Inhibitory Effect of Substances Produced by Native Lactococcus lactis Strains of Tropical Fruits towards Food Pathogens

Recent Advancements in Harnessing Lactic Acid Bacterial Metabolites for Fruits and Vegetables Preservation

Postharvest losses in fruits and vegetables exert substantial economic and environmental repercussions. Chemical interventions are being widely utilized for the past six decades which may lead to significant health complications. Bioprotection of fruits and vegetables is the need of the hour in which use of lactic acid bacteria (LAB) with GRAS status predominantly stands out. Incorporation of LAB in postharvest fruits and vegetables suppresses the growth of spoilage organisms by synthesizing various antimicrobial compounds such as bacteriocins, organic acids, hydrogen peroxide (H2O2), exopolysaccharides (EPS), and BLIS. For example, Pediococcus acidilactici, Lactobacillus plantarum, and Limosilactobacillus fermentum convert natural sugars in fruits and vegetables to lactic acid and create an acidic environment that do not favour spoilage organisms. LAB can improve the bioavailability of vitamins and minerals and enrich the phenolic profile and bioactivity components. LAB has remarkable physiological characteristics like resistance towards bacteriophage, proteolytic activity, and polysaccharide production which adds to the safety of foods. They modify the sensory properties and preserve the nutritional quality of fruits and vegetables. They can also perform therapeutic role in the intestinal tract as they tolerate low pH, high salt concentration. Thus application of LAB, whether independently or in conjunction with stabilizing agents as edible coatings, is regarded as an exceptionally promising methodology for ensuring safer consumption of fruits and vegetables. This review addresses the most recent research findings that harness the antagonistic property of lactic acid bacterial metabolites, formulations and coatings containing their bioactive compounds for extended shelf life of fruits and vegetables.

Metabiotics Signature through Genome Sequencing and In Vitro Inhibitory Assessment of a Novel Lactococcus lactis Strain UTNCys6-1 Isolated from Amazonian Camu-Camu Fruits

Metabiotics are the structural components of probiotic bacteria, functional metabolites, and/or signaling molecules with numerous beneficial properties. A novel Lactococcus lactis strain, UTNCys6-1, was isolated from wild Amazonian camu-camu fruits (Myrciaria dubia), and various functional metabolites with antibacterial capacity were found. The genome size is 2,226,248 base pairs, and it contains 2248 genes, 2191 protein-coding genes (CDSs), 50 tRNAs, 6 rRNAs, 1 16S rRNA, 1 23S rRNA, and 1 tmRNA. The average GC content is 34.88%. In total, 2148 proteins have been mapped to the EggNOG database. The specific annotation consisted of four incomplete prophage regions, one CRISPR-Cas array, six genomic islands (GIs), four insertion sequences (ISs), and four regions of interest (AOI regions) spanning three classes of bacteriocins (enterolysin_A, nisin_Z, and sactipeptides). Based on pangenome analysis, there were 6932 gene clusters, of which 751 (core genes) were commonly observed within the 11 lactococcal strains. Among them, 3883 were sample-specific genes (cloud genes) and 2298 were shell genes, indicating high genetic diversity. A sucrose transporter of the SemiSWEET family (PTS system: phosphoenolpyruvate-dependent transport system) was detected in the genome of UTNCys6-1 but not the other 11 lactococcal strains. In addition, the metabolic profile, antimicrobial susceptibility, and inhibitory activity of both protein–peptide extract (PPE) and exopolysaccharides (EPSs) against several foodborne pathogens were assessed in vitro. Furthermore, UTNCys6-1 was predicted to be a non-human pathogen that was unable to tolerate all tested antibiotics except gentamicin; metabolized several substrates; and lacks virulence factors (VFs), genes related to the production of biogenic amines, and acquired antibiotic resistance genes (ARGs). Overall, this study highlighted the potential of this strain for producing bioactive metabolites (PPE and EPSs) for agri-food and pharmaceutical industry use.

Potential Probiotic Lactic Acid Bacteria with Anti-Penicillium expansum Activity from Different Species of Tunisian Edible Snails

