Genotypic and phenotypic characterization of a Salmonella Typhimurium strain resistant to pulsed electric fields

Pulsed Electric Fields (PEF) technology is regarded as one of the most interesting alternatives to current food preservation methods, due to its capability to inactivate vegetative microorganisms while leaving the product's organoleptic and nutritional properties mostly unchanged. However, many aspects regarding the mechanisms of bacterial inactivation by PEF are still not fully understood. The aim of this study was to obtain further insight into the mechanisms responsible for the increased resistance to PEF of a Salmonella Typhimurium SL1344 variant (SL1344-RS, Sagarzazu et al., 2013), and to quantify the impact that the acquisition of PEF resistance has on other aspects of S. enterica physiology, such as growth fitness, biofilm formation ability, virulence and antibiotic resistance. WGS, RNAseq and qRT-PCR assays indicated that the increased PEF resistance of the SL1344-RS variant is due to a higher RpoS activity caused by a mutation in the hnr gene. This increased RpoS activity also results in higher resistance to multiple stresses (acidic, osmotic, oxidative, ethanol and UV-C, but not to heat and HHP), decreased growth rate in M9-Gluconate (but not in TSB-YE or LB-DPY), increased ability to adhere to Caco-2 cells (but no significant change in invasiveness) and enhanced antibiotic resistance (to six out of eight agents). This study significantly contributes to the understanding of the mechanisms of the development of stress resistance in Salmonellae and underscores the crucial role played by RpoS in this process. Further studies are needed to determine whether this PEF-resistant variant would represent a higher, equal or lower associated hazard than the parental strain.

Osmotolerance provided by the alternative sigma factors σB and rpoS to Staphylococcus aureus and Escherichia coli is solute dependent and does not result in an increased growth fitness in NaCl containing media

The aim of this work was to examine the role of the alternative general stress sigma factors σ(B) and rpoS on the ability of Staphylococcus aureus and Escherichia coli, respectively, to grow in liquid and solid media of different osmolarity. For this purpose, S. aureus strain Newman and its isogenic ΔsigB mutant IK84 and E. coli strain BJ4 and its isogenic ΔrpoS mutant BJ4L1 were grown in media (TSBYE) with different concentrations of NaCl. Growth parameters (lag phase duration, growth rate and maximum number of microorganisms) and limiting growth concentrations (Maximum Non-Inhibitory Concentration - MNIC - and Minimum Inhibitory Concentration - MIC-) were determined. The mechanisms underlying the differences observed between parental and mutant strains were also explored. The absence of the sigma factors σ(B) and rpoS led to a decrease in the MNICs and MICs calculated for S. aureus and E. coli, respectively. Conversely, neither σ(B) nor rpoS provided with increased growth fitness to S. aureus and E. coli cells at NaCl concentrations up to 1.36M and 1M, respectively. The decreased osmotolerance of the σ(B) and rpoS deficient strains, as compared to their parental strains, was compensated by the addition of glycine-betaine (1mM) to the growth medium. It was also observed that the decreased tolerance to NaCl of the mutant strains was coincident with a decreased tolerance to sucrose, KCl, and LiCl but not to glycerol, MgCl2, and CaCl2. Results obtained also demonstrate that the increased osmotolerance of stationary growth phase E. coli cells, as compared to exponential growth phase ones, would be due to the activation of both rpoS-independent and rpoS-dependent mechanisms. This work will help to understand the mechanisms of bacterial resistance to osmotic stress and the role of the alternative sigma factors σ(B) and rpoS in this process.

Bacterial maximum non-inhibitory and minimum inhibitory concentrations of different water activity depressing solutes

The NaCl MNICs (maximum non-inhibitory concentrations) and MICs (minimum inhibitory concentrations) for growth of various strains of six bacterial species were determined and then compared with those obtained for seven other solutes. The influence of prior growth conditions on the MNICs and MICs was also evaluated. No significant changes on the MNICs and MICs were found among the strains studied within each species. Among all factors investigated, only growth phase -for Gram-negatives- and growth at high NaCl concentrations led to a change in the NaCl MNICs. Species could be classified depending on its NaCl MNICs and MICs (in decreasing order) as follows: Staphylococcus aureus, Listeria monocytogenes, Cronobacter sakazakii, Enterococcus faecium, Escherichia coli and Salmonella Typhimurium. Similar results were obtained for KCl, LiCl, and sodium acetate, but not for the remaining solutes investigated (sucrose, glycerol, MgCl2 and CaCl2). Results obtained indicate that, in general, Gram-negatives showed lower MNICs and MICs than Gram-positives for all the solutes, S. aureus being the most solute tolerant microorganism. When compared on a molar basis, glycerol showed the highest MNICs and MICs for all the microorganisms -except for S. aureus- and LiCl the lowest ones. NaCl MNICs and MICs were not significantly different from those of KCl when compared on a molar basis. Therefore, the inhibitory action of NaCl could not be linked to the specific action of Na(+). Results also showed that the Na(+) tolerance of some species was Cl(-) dependent whereas for others it was not, and that factors others than aw-decrease contribute to the inhibitory action of LiCl, CaCl2 and MgCl2.

Emergence of pulsed electric fields resistance in Salmonella enterica serovar Typhimurium SL1344

In this investigation we selected and isolated a culture derived from Salmonella enterica serovar Typhimurium SL1344 with stable increased resistance to pulsed electric fields (PEF) after repeated rounds of PEF treatment and outgrowth of survivors. The resulting culture showed a higher resistance to PEF treatments under different treatment conditions. The acquisition of PEF resistance was only observed in stationary phase cells. The cytoplasmic membrane of the resistant variant showed a higher resilience against PEF treatments, since a lower permeabilization degree was observed after PEF treatments, in comparison to the parental strain. Resistance to PEF was also accompanied by a higher tolerance to acidic pH, hydrogen peroxide and ethanol, but not to heat. The occurrence of a PEF resistant variant in S. enterica serovar Typhimurium SL1344 emphasizes the need to further study the mechanisms of inactivation and resistance by PEF for an adequate design of safe treatments.

