Effects of combined exposure of micrococcus luteus to nisin and pulsed electric fields

Retention of nisin activity at elevated pH in an organic acid complex and gold nanoparticle composite

Biocompatible organic acids and citrate-stabilized gold nanoparticles were interacted with nisin to generate robust antimicrobial agents, which display archetypical nisin activity even at elevated pH.

Increased inactivation of exopolysaccharide-producing Pediococcus parvulus in apple juice by combined treatment with enterocin AS-48 and high-intensity pulsed electric field

The cyclic peptide bacteriocin enterocin AS-48 was tested (at final concentrations of 0.175, 0.613, and 1.05 AU/ml) against the exopolysaccharide-producing cider spoilage strain Pediococcus parvulus 48 in apple juice in combination with high-intensity pulsed electric field (HIPEF) treatment (35 kV/cm and 150 Hz for 4 mus and bipolar mode). The effect of the combined treatments was studied by surface response methodology, with AS-48 concentration and HIPEF treatment time as process variables. A bacteriocin concentration of 0.613 AU/ml in combination with HIPEF treatment time of 1,000 micros reduced the population of pediococci by 6.6 log cycles in apple juice and yielded an apple juice that was free from pediococci during a 30-day storage period at 4 and 22 degrees Celsius. In contrast, application of HIPEF treatment alone had no effect on the surviving pediococci during storage of juice at 22 degrees Celsius. The combined treatment significantly improved the stability of the juice against spoilage by exopolysaccharide-producing P. parvulus.

Effect of enterocin AS-48 in combination with high-intensity pulsed-electric field treatment against the spoilage bacterium Lactobacillus diolivorans in apple juice

Enterocin AS-48 was tested in apple juice against the cider-spoilage, exopolysaccharide-producing strain Lactobacillus diolivorans 29 in combination with high-intensity pulsed-electric field (HIPEF) treatment (35 kV/cm, 150 Hz, 4 micros and bipolar mode). A response surface methodology was applied to study the bactericidal effects of the combined treatment, with AS-48 concentration and HIPEF treatment time as process variables. At subinhibitory bacteriocin concentrations, microbial inactivation by the combined treatment increased as the bacteriocin concentration and the HIPEF treatment time increased (from 0.5 to 2.0 microg/ml and from 100 to 1000 micros, respectively). Highest inactivation (4.87 logs) was achieved by 1000 micros HIPEF treatment in combination with 2.0 microg/ml AS-48. While application of treatments separately did not protect juice from survivors during storage, survivors to the combined treatment were inactivated within the following 24 h of storage, and the treated samples remained free from detectable lactobacilli for at least 15 days at temperatures of 4 degrees C as well as 22 degrees C. The combined treatment could be useful for inactivation of exopolysaccharide-producing L. diolivorans in apple juice.

Enhanced bactericidal effect of enterocin AS-48 in combination with high-intensity pulsed-electric field treatment against Salmonella enterica in apple juice

The effect of the broad spectrum cyclic antimicrobial peptide enterocin AS-48 combination with high-intensity pulsed-electric field (HIPEF) treatment (35 kV/cm, 150 Hz, 4 micros and bipolar mode) was tested on Salmonella enterica CECT 915 in apple juice. A response surface methodology was applied to study the bactericidal effects of the combined treatment. The process variables were AS-48 concentration, temperature, and HIPEF treatment time. While treatment with enterocin AS-48 alone up to 60 microg/ml had no effect on the viability of S. enterica in apple juice, an increased bactericidal activity was observed in combination with HIPEF treatments. Survival fraction was affected by treatment time, enterocin AS48 concentration and treatment temperature. The combination of 100 micros of HIPEF treatment, 30 microg/ml of AS-48, and temperature of 20 degrees C resulted in the lowest inactivation, with only a 1.2-log reduction. The maximum inactivation of 4.5-log cycles was achieved with HIPEF treatment for 1000 micros in combination with 60 microg/ml of AS-48 and a treatment temperature of 40 degrees C. Synergism between enterocin AS-48 and HIPEF treatment depended on the sequence order application, since it was observed only when HIPEF was applied in the presence of previously-added bacteriocin. The combined treatment could improve the safety of freshly-made apple juice against S. enterica transmission.

