Inactivation of Escherichia coli and Listeria innocua in milk by combined treatment with high hydrostatic pressure and the lactoperoxidase system

High pressure processing plus technologies: Enhancing the inactivation of vegetative microorganisms

High pressure processing (HPP) is a non-thermal technology that can ensure microbial safety without compromising food quality. However, the presence of pressure-resistant sub-populations, the revival of sub-lethally injured (SLI) cells, and the resuscitation of viable but non-culturable (VBNC) cells pose challenges for its further development. The combination of HPP with other methods such as moderate temperatures, low pH, and natural antimicrobials (e.g., bacteriocins, lactate, reuterin, endolysin, lactoferrin, lactoperoxidase system, chitosan, essential oils) or other non-thermal processes (e.g., CO2, UV-TiO2 photocatalysis, ultrasound, pulsed electric fields, ultrafiltration) offers feasible alternatives to enhance microbial inactivation, termed as "HPP plus" technologies. These combinations can effectively eliminate pressure-resistant sub-populations, reduce SLI or VBNC cell populations, and inhibit their revival or resuscitation. This review provides an updated overview of microbial inactivation by "HPP plus" technologies and elucidates possible inactivation mechanisms.

Susceptibility of Campylobacter jejuni to Stressors in Agrifood Systems and Induction of a Viable-but-Nonculturable State

Many bacteria can become viable but nonculturable (VBNC) in response to stressors commonly identified in agrifood systems. Campylobacter is able to enter the VBNC state to evade unfavorable environmental conditions, but how food processing can induce Campylobacter jejuni to enter this state and the potential role of foods in inducing the VBNC state in C. jejuni remains largely unknown. In this study, the culturability and viability of C. jejuni cells were investigated under chlorine treatment (25 ppm), aerobic stress (atmospheric condition), and low-temperature (4°C) conditions that mimicked food processing. In addition, the behaviors of C. jejuni cells in ultrahigh-temperature (UHT) and pasteurized milk were also monitored during refrigerated storage. The numbers of viable and culturable C. jejuni cells in both the pure bacterial culture and food matrices were separately determined by propidium monoazide (PMA)-quantitative PCR (qPCR) and plating assay. The C. jejuni cells lost their culturability but partially retained their viability (1% to 10%) once mixed with chlorine. In comparison, ~10% of C. jejuni cells were induced to enter the VBNC state after 24 h and 20 days under aerobic and low-temperature conditions, respectively. The viability of the C. jejuni cells remained stable during the induction process in UHT (>10%) and pasteurized (>10%) milk. The number of culturable C. jejuni cells decreased quickly in pasteurized milk, but culturable cells could still be detected in the end (day 21). In contrast, the number of culturable C. jejuni cells slowly decreased, and they became undetectable after >42 days in UHT milk. The C. jejuni cells responded differently to various stress conditions and survived in high numbers in the VBNC state in agrifood systems. IMPORTANCE The VBNC state of pathogens can pose risks to food safety and public health because the pathogens cannot be detected using conventional microbiological culture-based methods but can resuscitate under favorable conditions to develop virulence. As a leading cause of human gastroenteritis worldwide, C. jejuni can enter the VBNC state to survive in the environment and food-processing chain with high prevalence. In this study, the effect of food-processing conditions and food products on the development of VBNC state in C. jejuni was investigated, providing a better understanding of the interaction between C. jejuni and the agroecosystem. The knowledge elicited from this study can aid in developing novel intervention strategies to reduce the food safety risks associated with this microbe.

Hypothiocyanite and host-microbe interactions

The pseudohypohalous acid hypothiocyanite/hypothiocyanous acid (OSCN^- /HOSCN) has been known to play an antimicrobial role in mammalian immunity for decades. It is a potent oxidant that kills bacteria but is non-toxic to human cells. Produced from thiocyanate (SCN^- ) and hydrogen peroxide (H₂ O₂ ) in a variety of body sites by peroxidase enzymes, HOSCN has been explored as an agent of food preservation, pathogen killing, and even improved toothpaste. However, despite the well-recognized antibacterial role HOSCN plays in host-pathogen interactions, little is known about how bacteria sense and respond to this oxidant. In this work, we will summarize what is known and unknown about HOSCN in innate immunity and recent advances in understanding the responses that both pathogenic and non-pathogenic bacteria mount against this antimicrobial agent, highlighting studies done with three model organisms, Escherichia coli, Streptococcus spp., and Pseudomonas aeruginosa.