Mechanism of the synergistic inactivation of Escherichia coli by UV-C light at mild temperatures

UV light only penetrates liquid food surfaces to a very short depth, thereby limiting its industrial application in food pasteurization. One promising alternative is the combination of UV light with mild heat (UV-H), which has been demonstrated to produce a synergistic bactericidal effect. The aim of this article is to elucidate the mechanism of synergistic cellular inactivation resulting from the simultaneous application of UV light and heat. The lethality of UV-H treatments remained constant below ∼45°C, while lethality increased exponentially as the temperature increased. The percentage of synergism reached a maximum (40.3%) at 55°C. Neither the flow regimen nor changes in the dose delivered by UV lamps contributed to the observed synergism. UV-H inactivation curves of the parental Escherichia coli strain obtained in a caffeic acid selective recovery medium followed a similar profile to those obtained with uvrA mutant cells in a nonselective medium. Thermal fluidification of membranes and synergistic lethal effects started around 40 to 45°C. Chemical membrane fluidification with benzyl alcohol decreased the UV resistance of the parental strain but not that of the uvrA mutant. These results suggest that the synergistic lethal effect of UV-H treatments is due to the inhibition of DNA excision repair resulting from the membrane fluidification caused by simultaneous heating.

Role of general stress-response alternative sigma factors σ(S) (RpoS) and σ(B) (SigB) in bacterial heat resistance as a function of treatment medium pH

This investigation aimed to determine the role of general stress-response alternative sigma factors σ(S) (RpoS) and σ(B) (SigB) in heat resistance and the occurrence of sublethal injuries in cell envelopes of stationary-phase Escherichia coli BJ4 and Listeria monocytogenes EGD-e cells, respectively, as a function of treatment medium pH. Given that microbial death followed first-order inactivation kinetics (R(2)>0.95) the traditional D(T) and z values were used to describe the heat inactivation kinetics. Influence of rpoS deletion was constant at every treatment temperature and pH, making a ΔrpoS deletion mutant strain approximately 5.5 times more heat sensitive than its parental strain for every studied condition. Furthermore, the influence of the pH of the treatment medium on the reduction of the heat resistance of E. coli was also constant and independent of the treatment temperature (average z value=4.9°C) in both parental and mutant strains. L. monocytogenes EGD-e z values obtained at pH 7.0 and 5.5 were not significantly different (p>0.05) in either parental or the ∆sigB deletion mutant strains (average z value=4.8°C). Nevertheless, at pH 4.0 the z value was higher (z=8.4°C), indicating that heat resistance of both L. monocytogenes strains was less dependent on temperature at pH 4.0. At both pH 5.5 and 7.0 the influence of sigB deletion was constant and independent of the treatment temperature, decreasing L. monocytogenes heat resistance approximately 2.5 times. In contrast, the absence of sigB did not decrease the heat resistance of L. monocytogenes at pH 4.0. The role of RpoS in protecting cell envelopes was more important in E. coli (4 times) than SigB in L. monocytogenes (1.5 times). Moreover, the role of σ(S) in increasing heat resistance seems more relevant in enhancing the intrinsic resilience of the cytoplasmic membrane, and to a lesser extent, outer membrane resilience. Knowledge of environmental conditions related to the activation of alternative sigma factors σ(S) and σ(B) and their effects on heat resistance would help us to avoid and/or identify situations that increase bacterial stress resistance. Therefore, more efficient food preservation processes might be designed.

High hydrostatic pressure resistance of Campylobacter jejuni after different sublethal stresses

AIMS

To study the development of resistance responses in Campylobacter jejuni to high hydrostatic pressure (HHP) treatments after the exposure to different stressful conditions that may be encountered in food-processing environments, such as acid pH, elevated temperatures and cold storage.

METHODS AND RESULTS

Campylobacter jejuni cells in exponential and stationary growth phase were exposed to different sublethal stresses (acid, heat and cold shocks) prior to evaluate the development of resistance responses to HHP. For exponential-phase cells, neither of the conditions tested increased nor decreased HHP resistance of C. jejuni. For stationary-phase cells, acid and heat adaptation-sensitized C. jejuni cells to the subsequent pressure treatment. On the contrary, cold-adapted stationary-phase cells developed resistance to HHP.

CONCLUSIONS

Whereas C. jejuni can be classified as a stress sensitive micro-organism, our findings have demonstrated that it can develop resistance responses under different stressing conditions. The resistance of stationary phase C. jejuni to HHP was increased after cells were exposed to cold temperatures.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of this study contribute to a better knowledge of the physiology of C. jejuni and its survival to food preservation agents. Results here presented may help in the design of combined processes for food preservation based on HHP technology.

Heat-adaptation induced thermotolerance in Staphylococcus aureus: Influence of the alternative factor sigmaB

The role of sigmaB in the Staphylococcus aureus heat-shock induced thermotolerance was investigated. Survival curves at 58 degrees C of S. aureus strain Newman and its isogenic DeltasigB mutant were obtained for native and heat-shocked cells (45 degrees C for 5-120 min) in exponential and stationary phase of growth. The magnitude of the acquisition of thermotolerance at 58 degrees C depended on the growth phase and on the duration of the heat shock. Stationary growth phase cells were always more heat tolerant than exponentially growing cells and thermotolerance increased with heat-shock duration up to 120 min. S. aureus cells were able to increase their heat tolerance in the absence of the sigma(B) factor. In stationary phase, whereas in the parental strain the thermotolerance was increased by a factor of 12 after a heat shock of 120 min at 45 degrees C (delta values at 58 degrees C for native and heat-shocked cells were 0.63 and 7.22 min, respectively), in the mutant strain it increased 43 fold (delta values 0.09 and 3.87 min). The addition of chloramphenicol to the adaptation medium resulted in a lower increase in heat tolerance but did not prevent it completely, suggesting that S. aureus can partially increase its thermotolerance without "de novo" protein synthesis. Both the number of non-damaged cells and the proportion of cells able to repair sublethal damage were higher for heat-shocked cells.

