Natural approach of using nisin and its nanoform as food bio-preservatives against methicillin resistant Staphylococcus aureus and E.coli O157:H7 in yoghurt

BACKGROUND

Natural antimicrobial agents such as nisin were used to control the growth of foodborne pathogens in dairy products. The current study aimed to examine the inhibitory effect of pure nisin and nisin nanoparticles (nisin NPs) against methicillin resistant Staphylococcus aureus (MRSA) and E.coli O157:H7 during the manufacturing and storage of yoghurt. Nisin NPs were prepared using new, natural, and safe nano-precipitation method by acetic acid. The prepared NPs were characterized using zeta-sizer and transmission electron microscopy (TEM). In addition, the cytotoxicity of nisin NPs on vero cells was assessed using the 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The minimum inhibitory concentrations (MICs) of nisin and its nanoparticles were determined using agar well-diffusion method. Further, fresh buffalo's milk was inoculated with MRSA or E.coli O157:H7 (1 × 106 CFU/ml) with the addition of either nisin or nisin NPs, and then the inoculated milk was used for yoghurt making. The organoleptic properties, pH and bacterial load of the obtained yoghurt were evaluated during storage in comparison to control group.

RESULTS

The obtained results showed a strong antibacterial activity of nisin NPs (0.125 mg/mL) against MRSA and E.coli O157:H7 in comparison with control and pure nisin groups. Notably, complete eradication of MRSA and E.coli O157:H7 was observed in yoghurt formulated with nisin NPs after 24 h and 5th day of storage, respectively. The shelf life of yoghurt inoculated with nisin nanoparticles was extended than those manufactured without addition of such nanoparticles.

CONCLUSIONS

Overall, the present study indicated that the addition of nisin NPs during processing of yoghurt could be a useful tool for food preservation against MRSA and E.coli O157:H7 in dairy industry.

Natural Antimicrobials in Dairy Products: Benefits, Challenges, and Future Trends

This review delves into using natural antimicrobials in the dairy industry and examines various sources of these compounds, including microbial, plant, and animal sources. It discusses the mechanisms by which they inhibit microbial growth, for example, by binding to the cell wall's precursor molecule of the target microorganism, consequently inhibiting its biosynthesis, and interfering in the molecule transport mechanism, leading to cell death. In general, they prove to be effective against the main pathogens and spoilage found in food, such as Escherichia coli, Staphylococcus aureus, Bacillus spp., Salmonella spp., mold, and yeast. Moreover, this review explores encapsulation technology as a promising approach for increasing the viability of natural antimicrobials against unfavorable conditions such as pH, temperature, and oxygen exposure. Finally, this review examines the benefits and challenges of using natural antimicrobials in dairy products. While natural antimicrobials offer several advantages, including improved safety, quality, and sensory properties of dairy products, it is crucial to be aware of the challenges associated with their use, such as potential allergenicity, regulatory requirements, and consumer perception. This review concludes by emphasizing the need for further research to identify and develop effective and safe natural antimicrobials for the dairy industry to ensure the quality and safety of dairy products for consumers.

Effect of antimicrobial treatments applied individually and in combination on the growth of Listeria monocytogenes in Queso Fresco at 3 different temperatures

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A combination of antimicrobial treatments provides a more effective approach against L. monocytogenes growth in QF.

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PlyP100 + NIS was the most effective treatment for L. monocytogenes growth in QF.

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Listeria monocytogenes can grow up to dangerously high levels regardless of the storage temperature in untreated QF.

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EPL + LAE are good candidates to further evaluate for improving safety of QF during cold storage.

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Temperature abuse dramatically reduces the effectiveness of the tested antilisterials in QF.

