In vitro toxicity of reuterin, a potential food biopreservative

Application of the Reuterin System as Food Preservative or Health-Promoting Agent: A Critical Review

The reuterin system is a complex multi-component antimicrobial system produced by Limosilactobacillus reuteri by metabolizing glycerol. The system mainly includes 3-hydroxypropionaldehyde (3-HPA, reuterin), 3-HPA dimer, 3-HPA hydrate, acrolein and 3-hydroxypropionic acid, and has great potential to be applied in the food and medical industries due to its functional versatility. It has been reported that the reuterin system possesses regulation of intestinal flora and anti-infection, anti-inflammatory and anti-cancer activities. Typically, the reuterin system exerts strong broad-spectrum antimicrobial properties. However, the antimicrobial mechanism of the reuterin system remains unclear, and its toxicity is still controversial. This paper presents an updated review on the biosynthesis, composition, biological production, antimicrobial mechanisms, stability, toxicity and potential applications of the reuterin system. Challenges and opportunities of the use of the reuterin system as a food preservative or health-promoting agent are also discussed. The present work will allow researchers to accelerate their studies toward solving critical challenges obstructing industrial applications of the reuterin system.

In vitro assessment of skin sensitization, irritability and toxicity of bacteriocins and reuterin for possible topical applications

Bacteriocins and reuterin are promising antimicrobials for application in food, veterinary, and medical sectors. In the light of their high potential for application in hand sanitizer, we investigated the skin toxicity of reuterin, microcin J25, pediocin PA-1, bactofencin A, and nisin Z in vitro using neutral red and LDH release assays on NHEK cells. We determined their skin sensitization potential using the human cell line activation test (h-CLAT). Their skin irritation potential was measured on human epidermal model EpiDerm™. We showed that the viability and membrane integrity of NHEK cells remained unaltered after exposure to bacteriocins and reuterin at concentrations up to 400 µg/mL and 80 mg/mL, respectively. Furthermore, microcin J25 and reuterin showed no skin sensitization at concentrations up to 100 µg/mL and 40 mg/mL, respectively, while pediocin PA-1, bactofencin A, and nisin Z caused sensitization at concentrations higher than 100 µg/mL. Tissue viability was unaffected in presence of bacteriocins and reuterin at concentrations up to 200 µg/mL and 40 mg/mL, respectively, which was confirmed by measuring cytokine IL-1α and IL-8 levels and by histological analysis. In conclusion, the current study provides scientific evidence that some bacteriocins and reuterin, could be safely applied topically as sanitizers at recommended concentrations.

In vitro investigation of gastrointestinal stability and toxicity of 3-hyrdoxypropionaldehyde (reuterin) produced by Lactobacillus reuteri

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Reuterin (3-hyrdoxypropionaldehyde (3-HPA)) is a highly potent metabolite of Lactobacillus reuteri.

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Reuterin is highly stable in gastrointestinal condition.

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Human colorectal adenocarcinoma cells’ viability and membrane integrity remained unaltered by reuterin.

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No significant hemolytic activity was detected.

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Reuterin is a promising therapeutic and/or food preservative.

Reuterin (3-hyrdoxypropionaldehyde (3-HPA)) is a highly potent metabolite of L. reuteri, which has applications in food, health, and veterinary sectors. Similar to other natural antimicrobial compounds, the approval of reuterin as a bio-preservative or therapeutic agent by regulatory agencies relies on sufficient data on its cytotoxicity and behavior in the gastrointestinal environment. Although the antimicrobial activity of reuterin has been broadly studied, its safety and toxicity are yet to be explored in detail. In this study, the stability and activity of reuterin were investigated in the gastrointestinal tract using in vitro models simulating gastrointestinal conditions. In addition, hemolytic activity and in vitro cytotoxicity of reuterin were evaluated by neutral red assay and lactate dehydrogenase (LDH) colorimetric assay using the same cell line. Activity of reuterin was observed to be stable during gastrointestinal transit. Viability and membrane integrity of cells remained unaltered by reuterin up to 1080 mM concentration. Furthermore, no hemolysis was observed in blood cells exposed to 270 mM reuterin. This study provides unique and highly relevant in vitro data regarding gastrointestinal behavior and toxicity of reuterin. In conclusion, the current study indicates that within a certain concentration range, reuterin can be safely used in bio-preservation and therapeutics applications. However, further in vivo studies are required to confirm these findings.