Biosynthetic requirements for the repair of sublethally injured Saccharomyces cerevisiae cells after pulsed electric fields

AIMS

The aim was to evaluate the biosynthetic requirements for the repair of sublethal membrane damages in Saccharomyces cerevisiae cells after exposure to pulsed electric fields (PEF).

METHODS AND RESULTS

The partial loss of the integrity and functionality of the cytoplasmic membrane was assessed by adding sodium chloride to the recovery medium. More than 2 log(10) cycles of survivors were sublethally injured after PEF. Repair of sublethal membrane damages occurred when survivors to PEF were incubated in Sabouraud Broth for 4 h at room temperature. The addition of inhibitors, such as chloramphenicol, rifampicin, 5-fluorocytosine, nalidixic acid, cycloheximide, cerulenin, miconazol and sodium azide to the liquid repair medium showed that the repair of PEF-injured cells required energy and protein synthesis. The extent of the sublethal damages was greater in PEF-treated cells at pH 4.0 than at pH 7.0.

CONCLUSIONS

This work confirms that membrane damage is an important event in the PEF-inactivation of yeast. The mechanism of yeast inactivation by PEF seems to differ from that of bacteria, as the repair of sublethal damages requires protein synthesis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Knowledge about the damages inflicted by PEF leads to a better description of the mechanism of yeast inactivation.

Heat and pulsed electric field resistance of pigmented and non-pigmented enterotoxigenic strains of Staphylococcus aureus in exponential and stationary phase of growth

The survival of four enterotoxigenic strains of Staphylococcus aureus (with different pigment content) to heat and to pulsed electric fields (PEF) treatments, and the increase in resistance to both processing stresses associated with entrance into stationary phase was examined. Survival curves to heat (58 degrees C) and to PEF (26 kV/cm) of cells in the stationary and in the exponential phase of growth were obtained. Whereas a wide variation in resistance to heat treatments was detected amongst the four strains, with decimal reduction time values at 58 degrees C (D(58 degrees C)) ranging from 0.93 to 0.20 min, the resistance to PEF was very similar. The occurrence of a higher tolerance to heat in stationary phase was coincident with a higher content in carotenoid pigmentation in S. aureus colonies. However, cells of the most heat resistant (pigmented) and the most heat sensitive (non-pigmented) strains in the mid-exponential phase of growth showed similar resistance to heat and to PEF. Therefore the increase in thermotolerance upon entrance into stationary phase of growth was more marked for the pigmented strains. Recovery in anaerobic conditions particularly enhanced survival to heat treatments in a non-pigmented strain. Strain CECT 4630, which possess a deficient sigma B activity, showed low heat resistance, low pigmentation, and reduced increase in thermotolerance in stationary phase. These results indicate that the magnitude of the development of a higher heat resistance in S. aureus in stationary phase is positively related to the carotenoid content of the strain. The development of tolerance to pulsed electric field was less relevant and not linked to the carotenoid content.

Relationship between sublethal injury and inactivation of yeast cells by the combination of sorbic acid and pulsed electric fields

The objective of this study was to investigate the occurrence of sublethal injury after the pulsed-electric-field (PEF) treatment of two yeasts, Dekkera bruxellensis and Saccharomyces cerevisiae, as well as the relation of sublethal injury to the inactivating effect of the combination of PEF and sorbic acid. PEF caused sublethal injury in both yeasts: more than 90% of surviving D. bruxellensis cells and 99% of surviving S. cerevisiae cells were sublethally injured after 50 pulses at 12 kV/cm in buffer at pHs of both 7.0 and 4.0. The proportion of sublethally injured cells reached a maximum after 50 pulses at 12.0 kV/cm (S. cerevisiae) or 16.5 kV/cm (D. bruxellensis), and it kept constant or progressively decreased at greater electric field strengths and with longer PEF treatments. Sublethally PEF-injured cells showed sensitivity to the presence of sorbic acid at a concentration of 2,000 ppm. A synergistic inactivating effect of the combination of PEF and sorbic acid was observed. Survivors of the PEF treatment were progressively inactivated in the presence of 2,000 ppm of sorbic acid at pH 3.8, with the combined treatments achieving more than log10 5 cycles of dead cells under the conditions investigated. This study has demonstrated the occurrence of sublethal injury after exposure to PEF, so yeast inactivation by PEF is not an all-or-nothing event. The combination of PEF and sorbic acid has proven to be an effective method to achieve a higher level of yeast inactivation. This work contributes to the knowledge of the mechanism of microbial inactivation by PEF, and it may be useful for improving food preservation by PEF technology.

Effect of a previous heat shock on the thermal resistance of Listeria monocytogenes and Pseudomonas aeruginosa at different pHs

In this work we study the effect of heat shocks of various durations up to 60 min, at different temperatures between 35 and 45 degrees C, in media of pH 4.0, 5.5 and 7.4 on the heat resistance of Listeria monocytogenes and Pseudomonas aeruginosa. The pattern of survival curves after heat treatment did not change with the application of a previous heat shock. However, the kinetics of inactivation was different for the two microorganisms studied. Whereas the inactivation of L. monocytogenes was similar to an exponential function of heating time and therefore straight survival curves were obtained, survival curves corresponding to P. aeruginosa showed convex profiles. All survival curves obtained in this investigation were fitted to Weibull-based Mafart equation: log(10)S(t)=-(t / delta)(p). The magnitude of the heat shock induced thermotolerance increased with treatment medium pH. At pH 7.4 the increase in heat tolerance depended on the duration and temperature of the heat shock. On the contrary, at pH 5.5 and pH 4.0, the heat-shock temperature did not exert any effect. The observed maximum delta values increased 2.3, 4.0 and 9.3 fold for L. monocytogenes, and 1.3, 2.1 and 8.4 fold for P. aeruginosa, at pH 4.0, 5.5 and 7.4, respectively. This research has proven that Mafart equation allows studying and quantifying the effect of heat shocks on bacterial heat resistance.