Queso fresco (QF), a fresh soft cheese, is one of the most popular Hispanic cheeses in the United States and is frequently associated with Listeria monocytogenes outbreaks. Listeria monocytogenes can grow and thrive at room temperature as well as refrigeration temperatures. A combination of antimicrobial agents provides a larger spectrum of listeriostatic and listeriocidal activity resulting in a more effective approach toward the control of L. monocytogenes. In this study, we evaluated the efficacy of 3 Food and Drug Administration-approved generally recognized as safe (GRAS) antimicrobials, nisin (NIS), lauric arginate ethyl ester (LAE), and ε-polylysine (EPL), and the endolysin PlyP100 individually and in combination for control of L. monocytogenes in QF at 4°C, 7°C, and 10°C. Additionally, growth curves of L. monocytogenes were obtained in BHI broth and QF at these temperatures. In order for an antimicrobial to be considered a postlethality treatment for L. monocytogenes, it should not allow an increase of more than 2-log over the product's shelf life. Three treatments, PlyP100, PlyP100 + NIS, and EPL + LAE, effectively kept the pathogen below the 2 log growth threshold at 4°C. However, at 7°C and 10°C, none of the antimicrobial treatments could inhibit L. monocytogenes growth (i.e., <2 log). Overall, our results suggest the importance of considering the effect of cold storage temperatures above 4°C on the antilisterial efficacy of antimicrobial treatments in QF.

Fabrication of zein-modified starch nanoparticle complexes via microfluidic chip and encapsulation of nisin

A microfluidic chip is a micro-reactor that precisely manipulates and controls fluids. Zein is a group of prolamines extracted from corn that can form self-assembled nanoparticles in water or a low concentration of ethanol in a microfluidic chip. However, the zein nanoparticles have stability issues, especially in a neutral pH environment due to the proximity of the isoelectric point. This study was designed 1) to evaluate the effect of octenyl succinic anhydride (OSA) modified starch on the stability of zein nanoparticles formed using a microfluidic chip and 2) to apply the zein-OSA starch for encapsulation of nisin and evaluate its anti-microbial activity in a model food matrix. A T-junction configuration of the microfluidic chip was used to fabricate the zein nanoparticles using 1% or 2% zein solution and 0–10% (w/w) of OSA starch solution. The stability of the nanoparticles in various ionic strength environments was assessed. Encapsulation efficiency and anti-microbial activity of nisin in the zein nanoparticles against Listeria monocytogenes in a fresh cheese were measured. As the concentration of OSA starch increased from 0 to 10%, effective diameter increased from 117.8 ± 14.5 to 198.7 ± 13.9 nm without affecting polydispersity indexes and zeta-potential changed toward that of the modified starch indicating the zein surface coverage by the OSA starch. The zein-OSA starch nanoparticle complexes were more stable at various sodium chloride concentrations than the zein nanoparticles without OSA starch. The encapsulation efficiency of nisin was positively correlated with the OSA starch concentration. The anti-microbial activity of nisin in the fresh cheese also increased until 3-days of storage as the concentration of the OSA starch increased, which presented both a potential and challenge toward applications.

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Microfluidic chip formed zein nanoparticles with OSA-modified starch.

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Zein nanoparticle size and stability were affected by zein and modified starch concentration.

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Nisin was encapsulated in the zein nanoparticles via microfluidic chip.

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Anti-microbial activity of nisin was improved by the encapsulation.

