Listeria monocytogenes in the Food Processing Environment

Early detection and population dynamics of Listeria monocytogenes in naturally contaminated drains from a meat processing plant

Listeria monocytogenes, a significant foodborne pathogen, often contaminates ready-to-eat foods through cross-contamination in food processing environments, and floor drains represent one of the most common sites of persistence. Subtyping of L. monocytogenes from food processing plants for the purpose of source tracking is usually performed on a single colony obtained after selective enrichment. This study investigates the temporal variation and population dynamics of L. monocytogenes in drains, focusing on the diversity of L. monocytogenes and the impact of the resident microbiota. Six different drains in a meat processing plant were each sampled four times over a period of 8 weeks and subjected to two-step selective enrichment in Half Fraser and Full Fraser broths. The clonal complexes (CCs) of at least 20 individual L. monocytogenes isolates from each positive sample (460 isolates in total) were determined using either the GenoListeria Multiplex qPCR assay or whole genome sequencing (WGS). The microbiota in drains and enrichment cultures was analyzed by 16S rRNA gene amplicon sequencing and metagenomic or quasimetagenomic sequencing. L. monocytogenes was detected in the majority of samples and four different CCs were identified - CC9, CC11 (ST451), CC121 and CC8 - with up to three CCs in the same sample and with different CCs dominating in different drains. The same clones of CC9, CC11, and CC121 had persisted in the facility for 3-5 years. The composition of the drain microbiota remained relatively stable over time, with Pseudomonas, Acinetobacter, Janthinobacterium, Chryseobacterium, Staphylococcus, and Sphingomonas as the most commonly identified genera. There were no apparent differences in the microbial genera present in L. monocytogenes positive and negative drains or samples. The study highlights the use of techniques such as qPCR and quasimetagenomics for monitoring and controlling the risk of L. monocytogenes contamination in processing environments.

Behaviour of Listeria monocytogenes and Yersinia enterocolitica during beef dry-aging up to 60 days

The Commission delegated Regulation (EU) 2024/1141 states that beef dry aging can be performed in a dedicated cabinet, at a surface temperature of -0.5 to 3 °C, maximum relative humidity of 85 %, air speed of 0.2-0.5 m/s and for a maximum of 35 days. Food business operators can apply different process parameters, even longer aging times, as long as they demonstrate to the competent authority that the aged meat is microbiologically safe. To support food business operators in the implementation of longer dry aging processes, in this study we investigated the behaviour of Listeria monocytogenes and Yersinia enterocolitica during beef dry aging in a patented cabinet up to 60 days. Beef loins were experimentally spiked with a mean load of 4.4 and 3.9 Log10 CFU/g of Listeria monocytogenes and Yersinia enterocolitica respectively and stored at 1 °C, relative humidity 78 % and ventilation 2.0 m/s for 60 days. L. monocytogenes counts did not significantly change during the dry aging process, while Y. enterocolitica significantly increased up to 3 Log10 CFU/g during the aging process and then tended to decrease at 60 days. The results of this study showed that beef dry aging at 1 ± 2 °C, relative humidity 78 ± 7 %, ventilation 2.0 m/s for 60 days does not support the growth of L. monocytogenes. On the contrary, the same process supported the growth of Y. enterocolitica up to 40 days in two of the three tested batches, showing that the impact of dry aging on the behaviour of this pathogen deserves further studies.

Antibiotic Resistance in Fermented Foods Chain: Evaluating the Risks of Emergence of Enterococci as an Emerging Pathogen in Raw Milk Cheese

Fermented foods, particularly fermented dairy products, offer significant health benefits but also present serious concerns. Probiotic bacteria, such as lactic acid bacteria (LAB), found in these foods have been strongly linked to the selection and dissemination of antibiotic resistance genes (ARGs). This study aims to examine the potential risks associated with fermented foods, despite their importance in human nutrition, by analyzing the entire production chain from raw material acquisition to storage. Focusing on cheese production as a key fermented food, the study will investigate various aspects, including dairy farm management, milk acquisition, milk handling, and the application of good manufacturing practices (GMP) and good hygiene practices (GHP) in cheese production. The findings of this review highlight that ARGs found in LAB are similar to those observed in hygiene indicator bacteria like E. coli and pathogens like S. aureus. The deliberate use of antibiotics in dairy farms and the incorrect use of disinfectants in cheese factories contribute to the prevalence of antibiotic-resistant bacteria in cheeses. Cheese factories, with their high frequency of horizontal gene transfer, are environments where the microbiological diversity of raw milk can enhance ARG transfer. The interaction between the raw milk microbiota and other environmental microbiotas, facilitated by cross-contamination, increases metabolic communication between bacteria, further promoting ARG transfer. Understanding these bacterial and ARG interactions is crucial to ensure food safety for consumers.

