Biocontrol of Listeria on cantaloupes with lactic acid bacteria

Pre-harvest biocontrol of Listeria and Escherichia coli O157 on lettuce and spinach by lactic acid bacteria

Recent outbreaks linked to contaminated leafy greens underline the need for identifying effective natural approaches to improve produce safety at pre-harvest level. Lactic acid bacteria (LAB) have been evaluated as biocontrol agents in food products. In this study, the efficacy of a cocktail of LAB including Lactococcus lactis, Lactiplantibacillus plantarum, Lactobacillus johnsonii, and Lactobacillus acidophilus as pre-harvest biocontrol agents against Listeria and Escherichia coli O157 on lettuce and spinach was investigated. Bacterial pathogens L. monocytogenes and E. coli O157:H7 and the non-pathogenic surrogates L. innocua and E. coli O157:H12 were used to spray-inoculate cultivars of lettuce and spinach grown in growth chamber and in field, respectively. Inoculated plants were spray-treated with water or a cocktail of LAB. On day 0, 3, and 5 post-inoculation, four samples from each group were collected and bacterial populations were determined by serial dilution and spiral plating on selective agars. LAB treatment exhibited an immediate antimicrobial efficacy against L. monocytogenes and E. coli O157:H7 on "Green Star" lettuce by ~2 and ~ 1 log reductions under growth chamber conditions, respectively (P < 0.05). The effect of LAB against E. coli O157:H7 on "New Red Fire" lettuce remained effective during the 5-day period in growth chamber (P < 0.05). Treatment of LAB delivered an effective bactericidal effect against E. coli O157:H12 immediately after application on the field-grown lettuce plants (P < 0.05). Approximately 1 log L. innocua reduction was observed on "Matador" and "Palco" spinach on day 5 (P < 0.05). Results of this study support that LAB could potentially be applied as biocontrol agents for controlling Listeria and E. coli O157 contamination on leafy greens at the pre-harvest level.

Ability of Two Strains of Lactic Acid Bacteria To Inhibit Listeria monocytogenes by Spot Inoculation and in an Environmental Microbiome Context

We evaluated the ability of two strains of lactic acid bacteria (LAB) to inhibit L. monocytogenes using spot inoculation and environmental microbiome attached-biomass assays. LAB strains (PS01155 and PS01156) were tested for antilisterial activity toward 22 phylogenetically distinct L. monocytogenes strains isolated from three fruit packing environments (F1, F2, and F3). LAB strains were tested by spot inoculation onto L. monocytogenes lawns (10⁸ and 10⁷ CFU/mL) and incubated at 15, 20, 25, or 30°C for 3 days. The same LAB strains were also cocultured at 15°C for 3, 5, and 15 days in polypropylene conical tubes with L. monocytogenes and environmental microbiome suspensions collected from F1, F2, and F3. In the spot inoculation assay, PS01156 was significantly more inhibitory toward less concentrated L. monocytogenes lawns than more concentrated lawns at all the tested temperatures, while PS01155 was significantly more inhibitory toward less concentrated lawns only at 15 and 25°C. Furthermore, inhibition of L. monocytogenes by PS01156 was significantly greater at 15°C than higher temperatures, whereas the temperature did not have an effect on the inhibitory activity of PS01155. In the assay using attached environmental microbiome biomass, L. monocytogenes concentration was significantly reduced by PS01156, but not PS01155, when cocultured with microbiomes from F1 and F3 and incubated for 3 days at 15°C. Attached biomass microbiota composition was significantly affected by incubation time but not by LAB strain. This study demonstrates that LAB strains that may exhibit inhibitory properties toward L. monocytogenes in a spot inoculation assay may not maintain antilisterial activity within a complex microbiome.

IMPORTANCEListeria monocytogenes has previously been associated with outbreaks of foodborne illness linked to consumption of fresh produce. In addition to conventional cleaning and sanitizing, lactic acid bacteria (LAB) have been studied for biocontrol of L. monocytogenes in food processing environments that are challenging to clean and sanitize. We evaluated whether two specific LAB strains, PS01155 and PS01156, can inhibit the growth of L. monocytogenes strains in a spot inoculation and in an attached-biomass assay, in which they were cocultured with environmental microbiomes collected from tree fruit packing facilities. LAB strains PS01155 and PS01156 inhibited L. monocytogenes in a spot inoculation assay, but the antilisterial activity was lower or not detected when they were grown with environmental microbiota. These results highlight the importance of conducting biocontrol challenge tests in the context of the complex environmental microbiomes present in food processing facilities to assess their potential for application in the food industry.