Lactose oxidase: An enzymatic approach to inhibit Listeria monocytogenes in milk

Listeria monocytogenes is a ubiquitous pathogen that can cause morbidity and mortality in immunocompromised individuals. Growth of L. monocytogenes is possible at refrigeration temperatures due to its psychrotrophic nature. The use of antimicrobials in dairy products is a potential way to control L. monocytogenes growth in processes with no thermal kill step, thereby enhancing the safety of such products. Microbial-based enzymes offer a clean-label approach for control of L. monocytogenes outgrowth. Lactose oxidase (LO) is a microbial-derived enzyme with antimicrobial properties. It oxidizes lactose into lactobionic acid and reduces oxygen, generating H₂O₂. This study investigated the effects of LO in UHT skim milk using different L. monocytogenes contamination scenarios. These LO treatments were then applied to raw milk with various modifications; higher levels of LO as well as supplementation with thiocyanate were added to activate the lactoperoxidase system, a natural antimicrobial system present in milk. In UHT skim milk, concentrations of 0.0060, 0.012, and 0.12 g/L LO each reduced L. monocytogenes counts to below the limit of detection between 14 and 21 d of refrigerated storage, dependent on the concentration of LO. In the 48-h trials in UHT skim milk, LO treatments were effective in a concentration-dependent fashion. The highest concentration of LO in the 21-d trials, 0.12 g/L, did not show great inhibition over 48 h, so concentrations were increased for these experiments. In the lower inoculum, after 48 h, a 12 g/L LO treatment reached levels of 1.7 log cfu/mL, a reduction of 1.3 log cfu/mL from the initial inoculum, whereas the control grew out to approximately 4 log cfu/mL, an increase of 1 log cfu/mL from the inoculum on d 0. When a higher challenge inoculum of 5 log cfu/mL was used, the 0.12 g/L and 1.2 g/L treatments reduced the levels by 0.2 to 0.3 log cfu/mL below the initial inoculum and the 12 g/L treatment by >1 log cfu/mL below the initial inoculum by hour 48 of storage at refrigeration temperatures. After the efficacy of LO was determined in UHT skim milk, LO treatments were applied to raw milk. Concentrations of LO were increased, and the addition of thiocyanate was investigated to supplement the effect of the lactoperoxidase system against L. monocytogenes. When raw milk was inoculated with 2 log cfu/mL, 1.2 g/L LO alone and combined with sodium thiocyanate reduced ~0.8 log cfu/mL from the initial inoculum on d 7 of storage, whereas the control grew out to >1 log cfu/mL from the initial inoculum. Furthermore, in the higher inoculum, 1.2 g/L LO combined with sodium thiocyanate reduced L. monocytogenes counts from the initial inoculum by >1 log cfu/mL, whereas the control grew out 2 log cfu/mL from the initial inoculum. Results from this study suggest that LO is inhibitory against L. monocytogenes in UHT skim milk and in raw milk. Therefore, LO may be an effective treatment to prevent L. monocytogenes outgrowth, increase the safety of raw milk, and be used as an effective agent to prevent L. monocytogenes proliferation in fresh cheese and other dairy products. This enzymatic approach is a novel application to control the foodborne pathogen L. monocytogenes in dairy products.

Enabling Cost-Effective Screening for Antimicrobials against Listeria monocytogenes in Ham

ABSTRACT

Ready-to-eat meat products, such as deli ham, can support the growth of Listeria monocytogenes (LM), which can cause severe illness in immunocompromised individuals. The objectives of this study were to validate a miniature ham model (MHM) against the ham slice method and to screen antimicrobial combinations to control LM on ham by using response surface methology (RSM) as a time- and cost-effective high-throughput screening tool. The effect of nisin (Ni), potassium lactate and sodium diacetate, lauric arginate (LAG), lytic bacteriophage (P100), and ε-polylysine (EPL) added alone, or in combination, were determined on the MHM over 12 days of storage. Results showed the MHM accurately mimics the ham slice method because no statistical differences were found (P = 0.526) in the change of LM cell counts in MHM and slice counts after 12 days of storage at 4°C for treated and untreated hams. The MHM was then used to screen antimicrobial combinations by using an on-face design and three center points in a central composite design. The RSM was tested by using a cocktail of five LM strains isolated from foodborne disease outbreaks. Three levels of the previously mentioned antimicrobials were used in combination for a total of 28 runs performed in triplicate. The change of LM cell counts were determined after 12 days of storage at 4°C. All tested antimicrobials were effective on reducing LM cell counts on ham when added alone. A significant antagonistic interaction (P = 0.002) was identified by the RSM between LAG and P100, where this antimicrobial combination caused a 2.2-log CFU/g change of LM cell counts after 12 days of storage. Two interactions, between Ni and EPL (P = 0.058), and Ni and P100 (P = 0.068), showed possible synergistic effects against LM on the MHM. Other interactions were clearly nonsignificant, suggesting additive effects. In future work, the developed MHM in combination with RSM can be used as a high-throughput method to analyze novel antimicrobial treatments against LM.