Induced thermotolerance under nonisothermal treatments of a heat sensitive and a resistant strain of Staphylococcus aureus in media of different pH

AIMS

The aim was to assess the induced thermotolerance under nonisothermal treatments of two strains of Staphylococcus aureus in media of different pH.

METHODS AND RESULTS

Staphylococcus aureus ATCC 25923 was more heat resistant than S. aureus ATCC 13565 at any pH investigated under isothermal conditions. At pH 7.4, the D58 value of the resistant strain was approx. 30 times greater. Both strains showed a higher heat resistance at pH 4.0 than at pH 7.4. In contrast, under nonisothermal treatments (0.5-2 degrees C min(-1)), both strains were more heat resistant when treated at pH 7.4 than at pH 4.0 due to heat adaptation at the higher pH. At the slowest heating up rate tested at pH 7.4, the initially heat-sensitive strain nearly reached the thermotolerance of the heat-resistant strain.

CONCLUSIONS

The induced thermotolerance under nonisothermal treatments depended on the treatment medium pH and the microbial strain tested. The induced thermotolerance in a sensitive strain can be greater than in a heat-resistant strain, showing similar resistance under nonisothermal conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work shows data of interest about mechanisms of microbial resistance and adaptation to heat. Moreover, it contributes to the development of more adequate combined processes for food preservation.

Variation in resistance of natural isolates of Staphylococcus aureus to heat, pulsed electric field and ultrasound under pressure

AIMS

To study and compare the resistance of 15 Staphylococcus aureus isolates to heat, pulsed electric field (PEF) and ultrasound (UW) under pressure (manosonication, MS).

METHODS AND RESULTS

Survival curves to heat (58 degrees C), to PEF (22 kV cm(-1), 2 micros square wave pulses) and to UW under pressure (117 microm, 20 kHz, 200 kPa) were obtained and inactivation parameters (decimal reduction times for heat and UW under pressure, and b-values for PEF) were calculated. A wide resistance variation to heat treatment, but not to PEF and MS, was observed amongst the 15 strains.

CONCLUSIONS

There was no relationship between the resistances to the three physical agents studied. Staphylococcus aureus was relatively resistant to MS but sensitive to PEF. Heat resistance varied with strain and was positively correlated to carotenoid pigment content.

SIGNIFICANCE AND IMPACT OF THE STUDY

Results would help in defining safe food preservation processes. Care should be taken to choose the most adequate strain of S. aureus to model food preservation processing.

Pulsed electric fields cause bacterial envelopes permeabilization depending on the treatment intensity, the treatment medium pH and the microorganism investigated

The relationship between membrane permeabilization and loss of viability by pulsed electric fields (PEF) depending on the treatment intensity and the treatment media pH in two gram-positive (Lactobacillus plantarum, Listeria monocytogenes) and two gram-negative (Escherichia coli, Salmonella senftenberg 775W) bacterial species has been investigated. Loss of membrane integrity was measured as increased uptake of the fluorescent dye propidium iodide (PI). Non-permanent/reversible permeabilization was detected when cells stained with PI during PEF resulted in higher fluorescence than that measured in cells stained after PEF. Whereas loss of viability of the two gram-negative bacteria was correlated with the sum of non-permanent and permanent membrane permeabilization when treated at pH 7.0, in the case of the two gram-positives, loss of viability was correlated with a permanent loss of membrane integrity. At pH 7.0, the four bacteria exhibited reversible permeabilization. However, whereas the gram-positives capable of reversing permeabilization survived, the gram-negative cells died, despite their capacity to reverse permeabilization immediately after PEF. Thus, resealing is not necessarily related to the survival of PEF-treated cells. In contrast, when cells were PEF-treated at pH 4.0 a more complicated picture emerged. Whereas loss of viability was correlated with a permanent loss of membrane integrity in L. monocytogenes cells, in L. plantarum the degree of permeabilization was higher, and in the gram-negative strains, much lower than the percentage of inactivated cells. These results support the view that membrane permeabilization is involved in the mechanism of bacterial inactivation by PEF, but the nature of membrane damage and its relationship with cell death depends on the bacterial species and the treatment medium pH.

Inactivation of Salmonella Senftenberg 775W by ultrasonic waves under pressure at different water activities

The inactivation of Salmonella Senftenberg 775W by ultrasonic waves (20 kHz, 117 microm) under pressure (175 kPa) treatments at sublethal (manosonication; MS) and lethal temperatures (manothermosonication; MTS) in media of different water activities has been investigated. Heat decimal reduction time values increased up to eighteen fold when the water activity was decreased from >0.99 to 0.93 at 65 degrees C, but hardly increased the MS resistance. In reduced water activity media (a(w) of 0.96 and 0.93) a synergistic lethal effect was observed between heat and ultrasound under pressure, being the inactivation rate of Salmonella Senftenberg 775W three times faster than the expected additive rate considering an effect of both bacterial lethal processes. An empirical mathematical equation enabled to predict the D(MS) and D(MTS) values obtained at different temperatures and a(w) in the ranges investigated of Salmonella serovars and also the microbial level of inactivation due to the synergistic lethal effect of MTS treatments in media of reduced a(w). This work could be useful for improving sanitation and preservation treatments of foods, especially those in which components protect microorganisms to heat.

Biosynthetic requirements for the repair of sublethal membrane damage in Escherichia coli cells after pulsed electric fields

AIMS

The aim was to evaluate the biosynthetic requirements for the repair of sublethal membrane damages in Escherichia coli cells after exposure to pulsed electric fields (PEF).