[Molecular characterization of Listeria monocytogenes isolates from human and food sources in Argentina, 2018-2023]

Human listeriosis is an infectious disease caused by Listeria monocytogenes. The invasive form of this disease leads to a high rate of hospitalizations and fatality. The main mode of transmission is through contaminated ready-to-eat foods such as dairy, vegetables and meat products. The knowledge of the diversity and population dynamics of isolates collected from human and food sources is essential for the detection of clusters and the identification of common sites of infection. The aim of this study was the molecular characterization of L. monocytogenes isolates in Argentina. We sequenced a total of 63 isolates, 35 from human and 28 from food sources, collected between 2018 and 2023. Our genomic study divided the isolates into two lineages, four serogroups, 17 sequence types and 15 clonal complexes (CCs). The hypervirulent clone CC1 (lineage I; serogroup IVb) predominated in human and food samples. The phylogenomic analysis showed a high and possible epidemiological relationship between isolates from human and/or food sources, suggesting the presence of transmission chains in our country. These findings highlight the need to strengthen genomic surveillance of L. monocytogenes in Argentina. The identification of geographic distribution and characteristics of predominant and emerging clones from human and food sources might help to focus action plans and public health policies better directed at the control and prevention of listeriosis.

Nanocoating and its application as antimicrobials in the food industry: A review

Nanocoatings are ultra-thin layers on the nanoscale (<100 nm) that are deposited on the substrate to improve their properties and functionality. These nanocoatings provide significant advantages compared to traditional coating, including stain resistance, antimicrobial and antioxidant activities, odor control and delivery of active agents, and liquid repellence properties. In the food industry, nanocoating is widely used in the food packaging sector. In this regard, nanocoating offers antimicrobials and antioxidant properties to active food packaging by incorporating active bioactive compounds into materials used in already existing packaging. The application of nanocoating is applied to these kinds of food packaging with nano coating to improve shelf life, safety, and quality of food packaging. In smart/intelligent packaging, the active packaging coating is promising food packaging, which is designed by releasing preservatives and nanocoating as an antimicrobial, antifungal, antioxidant, barrier coating, and self-cleaning food contact surfaces. In addition, nanocoating can be used for food contact surfaces, kitchen utensils, and food processing equipment to create antimicrobial, antireflective, and dirt-repellent properties. These are critical properties for food processing, especially for meat and dairy processing facilities, which can reduce biofilm formation and prevent cross-contamination. Recently, appreciable growth in the development of the application of nanocoating as edible films for coating food products has emerged to improve food safety issues. In this regard, much scientific research in the area of nanocoating fruits and vegetables, and other food products was performed to address food safety issues. Hence, this promising technology can be a great addition to the agricultural and food industries. Thus, this review addresses the most relevant information about this technology and the applications of nanocoating in the food industry.

Listeria monocytogenes, a silent foodborne pathogen in Ecuador

Listeria monocytogenes is a foodborne pathogen that can produce serious, even fatal, infections. Among other foods, it can be found in unpasteurized dairy and ready-to-eat products. Surveillance of L. monocytogenes is of great interest since sources of infection are difficult to determine due to the long incubation period, and because the symptoms of listeriosis are similar to other diseases. We performed a genomic study of L. monocytogenes isolated from fresh cheeses and clinical samples from Ecuador. Sixty-five isolates were evaluated and sequenced, 14 isolates from cheese samples and 20 from clinical listeriosis cases from the National Institute of National Institute of Public Health Research, and 31 isolates from artisanal cheese samples from 8 provinces. All isolates exhibited heterogeneous patterns of the presence of pathogenicity islands. All isolates exhibited at least 4 genes from LIPI-1, but all references (26 L. monocytogenes closed genomes available in the NCBI database) showed the complete island, which encompasses 5 genes but is present in only two Ecuadorian isolates. Most isolates lacked gene actA. Genes from LIPI-2 were absent in all isolates. LIPI-3 and LIPI-4 were present in only a few references and isolates. With respect to the stress survival islets, our samples either presented SSI-1 or SSI-F2365, except for one isolate that presented SSI-F2365 and also one gene from SSI-1. None of the samples presented SSI-2. The predominant ST (sequence type) was ST2 (84.62% 55/65), and the only ST found in food (93.33% 42/45) and clinical samples (65% 13/20). Isolates were not grouped according to their sampling origin, date, or place in a phylogenetic tree obtained from the core alignment. The presence of ST2 in food and clinical samples, with high genomic similarity, suggests a foodborne infection risk linked to the consumption of fresh cheeses in Ecuador.