HIGHLIGHTS

Lytic characterization and application of listerial endolysins PlyP40 and PlyPSA in queso fresco

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PlyP40 had lytic efficacy against a broad range of Listeria

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PlyP40 and PlyPSA were able to maintain lytic activity at refrigeration temperature

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Lytic activity of PlyP40 decreased as pH increased, whereas that of PlyPSA increased

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PlyP40 and PlyPSA maintained lytic activity within the queso fresco (QF) salt range

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PlyP40 and PlyPSA were able to decrease counts of Listeria monocytogenes in QF

Queso fresco (QF) is a fresh Hispanic-style cheese that is commonly associated with the human foodborne pathogen Listeria monocytogenes and outbreaks of listeriosis in the United States. Endolysins, cell wall hydrolases derived from bacteriophages, are promising candidates for controlling bacterial pathogens in food systems. In this study, we characterized the lytic capabilities of 2 endolysins, PlyP40 and PlyPSA, under varying conditions (pH, temperature, salt concentration) and compared their activities with those of the previously described endolysin PlyP100. We showed that PlyP40 was effective, showing at least a 33% reduction in cellular debris, against a broader range of Listeria than PlyPSA, which showed little lytic activity toward Listeria strains not from serovar 4. Both endolysins were also capable of maintaining lytic activity to varying extents at refrigeration temperature. The effect of salt concentration and pH differed between PlyP40 and PlyPSA. Furthermore, we added the endolysins to QF and monitored their ability to control L. monocytogenes contamination over 28 d of cold storage. Both PlyP40 and PlyPSA were capable of lowering QF inoculum cell counts compared with the control; however, both were less effective than the previously characterized PlyP100. Further characterization of endolysins will continue to open opportunities to optimization and implementation in a variety of food matrices for controlling pathogen contamination.

From Cheese-Making to Consumption: Exploring the Microbial Safety of Cheeses through Predictive Microbiology Models

Cheeses are traditional products widely consumed throughout the world that have been frequently implicated in foodborne outbreaks. Predictive microbiology models are relevant tools to estimate microbial behavior in these products. The objective of this study was to conduct a review on the available modeling approaches developed in cheeses, and to identify the main microbial targets of concern and the factors affecting microbial behavior in these products. Listeria monocytogenes has been identified as the main hazard evaluated in modelling studies. The pH, a_w, lactic acid concentration and temperature have been the main factors contemplated as independent variables in models. Other aspects such as the use of raw or pasteurized milk, starter cultures, and factors inherent to the contaminating pathogen have also been evaluated. In general, depending on the production process, storage conditions, and physicochemical characteristics, microorganisms can grow or die-off in cheeses. The classical two-step modeling has been the most common approach performed to develop predictive models. Other modeling approaches, including microbial interaction, growth boundary, response surface methodology, and neural networks, have also been performed. Validated models have been integrated into user-friendly software tools to be used to obtain estimates of microbial behavior in a quick and easy manner. Future studies should investigate the fate of other target bacterial pathogens, such as spore-forming bacteria, and the dynamic character of the production process of cheeses, among other aspects. The information compiled in this study helps to deepen the knowledge on the predictive microbiology field in the context of cheese production and storage.

Non-Destructive Luminescence-Based Screening Tool for Listeria monocytogenes Growth on Ham

Listeria monocytogenes is a food-borne pathogen often associated with ready-to-eat (RTE) food products. Many antimicrobial compounds have been evaluated in RTE meats. However, the search for optimum antimicrobial treatments is ongoing. The present study developed a rapid, non-destructive preliminary screening tool for large-scale evaluation of antimicrobials utilizing a bioluminescent L. monocytogenes with a model meat system. Miniature hams were produced, surface treated with antimicrobials nisin (at 0-100 ppm) and potassium lactate sodium diacetate (at 0-3.5%) and inoculated with bioluminescent L. monocytogenes. A strong correlation (r = 0.91) was found between log scale relative light units (log RLU, ranging from 0.00 to 3.35) read directly from the ham surface and endpoint enumeration on selective agar (log colony forming units (CFU)/g, ranging from 4.7 to 8.3) when the hams were inoculated with 6 log CFU/g, treated with antimicrobials, and L. monocytogenes were allowed to grow over a 12 d refrigerated shelf life at 4 °C. Then, a threshold of 1 log RLU emitted from a ham surface was determined to separate antimicrobial treatments that allowed more than 2 log CFU/g growth of L. monocytogenes (from 6 log CFU/g inoculation to 8 log CFU/g after 12 d). The proposed threshold was utilized in a luminescent screening of antimicrobials with days-to-detect growth monitoring of luminescent L. monocytogenes. Significantly different (p < 0.05) plate counts were found in antimicrobial treated hams that had reached a 1 log RLU increase (8.1-8.5 log(CFU/g)) and the hams that did not reach the proposed light threshold (5.3-7.5 log(CFU/g)). This confirms the potential use of the proposed light threshold as a qualitative tool to screen antimicrobials with less than or greater than a 2 log CFU/g increase. This screening tool can be used to prioritize novel antimicrobials targeting L. monocytogenes, alone or in combination, for future validation.