Nanoformulation approach for improved antimicrobial activity of bovine lactoperoxidase

Lactoperoxidase (LPO) is a glycosylated antimicrobial protein present in milk with a molecular mass of 78 kDa. LPO is included in many biological processes and is well-known to have biocidal actions, acting as an active antibiotic and antiviral agent. The wide spectrum biocidal activity of LPO is mediated via a definite inhibitory system named lactoperoxidase system which plays a potent role in the innate immune response. With the current advancement in nanotechnology, nanoformulations can be developed for stabilizing and potentiating the activity of LPO for several applications. In the research described in this Research Communication, fresh LPO purified from bovine mammary gland secretions was used for nanoparticle synthesis using a simple thermal process at different pH and temperatures. The round-shaped nanoparticles (average size 229 nm) were successfully synthesized at pH 7.0 and a temperature of 75°C. These nanoparticles were tested against four different bacterial species namely S. flexineri, P. aeruginosa, S. aureus, and E. coli. The prepared nanoparticles exhibited strong inhibition of the growth against all four bacterial species as stated by their MIC and ZOI values. These results may help in increasing the efficiency of lactoperoxidase system and will assist in identifying novel avenues to enhance the stability and antimicrobial function of LPO in drug discovery and industrial processes.

The efficacy and safety of high-pressure processing of food

High-pressure processing (HPP) is a non-thermal treatment in which, for microbial inactivation, foods are subjected to isostatic pressures (P) of 400-600 MPa with common holding times (t) from 1.5 to 6 min. The main factors that influence the efficacy (log10 reduction of vegetative microorganisms) of HPP when applied to foodstuffs are intrinsic (e.g. water activity and pH), extrinsic (P and t) and microorganism-related (type, taxonomic unit, strain and physiological state). It was concluded that HPP of food will not present any additional microbial or chemical food safety concerns when compared to other routinely applied treatments (e.g. pasteurisation). Pathogen reductions in milk/colostrum caused by the current HPP conditions applied by the industry are lower than those achieved by the legal requirements for thermal pasteurisation. However, HPP minimum requirements (P/t combinations) could be identified to achieve specific log10 reductions of relevant hazards based on performance criteria (PC) proposed by international standard agencies (5-8 log10 reductions). The most stringent HPP conditions used industrially (600 MPa, 6 min) would achieve the above-mentioned PC, except for Staphylococcus aureus. Alkaline phosphatase (ALP), the endogenous milk enzyme that is widely used to verify adequate thermal pasteurisation of cows' milk, is relatively pressure resistant and its use would be limited to that of an overprocessing indicator. Current data are not robust enough to support the proposal of an appropriate indicator to verify the efficacy of HPP under the current HPP conditions applied by the industry. Minimum HPP requirements to reduce Listeria monocytogenes levels by specific log10 reductions could be identified when HPP is applied to ready-to-eat (RTE) cooked meat products, but not for other types of RTE foods. These identified minimum requirements would result in the inactivation of other relevant pathogens (Salmonella and Escherichia coli) in these RTE foods to a similar or higher extent.

High pressure processing combined with selected hurdles: Enhancement in the inactivation of vegetative microorganisms

High pressure processing (HPP) as a nonthermal processing (NTP) technology can ensure microbial safety to some extent without compromising food quality. However, for vegetative microorganisms, the existence of pressure-resistant subpopulations, the revival of sublethal injury (SLI) state cells, and the resuscitation of viable but nonculturable (VBNC) state cells may constitute potential food safety risks and pose challenges for the further development of HPP application. HPP combined with selected hurdles, such as moderately elevated or low temperature, low pH, natural antimicrobials (bacteriocin, lactate, reuterin, endolysin, lactoferrin, lactoperoxidase system, chitosan, essential oils), or other NTP (CO₂ , UV-TiO₂ photocatalysis, ultrasound, pulsed electric field, ultrafiltration), have been highlighted as feasible alternatives to enhance microbial inactivation (synergistic or additive effect). These combinations can effectively eliminate the pressure-resistant subpopulation, reduce the population of SLI or VBNC state cells and inhibit their revival or resuscitation. This review provides an updated overview of the microbial inactivation by the combination of HPP and selected hurdles and restructures the possible inactivation mechanisms.