The Complex Role of Lactic Acid Bacteria in Food Detoxification

Toxic ingredients in food can lead to serious food-related diseases. Such compounds are bacterial toxins (Shiga-toxin, listeriolysin, Botulinum toxin), mycotoxins (aflatoxin, ochratoxin, zearalenone, fumonisin), pesticides of different classes (organochlorine, organophosphate, synthetic pyrethroids), heavy metals, and natural antinutrients such as phytates, oxalates, and cyanide-generating glycosides. The generally regarded safe (GRAS) status and long history of lactic acid bacteria (LAB) as essential ingredients of fermented foods and probiotics make them a major biological tool against a great variety of food-related toxins. This state-of-the-art review aims to summarize and discuss the data revealing the involvement of LAB in the detoxification of foods from hazardous agents of microbial and chemical nature. It is focused on the specific properties that allow LAB to counteract toxins and destroy them, as well as on the mechanisms of microbial antagonism toward toxigenic producers. Toxins of microbial origin are either adsorbed or degraded, toxic chemicals are hydrolyzed and then used as a carbon source, while heavy metals are bound and accumulated. Based on these comprehensive data, the prospects for developing new combinations of probiotic starters for food detoxification are considered.

Biocontrol of Listeria monocytogenes and Salmonella enterica on Fresh Strawberries with Lactic Acid Bacteria During Refrigerated Storage

Small fruits such as strawberries have been increasingly implicated in outbreaks of foodborne illnesses. Salmonella enterica and Listeria monocytogenes may contaminate strawberries leading to potential public health concern. The objective of this study was to investigate the efficacy of a combined lactic acid bacteria (LAB) treatment of Lactobacillus plantarum and Pediococcus pentosaceus for controlling S. enterica and L. monocytogenes on fresh strawberries during storage at 4°C and 10°C. Strawberries purchased from a local grocery store were separately dip inoculated with Salmonella Newport, Salmonella Tennessee, Salmonella Thompson, or a three-strain cocktail of L. monocytogenes at ∼9 log colony-forming unit (CFU)/mL and allowed to air-dry for 1 h. Inoculated strawberries were then divided into three groups: (1) Control (pathogen alone), (2) Man, Rogosa, Sharpe (MRS) control (dipping in MRS broth), and (3) LAB treatment (dipping in a LAB cocktail of L. plantarum and P. pentosaceus). After treatment, strawberries were stored at 4°C or 10°C for 7 d in vented clamshell containers. Surviving Listeria, Salmonella, and LAB populations on strawberries were determined on 0, 1, 3, and 7 d post-treatment by plating on selective agars. At both 4°C and 10°C, LAB treatment significantly decreased Listeria populations by up to 2 log CFU/g compared to controls after 3 d of storage (p < 0.05). When strawberries were stored at 4°C, LAB treatment reduced ∼2.5 log, ∼2.7 log, and ∼2.9 log CFU/g in Salmonella Newport, Salmonella Tennessee, and Salmonella Thompson populations, respectively, compared to control on day 7. Similarly, ∼2.5 log CFU/g reductions of Salmonella populations were observed with LAB treatment at 10°C on day 7. LAB populations remained at ∼7.5 log CFU/g levels on strawberries at both temperatures throughout the entire study. Results of this study suggest that a combined LAB treatment can be potentially used as biocontrol agents against Salmonella and L. monocytogenes on strawberries at postharvest level.

Antimicrobial Potential of Food Lactic Acid Bacteria: Bioactive Peptide Decrypting from Caseins and Bacteriocin Production

Lactic acid bacteria (LAB) potential in the food industry and in the biotechnological sector is a well-established interest. LAB potential in counteracting especially food-borne infections has received growing attention, but despite being a road full of promises is yet poorly explored. Furthermore, the ability of LAB to produce antimicrobial compounds, both by ribosomal synthesis and by decrypting them from proteins, is of high value when considering the growing impact of multidrug resistant strains. The antimicrobial potential of 14 food-derived lactic acid bacteria strains has been investigated in this study. Among them, four strains were able to counteract Listeria monocytogenes growth: Lactococcus lactis SN12 and L. lactis SN17 by high lactic acid production, whereas L. lactis 41FLL3 and Lactobacillus sakei I151 by Nisin Z and Sakacin P production, respectively. Strains Lactococcus lactis MG1363, Lactobacillus rhamnosus 17D10 and Lactobacillus helveticus 4D5 were tested and selected for their potential attitude to hydrolyze caseins. All the strains were able to release bioactive peptides with already known antimicrobial, antihypertensive and opioid activities. These features render these strains or their bioactive molecules suitable for use in food as biocontrol agents, or as nutraceutical supplements to treat mild disorders such as moderate hypertension and children insomnia. These results highlight once again that LAB potential in ensuring food safety, food nutraceutical value and ultimately in favoring human health is still underexplored and underexploited.