Bacteriocin-based strategies for food biopreservation

Bacteriocins are ribosomally-synthesized peptides or proteins with antimicrobial activity, produced by different groups of bacteria. Many lactic acid bacteria (LAB) produce bacteriocins with rather broad spectra of inhibition. Several LAB bacteriocins offer potential applications in food preservation, and the use of bacteriocins in the food industry can help to reduce the addition of chemical preservatives as well as the intensity of heat treatments, resulting in foods which are more naturally preserved and richer in organoleptic and nutritional properties. This can be an alternative to satisfy the increasing consumers demands for safe, fresh-tasting, ready-to-eat, minimally-processed foods and also to develop "novel" food products (e.g. less acidic, or with a lower salt content). In addition to the available commercial preparations of nisin and pediocin PA-1/AcH, other bacteriocins (like for example lacticin 3147, enterocin AS-48 or variacin) also offer promising perspectives. Broad-spectrum bacteriocins present potential wider uses, while narrow-spectrum bacteriocins can be used more specifically to selectively inhibit certain high-risk bacteria in foods like Listeria monocytogenes without affecting harmless microbiota. Bacteriocins can be added to foods in the form of concentrated preparations as food preservatives, shelf-life extenders, additives or ingredients, or they can be produced in situ by bacteriocinogenic starters, adjunct or protective cultures. Immobilized bacteriocins can also find application for development of bioactive food packaging. In recent years, application of bacteriocins as part of hurdle technology has gained great attention. Several bacteriocins show additive or synergistic effects when used in combination with other antimicrobial agents, including chemical preservatives, natural phenolic compounds, as well as other antimicrobial proteins. This, as well as the combined use of different bacteriocins may also be an attractive approach to avoid development of resistant strains. The combination of bacteriocins and physical treatments like high pressure processing or pulsed electric fields also offer good opportunities for more effective preservation of foods, providing an additional barrier to more refractile forms like bacterial endospores as well. The effectiveness of bacteriocins is often dictated by environmental factors like pH, temperature, food composition and structure, as well as the food microbiota. Foods must be considered as complex ecosystems in which microbial interactions may have a great influence on the microbial balance and proliferation of beneficial or harmful bacteria. Recent developments in molecular microbial ecology can help to better understand the global effects of bacteriocins in food ecosystems, and the study of bacterial genomes may reveal new sources of bacteriocins.

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.

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.

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.

Enhancing inactivation of Staphylococcus aureus in skim milk by combining high-intensity pulsed electric fields and nisin

High-intensity pulsed electric fields (HIPEF) can be used as a nonthermal preservation method that is believed to enhance the effect of nisin on microorganisms such as Staphylococcus aureus. The survival of S. aureus inoculated into skim milk and treated with nisin, with HIPEF, or with a combination of nisin-HIPEF was evaluated. Nisin dose, milk pH, and HIPEF treatment time were the controlled variables that were set up at 20 to 150 ppm, pH 5.0 to 6.8, and 240 to 2,400 micros, respectively. HIPEF strength and pulse width were kept constant at 35 kV/cm and 4 micros, respectively. No reduction in S. aureus concentration was observed in skim milk at its natural pH after treatment with nisin, but 1.1 log units were recovered after 90 min of treatment at pH 5.0 with 150 ppm nisin. A reduction in viable S. aureus counts of 0.3 and 1.0 log unit in skim milk treated with HIPEF at its natural pH was observed at 240 and 2,400 micros, respectively. The nisin-HIPEF treatment design was based on a response surface methodology. The combined effect of nisin and HIPEF was clearly synergistic. However, synergism depended on pH. A maximum microbial inactivation of 6.0 log units was observed at pH 6.8, 20 ppm nisin, and 2,400 micros of HIPEF treatment time, whereas a reduction of over 4.5 log units was achieved when pH, nisin concentration, and HIPEF treatment times were set at 5.0, 150 ppm, and 240 micros, respectively.