Nanostructured materials as a host matrix to develop robust peroxidases-based nanobiocatalytic systems

Nanostructured materials constitute an interesting and novel class of support matrices for the immobilization of peroxidase enzymes. Owing to the high surface area, robust mechanical stability, outstanding optical, thermal, and electrical properties, nanomaterials have been rightly perceived as immobilization matrices for enzyme immobilization with applications in diverse areas such as nano-biocatalysis, biosensing, drug delivery, antimicrobial activities, solar cells, and environmental protection. Many nano-scale materials have been employed as support matrices for the immobilization of different classes of enzymes. Nanobiocatalysts, enzymes immobilized on nano-size materials, are more stable, catalytically robust, and could be reused and recycled in multiple reaction cycles. In this review, we illustrate the unique structural/functional features and potentialities of nanomaterials-immobilized peroxidase enzymes in different biotechnological applications. After a comprehensive introduction to the immobilized enzymes and nanocarriers, the first section reviewed carbonaceous nanomaterials (carbon nanotube, graphene, and its derivatives) as a host matrix to constitute robust peroxidases-based nanobiocatalytic systems. The second half covers metallic nanomaterials (metals, and metal oxides) and some other novel materials as host carriers for peroxidases immobilization. The next section vetted the potential biotechnological applications of the resulted nanomaterials-immobilized robust peroxidases-based nanobiocatalytic systems. Concluding remarks, trends, and future recommendations for nanomaterial immobilized enzymes are also given.

Control of pathogenic and spoilage bacteria in meat and meat products by high pressure: Challenges and future perspectives

High-pressure processing is among the most widely used nonthermal intervention to reduce pathogenic and spoilage bacteria in meat and meat products. However, resistance of pathogenic bacteria strains in meats at the current maximum commercial equipment of 600 MPa questions the ability of inactivation by its application in meats. Pathogens including Escherichia coli, Listeria, and Salmonelle, and spoilage microbiota including lactic acid bacteria dominate in raw meat, ready-to-eat, and packaged meat products. Improved understanding on the mechanisms of the pressure resistance is needed for optimizing the conditions of pressure treatment to effectively decontaminate harmful bacteria. Effective control of the pressure-resistant pathogens and spoilage organisms in meats can be realized by the combination of high pressure with application of mild temperature and/or other hurdles including antimicrobial agents and/or competitive microbiota. This review summarized applications, mechanisms, and challenges of high pressure on meats from the perspective of microbiology, which are important for improving the understanding and optimizing the conditions of pressure treatment in the future.

Enhanced Antibacterial Activity of Lactoperoxidase-Thiocyanate-Hydrogen Peroxide System in Reduced-Lactose Milk Whey

The product of the lactoperoxidase system (LPOS) has been developed as a preservative agent to inhibit foodborne bacteria, but its action was, heretofore, limited to several original compounds in milk. This research was conducted to analyze the application of the lactoperoxidase system against Escherichia coli in fresh bovine milk and its derivative products to determine the strength of antibacterial activity. Lactoperoxidase was purified from bovine whey using the SP Sepharose Big Beads Column. The enzymatic reaction involving lactoperoxidase, thiocyanate, and hydrogen peroxide was used to generate the antibacterial agent from LPOS. This solution was then added to milk, skimmed milk, untreated whey, reduced-LPO whey, reduced-lactose whey, and high-lactose solution containing E. coli at an initial count of 6.0 log CFU/mL. LPOS showed the greatest reduction of bacteria (1.68 ± 0.1 log CFU/mL) in the reduced-lactose whey among the products tested. This result may lead to a method for enhancement of the antimicrobial activity of LPOS in milk and derived products.

Lactoperoxidase immobilization on silver nanoparticles enhances its antimicrobial activity

Lactoperoxidase (LPO) is an antimicrobial protein present in milk that plays an important role in natural defence mechanisms during neonatal and adult life. The antimicrobial activity of LPO has been commercially adapted for increasing the shelf life of dairy products. Immobilization of LPO on silver nanoparticles (AgNPs) is a promising way to enhance the antimicrobial activity of LPO. In the current study, LPO was immobilized on AgNPs to form LPO/AgNP conjugate. The immobilized LPO/AgNP conjugate was characterized by various biophysical techniques. The enhanced antibacterial activity of the conjugate was tested against E. coli in culture at 2 h intervals for 10 h. The results showed successful synthesis of spherical AgNPs. LPO was immobilized on AgNPs with agglomerate sizes averaging approximately 50 nm. The immobilized conjugate exhibited stronger antibacterial activity against E. coli in comparison to free LPO. This study may help in increasing the efficiency of lactoperoxidase system and will assist in identifying novel avenues to enhance the stability and antimicrobial function of LPO system in dairy and other industries.