Efficacy of nisin derivatives with improved biochemical characteristics, alone and in combination with endolysin PlyP100 to control Listeria monocytogenes in laboratory-scale Queso Fresco

Nisin is an antimicrobial peptide that is commonly used as a food preservative and capable of inhibiting the pathogen Listeria monocytogenes. However, nisin is ineffective in controlling L. monocytogenes in Queso Fresco (QF). To address the challenge, in this work, we used synthetic biology strategies to create a series of nisin A derivatives by substituting residues 27, 30, 31 and 32 with positively charged amino acids (H, K and R). Our results showed that nisin derivatives exhibited reduced antilisterial activity in vitro compared to nisin A; however, they were all more stable under QF-like experimental conditions (pH 7 + 22% milk fat), notably H27/31K. Compared to nisin A, the derivatives H31K and V32K exhibited slight antilisterial improvement in QF and H27/31K was able to reduce the initial population of L. monocytogenes by up to 1.5 Log CFU/g. L. monocytogenes isolates exhibited similar susceptibility to nisin A or H27/31K after 7 or 14 days of nisin exposure in QF. Notably, when combined with endolysin PlyP100, the application of H27/31K resulted in non-enumerable levels of L. monocytogenes after 14 days of cold storage. Our results highlight the potential of bioengineered nisin derivatives for stabilized and enhanced control of L. monocytogenes in QF.

Pre-growth conditions and strain diversity affect nisin treatment efficacy against Listeria monocytogenes on cold-smoked salmon

Listeria monocytogenes is a human pathogen that is commonly found in environments associated with cold-smoked salmon. Nisin is a natural antimicrobial that can be used as a food preservative. While nisin is active against a number of Gram-positive bacteria, including L. monocytogenes, environmental stresses encountered in cold-smoked salmon processing facilities might affect L. monocytogenes' nisin susceptibility. The objective of this study was to investigate the effect of seafood-relevant pre-growth conditions and L. monocytogenes strain diversity on nisin treatment efficacy on cold-smoked salmon. Six L. monocytogenes strains representing serotypes most commonly associated with cold-smoked salmon (1/2a, 1/2b, and 4b) were initially pre-grown under a number of seafood-relevant conditions and challenged with nisin in growth media modified to represent the characteristics of cold-smoked salmon. The pre-growth conditions with the lowest mean log reduction due to nisin and the highest strain-to-strain variability were selected for experiments on cold-smoked salmon; these included: (i) 4.65% w.p. NaCl ("NaCl"); (ii) pH = 6.1 ("pH"); (iii) 0.5 μg/ml benzalkonium chloride ("Quat"); and a control ("BHI"). Cold-smoked salmon slices with or without nisin were inoculated with L. monocytogenes pre-grown in one of the conditions above, vacuum-packed, and incubated at 7 °C. L. monocytogenes were enumerated on days 1, 15, and 30. A linear mixed effects model was constructed to investigate the effect of pre-growth condition, day in storage, serotype, source of isolation as well as their interactions on nisin efficacy against L. monocytogenes. Compared to pre-growth in "BHI", significant reduction (P < 0.05) in nisin efficacy was induced by pre-growth in "pH" and "Quat" on both days 15 and 30, and by pre-growth in "NaCl" on day 30, indicating a time-dependent cross-protection effect. Additionally, an effect of L. monocytogenes' serotype on the cross-protection to nisin was observed; pre-growth in "pH" significantly reduced nisin efficacy against serotype 1/2a and 4b strains, but not against 1/2b strains. In conclusion, pre-exposure to mildly acidic environment, high salt content, and sublethal concentrations of quaternary ammonium compounds, is likely to provide cross-protection against a subsequent nisin treatment of L. monocytogenes on cold-smoked salmon. Therefore, challenge studies that use pre-growth in "BHI", as well as more susceptible L. monocytogenes strains, may overestimate the efficacy of nisin as a control strategy for cold-smoked salmon.