METHODS AND RESULTS

The partial loss of the barrier and homeostatic functions of the cytoplasmic membrane was examined by adding sodium chloride to the recovery media. More than 4 log10 cycles of survivors were sublethally injured after PEF. Repair of such sublethal membrane damages occurred when survivors to PEF were incubated in peptone water for 2 h. Two different types of sublethally injured cells were detected. Whereas a small proportion (<5%) repaired after PEF in less than 2 min, the repair of the remaining 95% injured cells lasted 2 h and was dependent on biosynthetic requirements. The addition of inhibitors such as chloramphenicol, cerulenin, penicillin G, rifampicin and sodium azide to the liquid repair medium showed that the repair required energy and lipid synthesis, and was not dependent on protein, peptidoglican or RNA synthesis.

CONCLUSIONS

Cell survival after PEF is dependent on the repair of the cytoplasmic membrane. Requirement of lipid synthesis for the repair of sublethally injured cells confirms that the cytoplasmic membrane is a target directly involved in the mechanism of inactivation by PEF.

SIGNIFICANCE AND IMPACT OF THE STUDY

Knowledge about the damages inflicted by PEF might help in the design of more efficient treatments.

Occurrence of sublethal injury after pulsed electric fields depending on the micro-organism, the treatment medium ph and the intensity of the treatment investigated

AIMS

The objective was to investigate the occurrence of sublethal injury after pulsed electric field (PEF) depending on the treatment time, the electric field strength and the pH of the treatment media in two Gram-positive (Bacillus subtilis ssp. niger, Listeria monocytogenes) and six Gram-negative (Escherichia coli, Escherichia coli O157:H7, Pseudomonas aeruginosa, Salmonella serotype Senftenberg 775W, Salmonella serotype Typhimurium, Yersinia enterocolitica) bacterial strains.

METHODS AND RESULTS

A characteristic behaviour was observed for the Gram-positive and Gram-negative bacteria studied. Whereas Gram-positive bacteria showed a higher PEF resistance at pH 7.0, the Gram-negative were more resistant at pH 4.0. In these conditions, in which bacteria showed their maximum resistance, a large proportion of sublethally injured cells were detected. In most cases, the longer the treatment time and the higher the electric field applied, the greater the proportion of sublethally injured cells that were detected. No sublethal injury was detected when Gram-positive bacteria were treated at pH 4.0 and Gram-negative at pH 7.0.

CONCLUSIONS

Sublethal injury was detected after PEF so, bacterial inactivation by PEF is not an 'all or nothing' event.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work could be useful for improving food preservation by PEF.

Membrane damage and microbial inactivation by chlorine in the absence and presence of a chlorine-demanding substrate

The relationship between cell inactivation and membrane damage was studied in two gram-positive organisms, Listeria monocytogenes and Bacillus subtilis, and two gram-negative organisms, Yersinia enterocolitica and Escherichia coli, exposed to chlorine in the absence and presence of 150 ppm of organic matter (Trypticase soy broth). L. monocytogenes and B. subtilis were more resistant to chlorine in distilled water. The addition of small amounts of organic matter to the chlorination medium drastically increased the resistance of both types of microorganisms, but this effect was more marked in Y. enterocolitica and E. coli. In addition, the survival curves for these microorganisms in the presence of organic matter had a prolonged shoulder. Sublethal injury was not detected under most experimental conditions, and only gram-positive cells treated in distilled water showed a relevant degree of injury. The exposure of bacterial cells to chlorine in distilled water caused extensive permeabilization of the cytoplasmic membrane, but the concentrations required were much higher than those needed to inactivate cells. Therefore, there was no relationship between the occurrence of membrane permeabilization and cell death. The addition of organic matter to the treatment medium stabilized the cytoplasmic membrane against permeabilization in both the gram-positive and gram-negative bacteria investigated. Exposure of E. coli cells to the outer membrane-permeabilizing agent EDTA increased their sensitivity to chlorine and caused the shoulders in the survival curves to disappear. Based on these observations, we propose that bacterial envelopes could play a role in cell inactivation by modulating the access of chlorine to the key targets within the cell.

Predicting heat inactivation of Listeria monocytogenes under nonisothermal treatments

The aim of this study was to find a model that accurately predicts the heat inactivation of Listeria monocytogenes (ATCC 15313) at constantly rising heating rates (0.5 to 9 degrees C/min) in media of different pH values (4.0 to 7.4). Survival curves of L. monocytogenes obtained under isothermal treatments at any temperature were nearly linear. Estimations of survival curves under nonisothermal treatments obtained from heat resistance parameters of isothermal treatments adequately fit experimental values obtained at pH 4.0. On the contrary, survivors were much higher than estimations at pH 5.5 and 7.4. The slower the heating rate and the longer the treatment time, the greater the differences between the experimental and estimated values. An equation based on the Weibullian-like distribution, log S(t) = (t/delta)p, accurately described survival curves of L. monocytogenes obtained under nonisothermal conditions within the range of heating rates investigated. A nonlinear relationship was observed between the scale parameter (delta) and the heating rate, which allowed the development of an equation capable of predicting the inactivation rate of L. monocytogenes under nonisothermal treatments at pH 5.5 and 7.4. The model predictions were a good fit to the measured data independent of the magnitude of the thermotolerance increase. This work might contribute to the increase in safety of those food products that require long heating lag phases during the pasteurization process.