Synergistic Effect of the Lactoperoxidase System and Cinnamon Essential Oil on Total Flora andSalmonellaGrowth Inhibition in Raw Milk

Despite its antibacterial and antipathogenic effects, the heat treatment of milk induces undesirable changes that can be noted in the overall properties of ultrahigh temperature (UHT) milk, such as changes in nutritional and organoleptic properties. Our goal is to find new nonthermal antibacterial technologies for the preservation of raw milk (RM). This study investigates the possible synergistic effect of using a combination of the lactoperoxidase system (LS) and 3 μg mL−1of cinnamon essential oil (cinnamon EO) to inactivate the total flora of milk andSalmonellaHadar (S. Hadar). The LS was activated with 30 mg L−1sodium percarbonate and 14 mg L−1of sodium thiocyanate. Using this approach, we obtained a synergistic effect with a complete inhibition of the activity of the total flora of the milk andS.Hadar after 12 hours at 25°C. In addition, the attainment of synergy was defined when the inhibitory effect of the two compounds together was greater than the effect observed by each compound added alone. Moreover, the monitoring of the synergistic effect at 4°C for 5 days showed complete inhibition of total flora for 3 days and forS. Hadar it was up to 5 days. To summarize, the current study clearly identified a new inhibitory combination that may be used in food-based applications.

Antimicrobial efficacy of sequentially applied eugenol against food spoilage micro-organisms

AIMS

Compare the survival behaviour of food spoilage micro-organisms treated with sequential doses or all at once treatments of eugenol.

METHODS AND RESULTS

Staphylococcus carnosus, Listeria innocua, Escherichia coli and Pseudomonas fluorescens were exposed to a minimal lethal concentration (MLC) initially, or to half doses over time, with first half dose applied immediately and a second half dose applied after 3, 4, 6 and 8 h, of eugenol. Direct plate counts were determined at regular time intervals. Population dynamics were analysed using a combined growth and mortality model. Effect of sequential dosing varied significantly between tested organisms. High antimicrobial efficacy on E. coli K12 was observed regardless of timing of the two doses. Reduced effectiveness was observed against Staph. carnosus and L. innocua the later the second half dose was applied. Complete cell reduction occurred after immediate exposure to full MLC dose, while sequential half doses were bacteriostatic regardless of application times.

CONCLUSION

Time in between antimicrobial dose application had substantial impact on effectiveness, attributed to organisms becoming more tolerant.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study contributes to the evaluation of encapsulated antimicrobial systems, where antimicrobials are released over time and antimicrobial concentrations may only reach MLC levels after some time.

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

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

Impact of high hydrostatic pressure processing on individual cellular resuscitation times and protein aggregates in Escherichia coli

Live cell biology approaches can contribute to a more comprehensive understanding of heterogeneous injury and resuscitation phenomena in stressed populations of foodborne pathogens and spoilage microorganisms, and in turn lead to better insights in the mechanisms and dynamics of inactivation that can improve food safety and preservation measures. Especially in the context of designing minimal processing strategies, which depend on a synergistic combination of different mild stresses to ensure sufficient microbial reduction, a more profound understanding of the impact of each such stress or hurdle is mandatory. High hydrostatic pressure (HHP) stress is an interesting hurdle in this concept since cells that manage to survive this stress nevertheless tend to be injured and sensitized to subsequent stresses. In this study, populations of Escherichia coli were subjected to different HHP intensities and studied at the single-cell level with time-lapse fluorescence microscopy while monitoring resuscitation times and protein aggregate integrity at the single-cell level. This approach revealed that higher pressure intensities lead to longer and more variable resuscitation times of surviving cells as well as an increased dispersal of intracellular protein aggregates. Interestingly, at mild HHP exposure, cells within the population incurring less dispersion of protein aggregates appeared to have a higher probability of survival.

Combined treatments of high-pressure with the lactoperoxidase system or lactoferrin on the inactivation of Listeria monocytogenes, Salmonella Enteritidis and Escherichia coli O157:H7 in beef carpaccio

The effect of high hydrostatic pressure (HHP) treatments in combination with the lactoperoxidase system (LPOS) or activated lactoferrin (ALF) on Listeria monocytogenes, Salmonella enterica subsp. enterica serovar Enteritidis and Escherichia coli O157:H7 was investigated in cured beef carpaccio stored at 8 °C or 22 °C during 7 d. HHP (450 MPa for 5 min) reduced pathogen levels by 1-3 log units and the antimicrobial effect remained during 7 d of storage under temperature abuse conditions at 8 °C and at 22 °C. The individual application of LPOS and ALF did not affect the survival of the three pathogens studied during storage. However, a synergistic bactericidal interaction between LPOS and HHP was observed against S. Enteritidis and E. coli O157:H7. Combined treatments of HHP with LPOS would be useful to reduce the intensity of pressurization treatments diminishing changes in the quality of meat products.