Antagonistic effects of Lactobacillus reuteri against Escherichia coli O157:H7 in white-brined cheese under different storage conditions

This study aimed to investigate the survival of the foodborne pathogen Escherichia coli O157:H7 in white-brined cheeses as influenced by the presence of Lactobacillus reuteri. The white cheeses were made from pasteurized bovine milk inoculated with E. coli O157:H7 (cocktail of 3 strains) to achieve ∼5 log₁₀ cfu/g with absence or presence of Lb. reuteri (∼6 log₁₀ cfu/g). Cheese samples were brined in 10% or 15% NaCl solution and stored at 10°C and 25°C for 28 d. The white-brined cheeses were assessed for salt content, pH, water activity (A_w), and numbers of E. coli O157:H7, Lb. reuteri, nonstarter lactic acid bacteria (NSLAB), yeasts, and molds. Results showed that E. coli O157:H7 survived in cheese stored in both brine solutions at 10°C and 25°C regardless of the presence of Lb. reuteri. A substantial reduction was observed in cheese stored in 10% NaCl brine at 25°C, followed by cheese stored in 15% NaCl brine at 10°C by 2.64 and 2.16 log₁₀ cfu/g, respectively, in the presence of Lb. reuteri and by 1.02 and 1.87 log₁₀ cfu/g, respectively, in the absence of Lb. reuteri under the same conditions. The pathogen in brine solutions survived but at a lower rate. Furthermore, the growth of Lb. reuteri and NSLAB were enhanced or slightly decreased in cheese and brine by 28 d, respectively. The salt concentrations of cheese ranged from 4 to 6% and 5 to 7% (wt/wt), during 28-d ripening in 10 and 15% brine, respectively. Values of pH and A_w slightly increased at d 1 after exposure to brine and reached 4.69 to 6.08 and 0.91 to 0.95, respectively, in all treatments. Therefore, the addition of Lb. reuteri can be used as a biopreservation method to inhibit the survival of E. coli O157:H7 in white-brined cheese when combined with the appropriate temperature, NaCl level, and storage time.

A Phylogenetic View on the Role of Glycerol for Growth Enhancement and Reuterin Formation in Limosilactobacillus reuteri

Lineages within the species Limosilactobacillus reuteri have specialized to various hosts and their genomes reflect these adaptations. The pdu-cbi-cob-hem gene cluster is conserved in most human and poultry isolates but is infrequent in rodent and porcine isolates. This gene cluster confers the transformation of glycerol into 3-hydroxy-propionaldehyde (reuterin), which can either be secreted and function as precursor of the antimicrobial compound acrolein or serve as an electron acceptor that enhances the organisms’ growth rate. However, it remains unclear which of these two functions is more relevant for L. reuteri evolution and ecology. Here we characterized the effect of glycerol on growth rate and reuterin formation in L. reuteri strains across different phylogenetic lineages during growth on ecologically relevant carbohydrates. We further evaluated the innate reuterin resistance among these strains to infer a possible role of reuterin in the evolution of strains. Results revealed that the poultry/human lineage VI strain, L. reuteri DSM 17938 shows more growth enhancement through glycerol and greater capacity for reuterin production on glucose and maltose as compared to human lineage II strains. Interestingly, reuterin production in lineage II strains was significantly elevated on raffinose and lactose, reaching levels similar to DSM 17938. On all carbohydrates tested, reuterin production occurred during the exponential growth phase and became undetectable during the stationary growth phase. The amount of reuterin produced was sufficient to inhibit E. coli, suggesting that it could be ecologically relevant, but the resistance towards reuterin among L. reuteri strains was highly variable and, for the most part, unrelated to the strain’s capacity for reuterin production. Overall, the findings suggest differences in the substrate-specific regulation of the pdu cluster in L. reuteri lineages that might be reflective of their ecological niches, e.g., chicken foregut versus human infant and adult large intestine. Such information can inform future studies on the ecology of L. reuteri and guide the development of synbiotic applications to improve the therapeutic use of this species.

Edible Coating and Pulsed Light to Increase the Shelf Life of Food Products

The application of edible coatings (EC) in combination with pulsed light (PL) treatments represents an emerging approach for extending the shelf life of highly perishable but high value-added products, such as fresh-cut fruits and vegetables. The surface of these products would benefit from the protective effects of ECs and the PL decontamination capability. This review describes in detail the fundamentals of both EC and PL, focusing on the food engineering principles in the formulation and application of EC and the delivery of efficient PL treatments and the technological aspects related to the food characterization following these treatments and discussing the implementation of the two technologies, individually or in combination. The advantages of the combination of EC and PL are extensively discussed emphasizing the potential benefits that may be derived from their combination when preserving perishable foods. The downsides of combining EC and PL are also presented, with specific reference to the potential EC degradation when exposed to PL treatments and the screening effect of PL transmittance through the coating layer. Finally, the potential applications of the combined treatments to food products are highlighted, comparatively presenting the treatment conditions and the product shelf-life improvement.

Review on Natural Preservatives for Extending Fish Shelf Life

Fish is extremely perishable as a result of rapid microbial growth naturally present in fish or from contamination. Synthetic preservatives are widely used in fish storage to extend shelf life and maintain quality and safety. However, consumer preferences for natural preservatives and concerns about the safety of synthetic preservatives have prompted the food industry to search natural preservatives. Natural preservatives from microorganisms, plants, and animals have been shown potential in replacing the chemical antimicrobials. Bacteriocins and organic acids from bacteria showed good antimicrobial activities against spoilage bacteria. Plant-derived antimicrobials could prolong fish shelf life and decrease lipid oxidation. Animal-derived antimicrobials also have good antimicrobial activities; however, their allergen risk should be paid attention. Moreover, some algae and mushroom species can also provide a potential source of new natural preservatives. Obviously, the natural preservatives could perform better in fish storage by combining with other hurdles such as non-thermal sterilization processing, modified atmosphere packaging, edible films and coatings.