Nevertheless, She Resisted - Role of the Environment on Listeria monocytogenes Sensitivity to Nisin Treatment in a Laboratory Cheese Model

The growth of Listeria monocytogenes on refrigerated, ready-to-eat food products is a major health and economic concern. The natural antimicrobial nisin targets the bacterial cell wall and can be used to inhibit L. monocytogenes growth on cheese. Cell wall composition and structure, and therefore the efficacy of cell wall acting control strategies, can be severely affected by environmental and stress conditions. The goal of this study was to determine the effect of a range of pH and temperatures on the efficacy of nisin against several strains of L. monocytogenes in a lab-scale, cheese model. Cheese was made with or without the addition of nisin at different pH and then inoculated with L. monocytogenes; L. monocytogenes numbers were quantified after 1, 7, and 14 days of incubation at 6, 14, or 22°C. While our data show that nisin treatment is able to reduce L. monocytogenes numbers, at least initially, growth of this pathogen can occur even in the presence of nisin, especially when cheese is stored at higher temperatures. Several environmental factors were found to affect nisin efficacy against L. monocytogenes. For example, nisin is more effective when cheese is stored at lower temperatures. Nisin is also more effective when cheese is made at higher pH (6 and 6.5), compared to cheese made at pH 5.5, and this effect is at least partially due to the activity of cell envelope modification genes dltA and mprF. Serotype was also found to affect nisin efficacy against L. monocytogenes; serotype 4b strains showed lower susceptibility to nisin treatment compared to serotype 1/2 strains. Overall, our results highlight the importance of considering environmental conditions specific to a food matrix when developing and applying nisin-based intervention strategies against L. monocytogenes.

Invited review: Advances in nisin use for preservation of dairy products

Dairy product safety is a global public health issue that demands new approaches and technologies to control foodborne pathogenic microorganisms. Natural antimicrobial agents such as nisin can be added to control the growth of pathogens of concern in dairy foods, namely Listeria monocytogenes and Staphylococcus aureus. However, several factors affect the antimicrobial efficacy of nisin when directly added into the food matrix such as lack of stability at neutral pH, interaction with fat globules, casein, and divalent cations. To overcome these limitations, new and advanced strategies are discussed including nisin encapsulation technology, addition to active packaging, bioengineering, and combination with other antimicrobials. This review highlights advanced technologies with potential to expand and improve the use of nisin as a dairy preservative.

Ferulic Acid and Eugenol Have Different Abilities to Maintain Their Inhibitory Activity Against Listeria monocytogenes in Emulsified Systems

Natural phenolic compounds are found in large quantities in plants and plant extracts and byproducts from agro-industries. They could be used to ensure food quality and safety due to their antimicrobial properties demonstrated in systems such as culture media. The aim of this study was to evaluate the ability of two natural phenolic compounds, ferulic acid and eugenol, to maintain their inhibitory activity against the growth of Listeria monocytogenes in an oil-in-water emulsion, simulating a complex food system. The minimum inhibitory concentration (MIC) of each phenolic compound was first determined in culture medium, consisting of TS broth and an added emulsifier. Whey proteins and Tween 80 increased the MIC of the antimicrobial activity of eugenol. The MIC of ferulic acid was less affected by the addition of Tween 80. The inhibitory activities of both phenolic compounds were then compared at the same concentration in emulsions and their corresponding aqueous phases by following the growth of L. monocytogenes by plate counting. In emulsified systems, eugenol lost the high inhibitory activity observed in the aqueous phase, whereas ferulic acid retained it. The partition coefficient (logP_oct/wat) appears to be a key factor. Eugenol (logP_oct/wat = 2.61) dispersed in the aqueous phase intercalates into the bacterial membrane and has high antimicrobial activity. In contrast, it likely preferentially partitions into the lipid droplets when dispersed in an emulsion, consequently losing its antimicrobial activity. As ferulic acid is more hydrophilic, a higher proportion probably remains in the aqueous phase of the emulsion, retaining its antimicrobial activity.