Lactoperoxidase: structural insights into the function,ligand binding and inhibition

Lactoperoxidase (LPO) is a member of a large group of mammalian heme peroxidases that include myeloperoxidase (MPO), eosinophil peroxidase (EPO) and thyroid peroxidase (TPO). The LPO is found in exocrine secretions including milk. It is responsible for the inactivation of a wide range of micro-organisms and hence, is an important component of defense mechanism in the body. With the help of hydrogen peroxide, it catalyzes the oxidation of halides, pseudohalides and organic aromatic molecules. Historically, LPO was isolated in 1943, nearly seventy years ago but its three-dimensional crystal structure has been elucidated only recently. This review provides various details of this protein from its discovery to understanding its structure, function and applications. In order to highlight species dependent variations in the structure and function of LPO, a detailed comparison of sequence, structure and function of LPO from various species have been made. The structural basis of ligand binding and distinctions in the modes of binding of substrates and inhibitors have been analyzed extensively.

New mild technologies in meat processing: high pressure as a model technology

As a consequence of market globalization, the production and manufacture of meat products is at a stage of innovative dynamics. Consumers demand high quality and convenient meat products, with natural flavour and taste, and very much appreciate the fresh appearance of minimally processed food. To harmonize or to blend all these demands without compromising safety, it is necessary to implement new preservation technologies in the meat industry and in the food industry in general. High hydrostatic pressure (HHP) represents an attractive non-thermal process for meat products to avoid post-processing contamination. When combined with antimicrobials, like bacteriocins, the death rate may be increased because of sublethal injuries to living cells. HPP is a powerful tool to control risks associated with Salmonella spp. and Listeria monocytogenes in raw or marinated meats. The HPP treatment could extend the shelf life of the marinated beef loin by controlling the growth of both spoilage and pathogenic bacteria. As a general conclusion it can be stated that from both a physico-chemical and microbiological point of view, cooked pork ham, dry cured pork ham and marinated beef loin, vacuum-packed and high pressure treated at 600 MPa for 10 min at 30 °C, are substantially equivalent to the same untreated products.

Pasteurization of fruit juices of different pH values by combined high hydrostatic pressure and carbon dioxide

The inactivation of the selected vegetative bacteria Escherichia coli, Listeria innocua, and Lactobacillus plantarum by high hydrostatic pressure (HHP) in physiological saline (PS) and in four fruit juices with pHs ranging from 3.4 to 6.3, with or without dissolved CO(2), was investigated. The inactivation effect of HHP on the bacteria was greatly enhanced by dissolved CO(2). Effective inactivation (>7 log) was achieved at 250 MPa for E. coli and 350 MPa for L. innocua and L. plantarum in the presence of 0.2 M CO(2) at room temperature for 15 min in PS, with additional inactivation of more than 4 log for all three bacteria species compared with the results with HHP treatment alone. The combined inactivation by HHP and CO(2) in tomato juice of pH 4.2 and carrot juice of pH 6.3 showed minor differences compared with that in PS. By comparison, the combined effect in orange juice of pH 3.8 was considerably promoted, while the HHP inactivation was enhanced only to a limited extent. In another orange juice with a pH of 3.4, all three strains lost their pressure resistance. HHP alone completely inactivated E. coli at relatively mild pressures of 200 MPa and L. innocua and L. plantarum at 300 MPa. Observations of the survival of the bacteria in treated juices also showed that the combined treatment caused more sublethal injury, which increased further inactivation at a relatively mild pH of 4.2 during storage. The results indicated that the combined treatment of HHP with dissolved CO(2) may provide an effective method for the preservation of low- or medium-acid fruit and vegetable juices at relatively low pressures. HHP alone inactivated bacteria effectively in high-acid fruit juice.