This study aimed to isolate lactic acid bacteria (LAB) from the digestive tract, meat and slime of edible snails (Helix lucorum, Helix aspersa and Eobania vermiculata) and investigate their antagonistic activity against Penicillium expansum. They were then characterized for their probiotic potential. Among 900 bacterial isolates, 47 LAB exhibiting anti-P. expansum activity were identified through matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) as Levilactobacillus brevis (25), Lactococcus lactis (3), Enterococcus faecium (12), Enterococcus faecalis (4), Enterococcus casseliflavus (1), and Enterococcus mundtii (2). Sixty-two percent of the strains were tolerant to 100 mg/L of lysozyme. Seventy two percent of the isolates were able to survive at pH 3 and most of them tolerate 2.5% bile salt concentration. Moreover, 23% of the strains displayed bile salt hydrolase activity. Interestingly, all strains were biofilm strong producers. However, their auto- and co-aggregation properties were time and pH dependent with high aggregative potentiality at pH 4.5 after 24 h. Remarkably, 48.94% of the strains showed high affinity to chloroform. The safety assessment revealed that the 47 LAB had no hemolytic activity and 64% of them lacked mucin degradation activity. All isolated strains were susceptible to gentamycin, streptomycin, tetracycline, chloramphenicol, and trimethoprim-sulfamethoxazole. Overall, 43 LAB strains showed inhibitory activity against a broad spectrum of pathogenic Gram-positive and gram-negative bacteria, fungi, and yeast. Our findings suggest that L. brevis (EVM12 and EVM14) and Ent. faecium HAS34 strains could be potential candidates for probiotics with interesting antibacterial and anti-P. expansum activities.

Combinations of Peptide-Protein Extracts from Native Probiotics Suppress the Growth of Multidrug-Resistant Staphylococcus aureus and Citrobacter freundii via Membrane Perturbation and Ultrastructural Changes

The occurrence of multidrug-resistant pathogens in the food chain causes health problems in humans, thus, research for novel antimicrobials to combat their growth is of interest. This study evaluates the antimicrobial potential of several combinations of peptide-protein extracts (PCs) consisting of peptide extracts from three native probiotic strains, Lactiplantibacillus plantarum UTNGt2, Lactococcus lactis UTNGt28, and L. plantarum UTNGt21A, alone or in combination with EDTA (ethylenediaminetetraacetic acid) against multidrug-resistant Staphylococcus aureus ATCC1026 and Citrobacter freundii UTNB3Sm1. Based on the antimicrobial assay, among the 19 tested PCs, two (PC11 and PC17) produced a greater zone of inhibition against both pathogens in vitro. Time-killing assays indicated the rapid death of S. aureus after exposure to PC11 and PC17, while C. freundii was rapidly inhibited by PC11 and PC1 (UTNGt2 only), suggesting that the inhibitory action is pathogen and dose-dependent of a particular molecule present in the extract. A marginal inhibitory effect was observed when the peptides were combined with EDTA. Transmission electron microscopy (TEM) revealed the structural membrane damage of both target strains upon interaction with individual peptide extracts. Different degrees of cell deformation, condensed cytoplasm, membrane blebbing, and ghost cell formation with visibly broken cell walls were observed in S. aureus. Likewise, the separation of the cytoplasmic membrane from the outer membrane, ghost cells, along with ovoid and deformed cells with undulated cell walls were observed for C. freundii. Furthermore, scanning electronic microscopy (SEM) analysis revealed different wrinkled and deformed cells covered by debris. A leakage of aromatic molecules was detected for both pathogens, indicating that PCs disrupted the cell wall integrity, inducing cell death. Given their inhibitory action and capacity to induce damage of the cytoplasmic membrane, the selected PCs may serve to slow bacterial growth in vitro; further research is required to prove their efficiency ex vitro to battle against food poisoning and subsequent human infection.

Bacteriocins: Properties and potential use as antimicrobials

A variety of bacteriocins originate from lactic acid bacteria, which have recently been modified by scientists. Many strains of lactic acid bacteria related to food groups could produce bacteriocins or antibacterial proteins highly effective against foodborne pathogens such as Staphylococcus aureus, Pseudomonas fluorescens, P. aeruginosa, Salmonella typhi, Shigella flexneri, Listeria monocytogenes, Escherichia coli O157:H7, and Clostridium botulinum. A wide range of bacteria belonging primarily to the genera Bifidobacterium and Lactobacillus have been characterized with different health‐promoting attributes. Extensive studies and in‐depth understanding of these antimicrobials mechanisms of action could enable scientists to determine their production in specific probiotic lactic acid bacteria, as they are potentially crucial for the final preservation of functional foods or for medicinal applications. In this review study, the structure, classification, mode of operation, safety, and antibacterial properties of bacteriocins as well as their effect on foodborne pathogens and antibiotic‐resistant bacteria were extensively studied.