Decontamination of Minimally-Processed Fresh Lettuce Using Reuterin Produced by Lactobacillus reuteri

Over the last years the demand for pre-washed, fresh-cut, and minimally-processed (MP) produce has increased. MP fresh vegetable are rapidly spoiled, whereas there is consumers' concern about chemical disinfection treatments such as with chlorine. A promising antimicrobial is reuterin, a broad-spectrum-antimicrobial compound produced by food-grade Lactobacillus reuteri from glycerol. In aqueous solution, reuterin is a dynamic system consisting of 3-hydroxypropionaldehyde (3-HPA), its hydrate, its dimer as well as acrolein, which was recently identified as the main antimicrobial component of the system. Here, we tested the use of reuterin containing similar 3-HPA levels but different acrolein concentrations for decontaminating and preserving fresh-cut lettuce. Crude reuterin (CR) was produced by biotransformation of 600 mM glycerol using L. reuteri DSM 20016T. CR preparations were further incubated for 16 h at 50°C to produce enhanced reuterin (ER) with raised concentration of acrolein. Fresh-cut iceberg lettuce (Lactuca sativa) was washed using CR (1.5-1.9 mM acrolein) and ER (7.2-21.9 mM acrolein) solutions at 4°C, or sodium hypochloride (250 mg/L) and tap water, and compared with unwashed lettuce. Washed lettuce samples were packed under modified atmosphere (2% O2, 5% CO2, and 93% N2) and stored for 13 days at 4°C. Application of ER containing 12.1, 20.9, or 21.9 mM acrolein reduced the initial viable plate counts of Enterobacteriaceae (by 2.1-2.8 log CFU/g), and yeasts and molds (by 1.3-2.0 log CFU/g) when compared with unwashed samples. In contrast, reuterin solutions containing 7.2 mM acrolein, sodium hypochlorite and tap water only showed very limited and transient, or no effects on the cell loads of lettuce after washing and during storage. Visual assessment of leaves washed with ER showed acrolein concentration-dependent discoloration noticeable already after 3 days of storage for the highest acrolein concentrations. Discoloration became severe for all ER treatments after 7 days, while the other treatments preserved the aspect of washed lettuce. Our data show the predominant role of acrolein as the main antimicrobial component of the reuterin system for food biopreservation. Reuterin preparations with enhanced acrolein concentration of 12.1 mM and higher were effective to reduce plate counts of Enterobacteriaceae and yeasts and molds washed lettuce until day 7 but induced pronounced discoloration of lettuce.

Metabolism of Foodborne Heterocyclic Aromatic Amines by Lactobacillus reuteri DSM 20016

The heterocyclic aromatic amine (HAA) 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is converted into 7-hydroxy-5-methyl-3-phenyl-6,7,8,9-tetrahydropyrido[3',2':4,5]imidazo[1,2-a]pyrimidin-5-ium chloride (PhIP-M1) via a chemical reaction with 3-hydroxypropionaldehyde or acrolein derived from glycerol by reuterin producing gut bacteria. Because it is unknown whether this reaction also applies to other HAAs, seven foodborne HAAs (2-amino-9H-pyrido[2,3-b]indole (AαC), 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethyl-3H-imidazo[4,5-f]quinoline (MeIQ), 2-amino-3,8-dimethyl-3H-imidazo[4,5-f]quinoxaline (MeIQx), 9H-pyrido[3,4-b]indole (norharman), and 1-methyl-9H-pyrido[3,4-b]indole (harman)) were anaerobically incubated with Lactobacillus reuteri DSM 20016 in the presence of glycerol. The extent of conversion, as analyzed by HPLC-DAD/FLD, was dependent on both the studied HAAs and the glucose/glycerol ratio, indicating reuterin to be involved in HAA metabolism. Based on HRMS analyses, PhIP-M1-type metabolites were detected for AαC, Trp-P-1, IQ, MeIQ, MeIQx, harman, and norharman. In the case of AαC, this was confirmed by metabolite isolation (AαC-M8, 2,3,4,10-tetrahydro-1H-indolo[2,3-b][1,8]naphthyridin-2-ol) and one- (¹H) and two-dimensional (HSQC, HMBC, COSY, DOSY) NMR spectroscopy. In addition, based on HRMS and/or NMR spectroscopy, a new type of HAA metabolite, resulting from the reaction with two molecules of 3-hydroxypropionaldehyde or acrolein, is hypothesized for AαC, Trp-P-1, IQ, MeIQ, and MeIQx.