Combined effect of enterocin AS-48 and high hydrostatic pressure to control food-borne pathogens inoculated in low acid fermented sausages

The single and combined effects of enterocin AS-48 and high hydrostatic pressure (HHP) on Listeria monocytogenes, Salmonellaenterica, and Staphylococcus aureus was investigated in fuet (a low acid fermented sausage) during ripening and storage at 7 degrees C or at room temperature. AS-48 (148 AU g(-1)) caused a drastic 5.5 log cfu g(-1) decrease in L. monocytogenes (P<0.001) and a significant (P<0.01) inhibition (1.79 logs) for Salmonella at the end of ripening (10 d). After pressurization (400 MPa) and storage Listeria counts remained below 5 cfu g(-1) in all fuets containing AS-48 (pressurized or not). HHP alone had no anti-Listeria effect. HHP treatment significantly reduced Salmonella counts, with lowest levels in pressurized fuets with AS-48. S. aureus showed similar growth for all treatments and storage conditions. These results indicate that AS-48 can be applied alone to control L. monocytogenes and combined with HHP treatment to control Salmonella in fuets.

Effect of lysozyme or nisin on survival of some bacteria treated with high pressure at subzero temperature

The aim of this work was to examine the inactivation of some Gram-positive and Gram-negative bacteria exposed to the pressure of 193 MPa at -20 °C in the presence of lysozyme or nisin at concentration of 400 µg/ml. The highest effect of pressure at subzero temperature and lysozyme was found with pressure sensitive Pseudomonas fluorescens; viable cells of this strain were not detected in 1 ml of sample after combined treatment. The action of pressure at subzero temperature and lysozyme or nisin against Escherichia coli led to synergistic reduction by 0.7 or 1.6 log cycles, respectively, while it was practically insignificant for two Staphylococcus aureus strains. Viability loss of E. coli and S. aureus occurred during storage for 20 h of the samples at 37 and 5 °C, which were previously pressurized with lysozyme or nisin. The synergistic effect of pressure and nisin at pH 5 against E. coli cells just after the pressure treatment was lower than that at pH 7, however, the extent of the lethal effect after storage was higher.

Application of natural antimicrobials for food preservation

In this review, antimicrobials from a range of plant, animal, and microbial sources are reviewed along with their potential applications in food systems. Chemical and biochemical antimicrobial compounds derived from these natural sources and their activity against a range of pathogenic and spoilage microorganisms pertinent to food, together with their effects on food organoleptic properties, are outlined. Factors influencing the antimicrobial activity of such agents are discussed including extraction methods, molecular weight, and agent origin. These issues are considered in conjunction with the latest developments in the quantification of the minimum inhibitory (and noninhibitory) concentration of antimicrobials and/or their components. Natural antimicrobials can be used alone or in combination with other novel preservation technologies to facilitate the replacement of traditional approaches. Research priorities and future trends focusing on the impact of product formulation, intrinsic product parameters, and extrinsic storage parameters on the design of efficient food preservation systems are also presented.

Synergistic effect between different milk-derived peptides and proteins

Antimicrobial peptides derived from food proteins constitute a new field in the combined use of antimicrobial agents in food. The best examples of milk-derived peptides are those constituted by bovine lactoferricin [lactoferrin f(17-41)] (LFcin-B) and bovine alpha(s2)-casein f(183-207). The aim of this work was to study if the antimicrobial activity of a natural compound employed in food preservation, nisin, could be enhanced by combination with the aforementioned milk-derived peptides. Furthermore, the possibility of a synergistic effect between these peptides and bovine lactoferrin (LF) against Escherichia coli and Staphylococcus epidermidis was also studied. Finally, the most active combinations were assayed against the foodborne pathogens Listeria monocytogenes and Salmonella choleraesuis. Results showed a synergistic effect when LFcin-B was combined with bovine LF against E. coli. In the same way, the combination of LFcin-B with bovine LF was synergistic against Staph. epidermidis. Bovine LF and nisin increased their antimicrobial activity when they were assayed together with bovine alpha(s2)-casein f(183-207). It is important to note the synergistic effect among LFcin-B and bovine LF, because both compounds might be simultaneously in the suckling gastrointestinal tract and could, therefore, have a protective effect on it. The other synergistic effect high-lighted is that between alpha(s2)-casein f(183-207) and nisin against L. monocytogenes because of the ability of L. monocytogenes to develop resistance to nisin.

The SOS Regulatory Network

All organisms possess a diverse set of genetic programs that are used to alter cellular physiology in response to environmental cues. The gram-negative bacterium, Escherichia coli, mounts what is known as the "SOS response" following DNA damage, replication fork arrest, and a myriad of other environmental stresses. For over 50 years, E. coli has served as the paradigm for our understanding of the transcriptional, and physiological changes that occur following DNA damage (400). In this chapter, we summarize the current view of the SOS response and discuss how this genetic circuit is regulated. In addition to examining the E. coli SOS response, we also include a discussion of the SOS regulatory networks in other bacteria to provide a broader perspective on how prokaryotes respond to DNA damage.