Antimicrobial Cocktail Combining Specific Peptide Extracts from Native Probiotic Bacteria Hamper Adulteration of Ready-to-Eat Mango Wedges

Consumption of ready-to-eat chopped fruits sold in the streets is a concern, as such activities are outside the regulation and protection in most developing countries. Ready-to-eat mangos are commonly sold as wedges in plastic cups at ambient temperature by mobile vendors in Ecuador, thus they are prone to contamination by bacteria, which poses a safety issue of concern. This work aimed to evaluate the effect of several antimicrobial cocktails consisting of previously designed specific peptide extract combinations from two probiotic bacteria Lactobacillus plantarum UTNCys5-4 and Lactococcus lactis subsp. lactis UTNGt28, along with nisin, a commercial food additive, on mango wedges artificially inoculated with a logarithmic phase culture of a five-strain bacterial mixture (FSBM). Preliminary bacteriological analysis of mango wedges purchased from mobile vendors showed the presence of multiple antibiotic-resistant isolates such E. coli spp., Enterobacter spp., Shigella spp., Salmonella spp., along with yeasts and molds, indicating non-compliance with the food safety standards. The results revealed that two antimicrobial cocktails, T2 and T5, containing cell-free supernatant based (CFS) and precipitated peptides (PP) based cocktails from UTNCys5-4 and UTNGt28 strains applied at dose 1:3 (v/v), were the most efficient combinations that inhibited the colonization of total bacterial counts with 56.03% and 55.61% in mango wedges stored with refrigeration. The reduction of total E. coli counts was 64.93%, while Salmonella and Shigella counts were reduced by 98.09% and 97.93%, respectively, when mango wedges were treated with T5-cocktail. The commercial nisin inhibited total Salmonella spp. counts by 40.13%, while E. coli spp. and Shigella spp. diminished by 28.20% and 37.22%, respectively. Moreover, we showed that T5 but not T7 (nisin) damaged the target cell integrity, thereby eventually inhibiting their growth and reproduction. The selected antimicrobial cocktails exerted a bacteriolytic effect by killing the FSBM simultaneously in a fruit matrix and preventing their accumulation in mango wedges. Furthermore, there is a possibility of using peptide combinatorial treatments to combat drug-resistant bacteria in ready-to-eat fruits.

Peptide Extracts from Native Lactic Acid Bacteria Generate Ghost Cells and Spheroplasts upon Interaction with Salmonella enterica, as Promising Food Antimicrobials

Protecting foods from contamination applying peptides produced by lactic acid bacteria is a promising strategy to increase the food quality and safety. Interacting with the pathogen membranes might produce visible changes in shape or cell wall damage. Previously, we showed that the peptides produced by Lactobacillus plantarum UTNGt2, Lactobacillus plantarum UTNCys5-4, and Lactococcus lactis subsp. lactis UTNGt28 exhibit a broad spectrum of antibacterial activity against several foodborne pathogens in vitro. In this study, their possible mode of action against the commensal microorganism Salmonella enterica subsp. enterica ATCC51741 was investigated. The target membrane permeability was determined by detection of beta-galactosidase release from ONPG (o-nitro-phenyl-L-D-galactoside) substrate and changes in the whole protein profile indicating that the peptide extracts destroy the membrane integrity and may induce breaks in membrane proteins to some extent. The release of aromatic molecules such as DNA/RNA was detected after the interaction of Salmonella with the peptide extract. Transmission electronic microscopy (TEM) micrographs depicted at least four simultaneous secondary events after the peptide extract treatment underlying their antimicrobial actions, including morphological alterations of the membrane. Spheroplast and filament formation, vacuolation, and DNA relaxation were identified as the principal events from the Gt2 and Cys5-4 peptide extracts, while Gt28 induced the formation of ghost cells by release of cytoplasmic content, filaments, and separation of cell envelope layers. Gel retarding assays indicate that the Gt2 and Gt28 peptide extracts are clearly binding the Salmonella DNA, while Cys5-4 partially interacted with Salmonella genomic DNA. These results increased our knowledge about the inhibitory mechanism employed by several peptide extracts from native lactic acid bacteria against Salmonella. Further, we shall develop peptide-based formulation and evaluate their biocontrol effect in the food chains.

A Novel Weissella cibaria Strain UTNGt21O Isolated from Wild Solanum quitoense Fruit: Genome Sequence and Characterization of a Peptide with Highly Inhibitory Potential toward Gram-Negative Bacteria