Characterization and Application of Antilisterial Enterocins on Model Fresh Cheese

Enterococcus faecalis strains isolated from an artisanal cheese were selected based on enterocin production against Listeria monocytogenes. The strains formed biofilms and presented high hydrophobic character and good autoaggregation and coaggregation capacity with L. monocytogenes. Strains L3A21M3 and L3B1K3 presented high survival under gastrointestinal conditions, were able to adhere to human intestinal cells (Caco-2 and HT-29), and blocked the adhesion and invasion of L. monocytogenes. The antilisterial activity of enterocins was not affected by pH (2 to 12), heating (100°C), and chemical and surfactant agents. However, strains L3A21M3 and L3A21M8 produced thermolabile enterocins, which were also sensible to extreme pH values. Enterocins exhibited a bacteriostatic mode of action against L. monocytogenes, and maximum production was observed during the stationary phase. Common enterocin structural genes were not detected by PCR amplification with specific primers, although an exhaustive screening was not performed. The enterocin produced by the L3B1K3 strain was purified and applied to model cheeses contaminated with L. monocytogenes. This enterocin reduced survival of L. monocytogenes on fresh cheeses in a dose-dependent manner. The highest dose tested (2,048 arbitrary units per g of cheese) was effective in reducing the pathogen counts to undetectable values throughout storage (6 to 72 h). These results suggest that these strains have great potential to be used as biopreservatives in the food industry and also as probiotics, with the potential to prevent L. monocytogenes gastrointestinal infection.

A Multibacteriocin Cheese Starter System, Comprising Nisin and Lacticin 3147 in Lactococcus lactis, in Combination with Plantaricin from Lactobacillus plantarum

Functional starter cultures demonstrating superior technological and food safety properties are advantageous to the food fermentation industry. We evaluated the efficacies of single- and double-bacteriocin-producing starters of Lactococcus lactis capable of producing the class I bacteriocins nisin A and/or lacticin 3147 in terms of starter performance. Single producers were generated by mobilizing the conjugative bacteriophage resistance plasmid pMRC01, carrying lacticin genetic determinants, or the conjugative transposon Tn5276, carrying nisin genetic determinants, to the commercial starter L. lactis CSK2775. The effect of bacteriocin coproduction was examined by superimposing pMRC01 into the newly constructed nisin transconjugant. Transconjugants were improved with regard to antimicrobial activity and bacteriophage insensitivity compared to the recipient strain, and the double producer was immune to both bacteriocins. Bacteriocin production in the starter was stable, although the recipient strain proved to be a more efficient acidifier than transconjugant derivatives. Overall, combinations of class I bacteriocins (the double producer or a combination of single producers) proved to be as effective as individual bacteriocins for controlling Listeria innocua growth in laboratory-scale cheeses. However, using the double producer in combination with the class II bacteriocin producer Lactobacillus plantarum or using the lacticin producer with the class II producer proved to be most effective for reducing bacterial load. As emergence of bacteriocin tolerance was reduced 10-fold in the presence of nisin and lacticin, we suggest that the double producer in conjunction with the class II producer could serve as a protective culture providing a food-grade, multihurdle approach to control pathogenic growth in a variety of industrial applications.IMPORTANCE We generated a suite of single- and double-bacteriocin-producing starter cultures capable of generating the class I bacteriocin lacticin 3147 or nisin or both bacteriocins simultaneously via conjugation. The transconjugants exhibited improved bacteriophage resistance and antimicrobial activity. The single producers proved to be as effective as the double-bacteriocin producer at reducing Listeria numbers in laboratory-scale cheese. However, combining the double producer or the lacticin-producing starter with a class II bacteriocin producer, Lactobacillus plantarum LMG P-26358, proved to be most effective at reducing Listeria numbers and was significantly better than a combination of the three bacteriocin-producing strains, as the double producer is not inhibited by either of the class I bacteriocins. Since the simultaneous use of lacticin and nisin should reduce the emergence of bacteriocin-tolerant derivatives, this study suggests that a protective starter system produced by bacteriocin stacking is a worthwhile multihurdle approach for food safety applications.