Inactivation of bacterial pathogens in human milk by high-pressure processing

Low-temperature, long-time (LTLT) pasteurization assures the safety of banked human milk; however, heat can destroy important nutritional biomolecules. High-pressure processing (HPP) shows promise as an alternative for pasteurization of breast milk. The purpose of this study was to investigate the efficacy of HPP for inactivation of selected bacterial pathogens in human milk. Human milk was inoculated with one of five pathogens (10(8) to 10(9) CFU/ml), while 0.1% peptone solution solutions with the same levels of each organism were used as controls. The samples were subjected to 400 MPa at 21 to 31 degrees C for 0 to 50 min or to 62.5 degrees C for 0 to 30 min (capillary tube method) to simulate LTLT pasteurization. Tryptic soy agar and selective media were used for enumeration. Traditional thermal pasteurization resulted in inactivation (> 7 log) of all pathogens within 10 min. In human milk and in peptone solution, a 6-log reduction was achieved after 30 min of HPP for Staphylococcus aureus ATCC 6538. After 30 min, S. aureus ATCC 25923 was reduced by 8 log and 6 log in human milk and peptone solution, respectively. Treatments of 4 and 7 min resulted in an 8-log inactivation of Streptococcus agalactiae ATCC 12927 in human milk and peptone solution, respectively, while Listeria monocytogenes ATCC 19115 required 2 min for an 8-log inactivation in human milk. Escherichia coli ATCC 25922 was inactivated by 8 log after 10 min in peptone solution and by 6 log after 30 min in human milk. These data suggest that HPP may be a promising alternative for pasteurization of human milk. Further research should evaluate the efficacy of HPP in the inactivation of relevant viral pathogens.

Inhibition of growth of Enterobacter sakazakii in reconstituted infant formula by the lactoperoxidase system

Neonatal bacteremia and meningitis caused by the opportunistic pathogen Enterobacter sakazakii have been associated with the consumption of reconstituted powdered infant formula. Lactoperoxidase (LPO), present in mammalian milk, is known to inhibit the growth of enteric pathogens. We undertook a study to determine if the lactoperoxidase system (LPOS) will inhibit the growth of E. sakazakii in a milk-based powdered infant formula reconstituted with water. Initially at 0.04 CFU/ml, E. sakazakii grew to 2.40 to 2.74 log CFU/ml in reconstituted infant formula held at 30 or 37 degrees C for 8 h and to 0.6 log CFU/ ml in formula held for 12 h at 21 degrees C. The pathogen was not detected (less than 1 CFU/227 ml) by enrichment of formula treated with 10 to 30 microg/ml LPO and stored for 24 h at 37 degrees C or 30 microg/ml LPO and stored for 24 h at 30 degrees C. Populations of E. sakazakii, initially at 4.40 log CFU/ml of reconstituted infant formula containing 5 microg/ml LPO, did not increase significantly (P > 0.05) for up to 12 h at 21 and 30 degrees C. Populations either decreased significantly or were unchanged in formula supplemented with 10 microg/ml LPO and stored at 21, 30, or 37 degrees C for up to 24, 8, and 8 h, respectively. Results indicate that LPOS can be used to control the growth of E. sakazakii in reconstituted infant formula, thereby potentially reducing the risk of neonatal infections resulting from consumption of formula that may be contaminated with the pathogen.

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.

Role of porins in sensitivity of Escherichia coli to antibacterial activity of the lactoperoxidase enzyme system

Lactoperoxidase is an enzyme that contributes to the antimicrobial defense in secretory fluids and that has attracted interest as a potential biopreservative for foods and other perishable products. Its antimicrobial activity is based on the formation of hypothiocyanate (OSCN-) from thiocyanate (SCN-), using H2O2 as an oxidant. To gain insight into the antibacterial mode of action of the lactoperoxidase enzyme system, we generated random transposon insertion mutations in Escherichia coli MG1655 and screened the resultant mutants for an altered tolerance of bacteriostatic concentrations of this enzyme system. Out of the ca. 5,000 mutants screened, 4 showed significantly increased tolerance, and 2 of these had an insertion, one in the waaQ gene and one in the waaO gene, whose products are involved in the synthesis of the core oligosaccharide moiety of lipopolysaccharides. Besides producing truncated lipopolysaccharides and displaying hypersensitivity to novobiocin and sodium dodecyl sulfate (SDS), these mutants were also shown by urea-SDS-polyacrylamide gel electrophoresis analysis to have reduced amounts of porins in their outer membranes. Moreover, they showed a reduced degradation of p-nitrophenyl phosphate and an increased resistance to ampicillin, two indications of a decrease in outer membrane permeability for small hydrophilic solutes. Additionally, ompC and ompF knockout mutants displayed levels of tolerance to the lactoperoxidase system similar to those displayed by the waa mutants. These results suggest that mutations which reduce the porin-mediated outer membrane permeability for small hydrophilic molecules lead to increased tolerance to the lactoperoxidase enzyme system because of a reduced uptake of OSCN-.