Investigating antimicrobial peptide RI12 (K3W) as an effective bio-preservative against Listeria monocytogenes: a major foodborne pathogen

Due to public apprehension regarding the use of chemical preservatives to prevent food spoilage and food-borne diseases, it is imperative to identify natural alternatives such as antimicrobial peptides as a potential solution. The study aimed at evaluating the effectiveness of the antimicrobial peptide RI12 (K3W) against Listeria monocytogenes. RI12 (K3W) exhibited potent antimicrobial properties, with a minimum inhibitory concentration and minimum bactericidal concentration of 16 µM and 32 µM, respectively. The time-kill assay revealed a consistent reduction in bacterial viability at 8, 16, and 24 h of study. Cytotoxicity testing on mammalian cells demonstrated no apparent change in morphology or cell count. Investigating how well it worked in a food matrix to replicate real-world conditions showed a significant decrease in the bacterial count. The study underscores the potential of RI12 (K3W) as a safe and effective antimicrobial against L. monocytogenes that might also serve as an alternative to chemical preservatives.

Combining High-Pressure Processing and Supercritical Carbon Dioxide for Inactivation of Listeria innocua

The effect of high-pressure treatment with supercritical CO2 on the inactivation of Listeria innocua in a fish soup was investigated. The soup was inoculated with L. innocua, packaged in modified atmosphere with 50:50 or 95:5 CO2:N2, high-pressure processed (300, 350, 400 and 600 MPa, 2 min) under subcritical (T < 304 K) or supercritical conditions (T > 304 K) and stored at 4 °C for up to 53 days. Treatment at 400 and 600 MPa had a significant (p < 0.05) effect on L. innocua under both supercritical and subcritical conditions. In contrast, pressurization at 350 MPa and supercritical conditions were needed to significantly (p < 0.05) inactive L. innocua. Increased levels of CO2 in the headspace significantly (p < 0.05) reduced the bacterial load during processing, and supercritical conditions had a significant (p < 0.01) interaction with both CO2 levels and pressure. Increased storage time gave significantly increased levels of L. innocua at 400 and 600 MPa. In addition, high levels of CO2 significantly decreased (p < 0.001) growth. However, 350 MPa under supercritical conditions seemed to set the L. innocua in a permanent lag phase, with slow and steadily decreasing numbers of bacteria during storage. All the design variables resulted in significant inactivation of L. innocua, and supercritical conditions combined with high levels of CO2 inhibited the recovery of L. innocua to a large degree.

Listeria monocytogenes Strains Persisting in a Meat Processing Plant in Central Italy: Use of Whole Genome Sequencing and In Vitro Adhesion and Invasion Assays to Decipher Their Virulence Potential

In this study, we used both a WGS and an in vitro approach to study the virulence potential of nine Listeria monocytogenes (Lm) strains belonging to genetic clusters persisting in a meat processing plant in Central Italy. The studied clusters belonged to CC1-ST1, CC9-ST9, and CC218-ST2801. All the CC1 and CC218 strains presented the same accessory virulence genes (LIPI-3, gltA, gltB, and aut_IVb). CC1 and CC9 strains presented a gene profile similarity of 22.6% as well as CC9 and CC218 isolates. CC1 and CC218 showed a similarity of 45.2% of the same virulence profile. The hypervirulent strains of lineage I (CC1 and CC218) presented a greater ability to adhere and invade Caco-2 cells than hypovirulent ones (CC9). CC1 strains were significantly more adhesive and invasive compared with CC9 and CC218 strains, although these last CCs presented the same accessory virulence genes. No statistically significant difference was found comparing CC218 with CC9 strains. This study provided for the first time data on the in vitro adhesiveness and invasiveness of CC218-ST2801 and added more data on the virulence characteristics of CC1 and CC9. What we observed confirmed that the ability of Lm to adhere to and invade human cells in vitro is not always decipherable from its virulence gene profile.

Growth performance of Listeria monocytogenes and background microbiota from mushroom processing environments