Inactivation of Salmonella enterica serovar enteritidis by ultrasonic waves under pressure at different water activities

The inactivation of Salmonella enterica serovar Enteritidis by ultrasonic waves (20 kHz; 117- microm wavelength) under pressure (175 kPa) at nonlethal temperatures (manosonication [MS]) and lethal temperatures (manothermosonication [MTS]) in media of different water activities has been investigated. Heat decimal reduction time values increased 30 times when the water activity was decreased from nearly 1 to 0.96, but the MS resistance was increased only twofold. The inactivation of Salmonella serovar Enteritidis by ultrasound under pressure at low water activities was a phenomenon of the "all-or-nothing" type. A synergistic lethal effect was observed between heat and ultrasound in media with reduced water activity; the lower the water activity, the greater the synergistic effect. This work could be useful for improving sanitation and preservation treatments of foods, especially those which are sensitive to temperature and those in which components protect microorganisms to heat. It also contributes to our knowledge of microbial inactivation mechanisms by MS and MTS treatments.

Role of membrane fluidity in pressure resistance of Escherichia coli NCTC 8164

The relationship among growth temperature, membrane fatty acid composition, and pressure resistance was examined in Escherichia coli NCTC 8164. The pressure resistance of exponential-phase cells was maximal in cells grown at 10 degrees C and decreased with increasing growth temperatures up to 45 degrees C. By contrast, the pressure resistance of stationary-phase cells was lowest in cells grown at 10 degrees C and increased with increasing growth temperature, reaching a maximum at 30 to 37 degrees C before decreasing at 45 degrees C. The proportion of unsaturated fatty acids in the membrane lipids decreased with increasing growth temperature in both exponential- and stationary-phase cells and correlated closely with the melting point of the phospholipids extracted from whole cells examined by differential scanning calorimetry. Therefore, in exponential-phase cells, pressure resistance increased with greater membrane fluidity, whereas in stationary-phase cells, there was apparently no simple relationship between membrane fluidity and pressure resistance. When exponential-phase or stationary-phase cells were pressure treated at different temperatures, resistance in both cell types increased with increasing temperatures of pressurization (between 10 and 30 degrees C). Based on the above observations, we propose that membrane fluidity affects the pressure resistance of exponential- and stationary-phase cells in a similar way, but it is the dominant factor in exponential-phase cells whereas in stationary-phase cells, its effects are superimposed on a separate but larger effect of the physiological stationary-phase response that is itself temperature dependent.

Effects of caffeine, vitamin D, and other nutrients on quantitative phalangeal bone ultrasound in postmenopausal women

OBJECTIVE

We investigated the controversial effects of coffee and other nutrients on bone mass.

METHODS

In a study of 93 healthy postmenopausal women (mean +/- standard deviation: 57.3 +/- 7.1 y old and 8.9 +/- 7.5 y since menopause) selected on the basis of not having changed their eating habits since premenopause, not smoking, not exercising, not receiving hormone-replacement therapy, and having a weight in the range of 70% to 130% of their ideal weights, amplitude-dependent speed of sound (Ad-SOS) was determined by quantitative bone ultrasound, and a prospective 7-d diet survey evaluated the intake of caffeine and nutrients involved in calcium metabolism. Women were stratified according to their caffeine, calcium, and vitamin D intakes and ratios of calcium to phosphorus and to protein. Ad-SOS differed only with vitamin D intake and was greater in the group taking at leasst 400 IU/d (P < 0.0001).

RESULTS

In simple and multiple regression analyses, the only significant variable that affected Ad-SOS and nutrient intake was vitamin D (P < 0.0001). Phalangeal bone Ad-SOS was influenced only by the intake of vitamin D, not of caffeine or other nutrients.

CONCLUSIONS

This lack of effect of caffeine and protein may be related to good nutritional intake or the low levels of caffeine consumed.

Ultrasound bone mass in paraplegic patients

STUDY DESIGN

A cross-sectional study.

OBJECTIVE

To clarify the existing controversy with regard to whether paraplegic patients suffer a loss of bone mass in the upper limbs.

SETTING

Madrid, Spain.

METHODS

We evaluated bone mass by phalangeal ultrasonography in 35 male patients with paraplegia (mean age 49+/-12 years), and 25(OH)D3 and PTH to exclude the presence of osteomalacia and secondary hyperparathyroidism. Spasticity was evaluated according to the Ashworth scale. Patients were compared with a control group of 35 healthy male subjects (mean age 48+/-13 years).

RESULTS

The patients had lower 25(OH)D3 levels and amplitude-dependent speed of sound (Ad-SOS) than controls (both P<0.001), and higher PTH levels (P<0.05). There was a statistically significant negative association between PTH and 25(OH)D3 levels (r=-0.52, P<0.0001, CI -0.73 to -0.22) and between 25(OH)D3 and injury duration (r=0.34, P<0.05, CI -0.60 to -0.01). There was no correlation between Ad-SOS values, levels of PTH or 25(OH)D3, and the injury duration. No significant difference in Ad-SOS values was found in patients grouped according to low-to-normal 25(OH)D3 level or according to normal-to-high PTH level. There were no differences in relation to muscle tone. Only alkaline phosphatase and tartrate-resistant acid phosphatase levels were higher in patients than in controls (both P<0.001).

CONCLUSION

Paraplegic patients had a loss of phalangeal bone mass that was unrelated to the levels of vitamin D or PTH, or to muscle tone, so it seems to be related to increased bone resorption rather than to deficient bone formation.

Low molecular weight milk whey components protect Salmonella senftenberg 775W against heat by a mechanism involving divalent cations

AIMS

To investigate which components of milk increase the heat resistance of Salmonella senftenberg 775W, and to explore the mechanisms that could be involved in this protective effect.

METHODS AND RESULTS

The heat resistance of Salm. senftenberg was determined in a specially designed resistometer in several heating media. The molecules responsible for the thermal protective effect of milk were in the protein fraction, even in the < 3000 Da ultrafiltrate. The protective effect was lost when whey was demineralized. The former protective effect was restored when calcium or magnesium was added. Milk components protected cell envelopes of Salm. senftenberg from heat damage.