A novel Weissella cibaria strain UTNGt21O from the fruit of the Solanum quitoense (naranjilla) shrub produces a peptide that inhibits the growth of both Salmonella enterica subsp. enterica ATCC51741 and Escherichia coli ATCC25922 at different stages. A total of 31 contigs were assembled, with a total length of 1,924,087 bases, 20 contig hits match the core genome of different groups within Weissella, while for 11 contigs no match was found in the database. The GT content was 39.53% and the genome repeats sequences constitute around 186,760 bases of the assembly. The UTNGt21O matches the W. cibaria genome with 83% identity and no gaps (0). The sequencing data were deposited in the NCBI Database (BioProject accessions: PRJNA639289). The antibacterial activity and interaction mechanism of the peptide UTNGt21O on target bacteria were investigated by analyzing the growth, integrity, and morphology of the bacterial cells following treatment with different concentrations (1×, 1.5× and 2× MIC) of the peptide applied alone or in combination with chelating agent ethylenediaminetetraacetic acid (EDTA) at 20 mM. The results indicated a bacteriolytic effect at both early and late target growth at 3 h of incubation and total cell death at 6 h when EDTA was co-inoculated with the peptide. Based on BAGEL 4 (Bacteriocin Genome Mining Tool) a putative bacteriocin having 33.4% sequence similarity to enterolysin A was detected within the contig 12. The interaction between the peptide UTNGt21O and the target strains caused permeability in a dose-, time- response manner, with Salmonella (3200 AU/mL) more susceptible than E. coli (6400 AU/mL). The results indicated that UTNGt21O may damage the integrity of the cell target, leading to release of cytoplasmic components followed by cell death. Differences in membrane shape changes in target cells treated with different doses of peptide were observed by transmission electronic microscopy (TEM). Spheroplasts with spherical shapes were detected in Salmonella while larger shaped spheroplasts with thicker and deformed membranes along with filamentous cells were observed in E. coli upon the treatment with the UTNGt21O peptide. These results indicate the promising potential of the putative bacteriocin released by the novel W. cibaria strain UTNGt21O to be further tested as a new antimicrobial substance.

Probiotic Potential and Technological Properties of Bacteriocinogenic Lactococcus lactis Subsp. Lactis UTNGt28 from a Native Amazonian Fruit as a Yogurt Starter Culture

A native Lactococcus lactis subsp. lactis UTNGt28 (GenBank accession no: MG675576.1) isolated from Amazonian fruit of the tropical Caimitillo (Chrysophyllum oliviforme) tree and the commercial strain Lactococcus lactis subsp lactis ATCC11454 (LacAT) were targeted ex vitro in whole milk in combination with Streptococcus thermophilus ATCC19258 to obtain a fermented probiotic beverage. Concomitant with cell viability determination during storage (28 days), the pH, titratable acidity, syneresis, protein and fat were evaluated. The results indicated that neither UTNGt28 nor LacAT displayed a high capacity to ferment whole milk and survive during storage; a statistically significant difference (p < 0.05) in cell viability was registered for UTNGt28 compared with LacAT when inoculated alone or in combination with S. thermophilus. A principal component analysis showed a clear difference between the yogurt formulations at day 1 and 28 of storage. The PC 1 explained 46.8% of the total variance (day 28), was loaded in the negative (-) direction with titratable acidity (% lactic acid), while the PC 2 explained 22.5% (day 1) with pH. PC 1 was loaded in the positive (+) direction with pH, cell viability, syneresis, fat and protein. Overall results indicated that UTNGt28 has the technological properties for further development of a new probiotic product.

Peptide-Based Formulation from Lactic Acid Bacteria Impairs the Pathogen Growth in Ananas Comosus (Pineapple)

Worldwide, street vending commerce has grown exponentially, representing in some countries, including Ecuador, a significant proportion of food consumed by the urban population. Pineapple is one of the common fruits sold as ready-to-eat slices by ambulant vendors in the street or on public transport at risk of contamination by various microorganisms. Previously, we selected Lactobacillus plantarum UTNCys5-4 and Lactococcus lactis subsp. lactis Gt28 strains producing peptides with high capacity to inhibit pathogen growth in vitro. In this study, the effect of different edited formulations containing a mixture of Cys5-4/Gt28 peptides was evaluated in vitro and ex vitro against a pathogenic cocktail containing E. coli (2), Salmonella (2) and Shigella (1). The growth of bacterial cocktail co-inoculated with cell-free supernatant containing peptides (formulation T1) and precipitated peptides (formulation T6), in a ratio of Cys5-4/Gt28:1:1 (v/v), results in a decrease of total cell viability with 1.85 and 1.2 log CFU/mL orders of magnitude at 6 h of incubation. About the same decrease (1.9 log CFU/g) was observed when pineapple slices artificially inoculated with the pathogenic cocktail were coated with T1 formulation, indicating the capacity to diminish simultaneous pathogens in situ, thus demonstrating its great biological control and protection. However, the E. coli cell counts reduced by 2.08 log CFU/g while Salmonella and Shigella cell counts reduced by 1.43 and 1.91 log CFU/g, respectively, at 5 days of refrigeration. In the untreated pineapple slices, the total cell density was maintained during storage, suggesting the adaptation of the pathogens to the fruit matrix. The peptide-based formulation exerted a bacteriolytic mode of action inducing pathogenic cell death. The results indicate that coating pineapple slices with peptide-based formulation is a promising approach to protect them from further contamination by microbial spoilage as well as an alternative to increase the food safety.