Mode of action of lactocin 160, a bacteriocin from vaginal Lactobacillus rhamnosus

OBJECTIVES: To determine the mechanism of antimicrobial action of lactocin 160, a bacteriocin produced by the healthy vaginal strain of Lactobacillus rhamnosus, using an established model, with Micrococcus luteus ATCC 10420 as a test organism. METHODS: Sensitivity of M. luteus to lactocin 160 was determined by the diffusion assay. Loss of cellular ATP in the lactocin-treated cells was elucidated using a commercially available ATP determination kit (luciferin-luciferase bioluminescence assay). Luminescence intensity as a reflection of ATP quantity was determined using a luminometer. Dissipation of membrane potential (Deltapsi) was studied using fluorophore DiSC3(5) with the fluorescence spectrum sensitive to changes in Deltapsi. RESULTS: Lactocin 160 inhibited growth of M. luteus ATCC 10420 at a concentration of 5 microg/ml. There were no significant changes in the intracellular ATP level of M. luteus upon the addition of 20 microg/ml of lactocin 160. However, the extracellular ATP level increased significantly. This means that the treatment of cells with lactocin 160 resulted in an efflux of ATP from inside the cells. Therefore, a partially purified lactocin 160 preparation (16 microg /ml of the bacteriocin in the sample) killed sensitive cells and dissipated 3.12 +/- 0.36% of Deltapsi. CONCLUSION: Lactocin 160 has a mode of action typical for bacteriocins. It disturbs the cellular membrane (Deltapsi dissipation) and induces ATP efflux, most likely because of the pore formation, which is a common mechanism of action for many bacteriocins.

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.

Pulsed electric fields cause sublethal injury in Escherichia coli

AIMS

The objective was to investigate the occurrence of sublethal injury in Escherichia coli by pulsed electric fields (PEF) at different pH values.

METHODS AND RESULTS

The occurrence of sublethal injury in PEF-treated E. coli cells depended on the pH of the treatment medium. Whereas a slight sublethal injury was detected at pH 7, 99.95% of survivors were injured when cells were treated at pH 4 for 400 micros at 19 kV. The PEF-injured cells were progressively inactivated by a subsequent holding at pH 4.

CONCLUSIONS

PEF cause sublethal injury in E. coli. The measurement of sublethal injury using a selective medium plating technique allowed prediction of the number of cells that would be inactivated by subsequent storage in acidic conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work could be useful for improving food preservation by PEF technology and contributes to the knowledge of the mechanism of microbial inactivation by PEF.

Comparison of sublethal injury induced in Salmonella enterica serovar Typhimurium by heat and by different nonthermal treatments

We have studied sublethal injury in Salmonella enterica serovar Typhimurium caused by mild heat and by different emerging nonthermal food preservation treatments, i.e., high-pressure homogenization, high hydrostatic pressure, pulsed white light, and pulsed electric field. Sublethal injury was determined by plating on different selective media, i.e., tryptic soy agar (TSA) plus 3% NaCl, TSA adjusted to pH 5.5, and violet red bile glucose agar. For each inactivation technique, at least five treatments using different doses were applied in order to cover an inactivation range of 0 to 5 log units. For all of the treatments performed with a technique, the logarithm of the viability reductions measured on each of the selective plating media was plotted against the logarithm of the viability reduction on TSA as a nonselective medium, and these points were fined by a straight line. Sublethal injury between different techniques was then compared by the slope and the y intercept of these regression lines. The highest levels of sublethal injury were observed for the heat and high hydrostatic pressure treatments. Sublethal injury after those treatments was observed on all selective plating media. For the heat treatment, but not for the high-pressure treatment, sublethal injury occurred at low doses, which were not yet lethal. The other nonthermal techniques resulted in sublethal injury on only some of the selective plating media, and the levels of injury were much lower. The different manifestations of sublethal injury were attributed to different inactivation mechanisms by each of the techniques, and a mechanistic model is proposed to explain these differences.