CONCLUSIONS

The protein fraction and divalent cations were responsible for the protective effect of milk. The whole protective effect on Salm. senftenberg was not the result of the addition of the protective effect of each component, but the result of a synergistic effect of some of them interacting.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work could be useful for improving food preservation and hygiene treatments. It also contributes to our knowledge of microbial physiology.

Quantitative phalangeal bone ultrasound is normal after long-term gluten-free diet in young coeliac patients

BACKGROUND AND AIMS

To determine bone mass using quantitative phalangeal bone ultrasound in young coeliac patients after a long-term gluten-free diet.

SUBJECTS

A total of 99 patients with coeliac disease (CD) (52 girls, mean age 15.5 +/- 9.7 years; 47 boys, mean age 14.4 +/- 8.4 years) and 44 healthy subjects matched for age, weight and height (26 girls, mean age 18.4 +/- 9.9 years; 18 boys, mean age 16.3 +/- 11.2 years) were included in the study.

METHODS

Bone status was assessed using an ultrasound device that measures amplitude-dependent speed of sound (Ad-SoS) in metres per second. Measurements were made on the phalanges (II-V) of the non-dominant hand, and an average value was computed. Nutrient intake was assessed using a 7-day record.

RESULTS

Analysis of variance (ANOVA) and the unpaired t-test showed that the intake of phosphates (P < 0.05), proteins, fat and carbohydrates (P < 0.005 in all) was higher in patients than in controls. No significant difference in Ad-SoS was found between patients and controls (95% CI -10.9 to 60.6) in the overall group, in girls or boys, or in patients following a gluten-free diet strictly or partially. In the patients, only age (beta 4.01, 95% CI 1.84-6.16, P < 0.0005) and weight (beta 2.62, 95% CI 1.53-3.70, P < 0.0001) showed a significant positive relation with Ad-SoS.

CONCLUSIONS

In patients with CD, gluten-free diet and increased nutritional intake were accompanied by normal bone mass values as determined by ultrasound on phalanges.

Effect of sodium chloride concentration on the heat resistance and recovery of Salmonella typhimurium

The survival of Salmonella typhimurium (ATCC 13311) heated and recovered in media with 0, 1, 2, 3, 4 or 5% (w/w) added sodium chloride was investigated. A protective effect in the heating medium and an inhibitory effect in the recovery medium were observed. The results showed an interaction between the effect on, D(58 degrees C) values, of sodium chloride concentration in both media. Lower concentration in the heating media led to a greater effect of the sodium chloride concentration in the recovery media. When the sodium chloride concentration was the same in both media, the protective effect exerted in the heating media dominated over its inhibitory effect in the recovery media.

The mnn2 mutant of Saccharomyces cerevisiae is affected in phosphorylation of N-linked oligosaccharides

We studied the phosphorylation of the inner core region of N-linked oligosaccharides in the mannan defective mutant Saccharomyces cerevisiae mnn2 which was described as unable to synthesize branches on the outer chain. We performed structural studies of the N-oligosaccharides synthesized by the strains mnn2, mnn1mnn2mnn9 and mnn1mnn9ldb8, and the results are compared with previously published structural data of mnn1mnn2mnn10 and mnn1mnn9 [Hernández, L.M., Ballou, L., Alvarado, E., Tsai, P.-K. and Ballou, C.E. (1989) J. Biol. Chem. 264, 13648-13659]. We conclude that the mnn2/ldb8 mutation is responsible for the inhibition of incorporation of phosphate to mannose A(3) (see below), a particular phosphorylation site of the inner core, while phosphorylation at the other possible site (mannose C(1)) is allowed, although it is also reduced. *Phosphorylation sites in mnn1mnn9. (see structure below)

Inactivation of Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Senftenberg by ultrasonic waves under pressure

The resistance of Salmonella Enteritidis (ATCC 13076), Salmonella Typhimurium (ATCC 13311), and Salmonella Senftenberg 775W (ATCC 43845) to ultrasonic waves under pressure treatments, at sublethal (manosonication) and lethal temperatures (manothermosonication) in citrate-phosphate buffer and in liquid whole egg was investigated. The influence of treatment parameters on the inactivation rate of manosonication was also studied. Decimal reduction times (Dt) of Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Senftenberg 775W corresponding to a heat treatment at 60 degrees C in pH 7 buffer and in liquid whole egg were 0.068, 0.12, and 1.0 min for buffer, and 0.12, 0.20, and 5.5 min for liquid whole egg. Those corresponding to a manosonication treatment (117 microns, 200 kPa, 40 degrees C) in both media were 0.73, 0.78, and 0.84 min, and 0.76, 0.84, and 1.4 min, respectively. When the amplitude of ultrasonic waves was increased linearly, the inactivation rate of manosonication increased exponentially. The inactivation rate also increased when pressure was raised. However, the magnitude of this increase was progressively smaller at higher pressures. The magnitude of the influence of the amplitude of ultrasonic waves and static pressure on the inactivation rate of manosonication was the same in the three serotypes investigated. Whereas a heat treatment at 60 degrees C only attained a 1/2-log cycle reduction in the number of Salmonella Senftenberg 775W survivors, a manothermosonication treatment (117 microns and 200 kPa) at this temperature attained a 3-log cycle reduction.

Heat resistance of Yersinia enterocolitica grown at different temperatures and heated in different media

In the range of 4-20 degrees C, growth temperature did not influence the heat resistance at 54-66 degrees C for Yersinia enterocolitica at pH 7 in citrate phosphate buffer. However, when cells were grown at 37 degrees C. the D62 increased from 0.044 to 0.17 min. This increase was constant at all heating temperatures tested (z = 5.7-5.8). Growth temperature did not influence the proportion of heat-damaged cells after a heat treatment, as measured by their response to a 2% of sodium chloride added to the recovery medium. The sensitivity of heat treated cells to nisin or lysozyme depended on growth temperature: Whereas the number of cells grown at 4 degrees C surviving heat treatment was the same regardless of the presence of 100 IU/ml of nisin or 100 microg/ml of lysozyme in the recovery medium, that of cells grown at 37 degrees C was, in these media, lower. The pH of maximum heat resistance in citrate phosphate buffer was pH 7 for cells grown at 37 degrees C, but pH 5 for those grown at 4 degrees C. In both suspensions the magnitude of the effect of pH on heat resistance was constant at all heating temperatures. For cells grown at 4 degrees C the heat resistance at 54-66 degrees C, in skimmed milk or pH 7 buffer, was the same. For cells grown at 37 degrees C this also applied for heat treatment at 66 degrees C but at 56 degrees C the heat resistance in skimmed milk was higher.

