to the conclusion that the inactivation of cells and membrane enzymes strongly depends on the thermodynamic properties of the membrane. Pressure treatment of cells with a liquid crystalline membrane at 0.1 MPa resulted in HorA inactivation and cell death more rapid than those of cells with a gel phase membrane at 0.1 MPa.

On-line fluorescence determination of pressure mediated outer membrane damage in Escherichia coli

The outer membrane (OM) of Gram-negative bacteria provides a protective barrier for natural occurring inhibitors. Pressure mediated OM permeabilisation therefore contributes to the elimination of Escherichia coli and Salmonella by pressure preservation processes. Pressure mediated inactivation, sublethal injury, and membrane permeabilisation of E. coli were determined using two strains differing in their barotolerance. Pressure treatment of E. coli TMW 2.427 at 300, 500 and 600 MPa for 40 min resulted in a 0, 1, and greater 6 log decrease of viable cell counts, respectively. The kinetics of OM and cytoplasmic membrane permeabilisation after pressure treatment were determined by staining of pressure treated cells with the fluorescent dyes propidium iodide (PI) and 1-N-phenylnaphtylamine (NPN), respectively. A slight increase of PI fluorescence was observed at conditions resulting in a greater 6 log decrease of viable cell counts only. In contrast, increased NPN fluorescence indicating OM permeabilisation was observed prior to cell death and sublethal injury. An on-line assay for determination of pressure mediated OM damage based on NPN fluorescence was established to distinguish between reversible and irreversible OM damage. Generally, the same degree of outer membrane damage was observed by either on line or off line determinations. However, whereas reversible membrane damage occurred fast and in thermodynamic equilibrium with pressure conditions, irreversible outer membrane damage was a time dependent process.