Unique stress response to the lactoperoxidase-thiocyanate enzyme system in Escherichia coli

Using a differential fluorescence induction approach, we screened a promoter trap library constructed in a vector with a promoterless gfp gene for Escherichia coli MG1655 promoters that are induced upon challenge with the antimicrobial lactoperoxidase-thiocyanate enzyme system. None of the thirteen identified lactoperoxidase-inducible open reading frames was inducible by H(2)O(2) or by the superoxide generator plumbagin. However, analysis of specific promoters of known stress genes showed some of these, including recA, dnaK and sodA, to be inducible by the lactoperoxidase-thiocyanate enzyme system. The results show that the lactoperoxidase-thiocyanate enzyme system elicits a distinct stress response different from but partly overlapping other oxidative stress responses. Several of the induced genes or pathways may be involved in bacterial defense against the toxic effects of the lactoperoxidase-thiocyanate enzyme system.

Effects of a lactoperoxidase–thiocyanate–hydrogen peroxide system on Salmonella enteritidis in animal or vegetable foods

Lactoperoxidase (LPO) from skim milk was purified by ion-exchange chromatography. The purified protein was used to catalyze the oxidation of thiocyanate by H2O2 in an antibacterial system (LPO system). The LPO system was used to inactivate or inhibit Salmonella enteritidis in tomato juice, carrot juice, milk, liquid whole egg, and chicken skin extract under various conditions. The system was found to be more effective against the organism in vegetable juices than in animal products, at low pH than at neutral pH, and at higher temperatures than at lower temperatures. Acid-adapted S. enteritidis cells were more susceptible than nonadapted cells. The system reduced numbers of S. enteritidis in vegetable products by up to 5.4 log units and inhibited growth of the organism in animal-derived foods during 4 h incubation at 30 degrees C. Sodium chloride (>100 mM) and polyphosphate (0.01-0.5%) enhanced the antibacterial effects of the system in tomato juice and chicken skin extract, respectively. The findings indicate that the LPO system could probably be used to prevent the growth and survival of salmonellae in minimally processed fruit and vegetable products, but combination of the system with other preservatives or treatments would be needed to effectively inhibit growth and survival of salmonellae in animal products.

High sucrose concentration protects E. coli against high pressure inactivation but not against high pressure sensitization to the lactoperoxidase system

The inactivation of Escherichia coli by high hydrostatic pressure treatment at up to 550 MPa and 20 degrees C was studied in potassium phosphate buffer containing high concentrations of sucrose. E. coli strain MG1655 was pressure-sensitive in the absence of sucrose, but became highly pressure resistant in the presence of 10% to 50% (w/v) sucrose. The pressure resistance of E. coli strain LMM1010, a previously described derivative of MG1655 that is pressure resistant in the absence of sucrose, was further increased in the presence of sucrose, to a similar level as for strain MG1655 in the presence of sucrose. When cell suspensions of either strain were stored after pressure treatment for 24 h at 20 degrees C, a further reduction of the plate counts indicative of pressure induced sublethal injury was observed, that was positively correlated with pressure intensity and negatively with sucrose concentration. Addition of the lactoperoxidase system to the cell suspensions strongly enhanced high pressure inactivation of E. coli at high sucrose concentrations. Using a pressure intensity of only 250 MPa, both E. coli strains were sensitized for the lactoperoxidase system in up to 30% (w/v) sucrose, resulting in at least 10(6)-fold inactivation within 24 h or less after pressure treatment. For comparison, a pressure treatment at 250 MPa in the absence of the lactoperoxidase system did not cause any inactivation of either strain even in the absence of sucrose. At sucrose concentrations above 30% (w/v), no or very little inactivation occurred even in the presence of the lactoperoxidase system.