Bacterial resistance to ultrasonic waves under pressure at nonlethal (manosonication) and lethal (manothermosonication) temperatures

The decimal reduction times of Streptococcus faecium, Listeria monocytogenes, Salmonella enteritidis, and Aeromonas hydrophila corresponding to heat treatment at 62 degrees C were 7.1, 0.34, 0.024, and 0.0096 min, and those corresponding to manosonication treatment (40 degrees C, 200 kPa, 117 microm) were 4.0, 1.5, 0.86, and 0.90 min, respectively. The manosonication decimal reduction times of the four species investigated decreased sixfold when the amplitude was increased from 62 to 150 microm and fivefold when the relative pressure was raised from 0 to 400 kPa. In L. monocytogenes, S. enteritidis, and A. hydrophila, the lethal effect of manothermosonication was the result of the addition of the lethal effects of heat and manosonication, whereas in S. faecium it was a synergistic effect.

Resistance of heat-shocked cells of Listeria monocytogenes to mano-sonication and mano-thermo-sonication

Heat shocks did not increase the resistance of Listeria monocytogenes to an ultrasonication treatment under pressure (Mano-Sonication; MS). While heat-shocked cells (180 min, 45 degrees C) became sixfold more heat resistant than native cells (D62 = 1.8 min vs D62 = 0.24 min), the resistance of native and heat-shocked cells to MS (200 kPa, 117 microns) was the same (DMS = 1.6 min). The inactivation rate of non-heat-shocked cells of L monocytogenes by a combined heat/ultrasonication treatment under pressure (Mano-Thermo-Sonication; MTS) was an additive effect. On the contrary, on heat-shocked cells, the inactivation rate of MTS was greater than that of heat added to the inactivation rate of MS (synergistic effect) in the range 62-68 degrees C.

Influence of different factors on the output power transferred into medium by ultrasound

The influence of several factors (amplitude of ultrasonic waves, external static pressure, temperature and viscosity of medium) acting, either individually or in combination, on the amount of power transferred to a liquid medium during ultrasonication (power output) was measured by calorimetry. At constant amplitude (150 microns) and pressure (200 kPa), the power output decreased as the temperature was raised. The effect of temperature could be compensated by increasing pressure. The magnitude of the increase in power output due to raising the pressure depended on the pressure range and the treatment temperature. At all temperatures and pressures studied, the power output increased exponentially when the amplitude was increased linearly. The magnitude of this power output did not depend on the temperature or pressure of treatment. At 40 degrees C the magnitude of the increase in power output due to increasing the pressure was not influenced by the amplitude of sonic waves. The power output increased as the viscosity of the medium was increased. The magnitude of this effect did not depend on the amplitude but on the static pressure.

Proteolytic processing of a secreted glycoprotein and O-glycosylation of mannoproteins are affected in the N-glycosylation mutant Saccharomyces cerevisiae ldb1

In a previous work [P.I. Mañas, I. Olivero, M. Avalos, L.M. Hernández, Glycobiology, 7 (1997) 487-497], we described the isolation and characterization of the Saccharomyces cerevisiae ldb1 mutant which is affected in several steps of the N-glycosylation of mannoproteins probably due to a malfunction of the Golgi apparatus. Here, we found that two further functions assigned to the Golgi cisternae are also affected in the mutant: proteolytic processing of a secreted protein and O-glycosylation. We found that around 70% of the exoglucanase activity that is secreted into the culture medium by ldb1 bears an extra tetrapeptide in its NH2-terminus due to incomplete proteolytic processing. The O-linked oligosaccharides from ldb1 mnn1 were indistinguishable from those synthesized by the parental strain mnn1. However, when the O-oligosaccharides from the wild type and ldb1 were compared, we found a significant decrease in the tetrasaccharide in the latter, as well as a concomitant increase in the disaccharide, suggesting a defect in the Kre2p/Mnt1p involved in the transfer of the third mannose of these residues.

Isolation of new nonconditional Saccharomyces cerevisiae mutants defective in asparagine-linked glycosylation

We describe the isolation and partial characterization of Saccharomyces cerevisiae nonconditional mutants that show defects in N-glycosylation of proteins. The selection method is based on the reduction of affinity for the ion exchanger QAE-Sephadex as a consequence of the decrease in the negative charge of the cell surface. This characteristic reflects a decrease in the incorporation of mannosylphosphate units into the N-linked oligosaccharides of the mannoproteins. The mutants exhibit low affinity for the basic dye alcian blue and for that reason we have called them Idb (low dye binding) mutants. Eight of the complementation groups seem to be new as shown by complementation studies with previously isolated mutants of similar phenotype. Four of the groups showed a significant reduction in the number and/or size of the N-linked oligosaccharides attached to secreted invertase. We have analyzed the N-linked oligosaccharides of Idb1 and Idb2, the mutants that show the most drastic reduction in the affinity for the alcian blue dye. In both cases, the purified endo H-released oligosaccharides from the mannoproteins lacked detectable amounts of phosphate groups as shown by ion exchange chromatography and the 1H NMR spectra. In addition, Ibd1 synthesizes a truncated and unbranched outer chain lacking any alpha (1,2) linked mannoses attached to the alpha (1,6) linear backbone.