Interaction between Listeria monocytogenes and resident background microbiota may occur in food processing environments and may influence the survival of this pathogen in a factory environment. Therefore the aim of this study was to characterize the growth performance of microbiota isolated from the processing environments of frozen sliced mushrooms, and to investigate the competitive performance of L. monocytogenes when co-cultured with accompanying environmental microbiota. Acinetobacter, Enterobacteriaceae, Lactococcus and Pseudomonas were the most prominent background microbiota isolated from the processing environment of frozen sliced mushrooms. All individual microbiota strains were able to grow and form biofilm in filter-sterilized mushroom medium, with the mannitol-consumers Raoultella and Ewingella as top performers, reaching up to 9.6 and 9.8 log CFU/mL after 48 h incubation at room temperature. When L. monocytogenes mushroom isolates were co-cultured with the microbiota strains, L. monocytogenes counts ranged from 7.6 to 8.9 log CFU/mL after 24 h of incubation, while counts of the microbiota strains ranged from 5.5 to 9.0 log CFU/mL. Prolonged incubation up to 48 h resulted in further increase of L. monocytogenes counts when co-cultured with non-acidifying species Pseudomonas and Acinetobacter reaching 9.1 to 9.2 log CFU/mL, while a decrease of L. monocytogenes counts reaching 5.8 to 7.7 log CFU/mL was observed in co-culture with Enterobacteriaceae and acidifying Lactococcus representatives. In addition, L. monocytogenes grew also in spent mushroom media of the microbiota strains, except in acidified spent media of Lactococcus strains. These results highlight the competitive ability of L. monocytogenes during co-incubation with microbiota in fresh and in spent mushroom medium, indicative of its invasion and persistence capacity in food processing factory environments.

Evidence of Virulent Multi-Drug Resistant and Biofilm-Forming Listeria Species Isolated from Various Sources in South Africa

Listeriosis is a foodborne disease caused by Listeria monocytogenes species and is known to cause severe complications, particularly in pregnant women, young children, the elderly, and immunocompromised individuals. The aim of this study was to investigate the presence of Listeria species in food and water using both biochemical and species-specific PCR analysis. L. monocytogenes isolates were further screened for the presence of various antibiotic resistance, virulence, and biofilm-forming determinants profiles using phenotypic and genotypic assays. A total of 207 samples (composed of meat, milk, vegetables, and water) were collected and analyzed for presence of L. monocytogenes using species specific PCR analysis. Out of 267 presumptive isolates, 53 (19.85%) were confirmed as the Listeria species, and these comprised 26 L. monocytogenes, 3 L. innocua, 2 L. welshimeri, and 1 L. thailandensis. The remaining 21 Listeria species were classified as uncultured Listeria, based on 16SrRNA sequence analysis results. A large proportion (76% to 100%) of the L. monocytogenes were resistant to erythromycin (76%), clindamycin (100%), gentamicin (100%), tetracycline (100%), novobiocin (100%), oxacillin (100%), nalidixic acid (100%), and kanamycin (100%). The isolates revealed various multi-drug resistant (MDR) phenotypes, with E-DA-GM-T-NO-OX-NA-K being the most predominant MDR phenotypes observed in the L. monocytogenes isolates. The virulence genes prfA, hlyA, actA, and plcB were detected in 100%, 68%, 56%, and 20% of the isolates, respectively. In addition, L. monocytogenes isolates were capable of forming strong biofilm at 4 °C (%) after 24 to 72 h incubation periods, moderate for 8% isolates at 48 h and 20% at 72 h (p < 0.05). Moreover, at 25 °C and 37 °C, small proportions of the isolates displayed moderate (8−20%) biofilm formation after 48 and 72 h incubation periods. Biofilm formation genes flaA and luxS were detected in 72% and 56% of the isolates, respectively. These findings suggest that proper hygiene measures must be enforced along the food chain to ensure food safety.

Surveillance of Listeria monocytogenes: Early Detection, Population Dynamics, and Quasimetagenomic Sequencing during Selective Enrichment

In this study, we addressed different aspects regarding the implementation of quasimetagenomic sequencing as a hybrid surveillance method in combination with enrichment for early detection of Listeria monocytogenes in the food industry. Different experimental enrichment cultures were used, comprising seven L. monocytogenes strains of different sequence types (STs), with and without a background microbiota community. To assess whether the proportions of the different STs changed over time during enrichment, the growth and population dynamics were assessed using dapE colony sequencing and dapE and 16S rRNA amplicon sequencing. There was a tendency of some STs to have a higher relative abundance during the late stage of enrichment when L. monocytogenes was enriched without background microbiota. When coenriched with background microbiota, the population dynamics of the different STs was more consistent over time. To evaluate the earliest possible time point during enrichment that allows the detection of L. monocytogenes and at the same time the generation of genetic information that enables an estimation regarding the strain diversity in a sample, quasimetagenomic sequencing was performed early during enrichment in the presence of the background microbiota using Oxford Nanopore Technologies Flongle and Illumina MiSeq sequencing. The application of multiple displacement amplification (MDA) enabled detection of L. monocytogenes (and the background microbiota) after only 4 h of enrichment using both applied sequencing approaches. The MiSeq sequencing data additionally enabled the prediction of cooccurring L. monocytogenes strains in the samples.

IMPORTANCE We showed that a combination of a short primary enrichment combined with MDA and Nanopore sequencing can accelerate the traditional process of cultivation and identification of L. monocytogenes. The use of Illumina MiSeq sequencing additionally allowed us to predict the presence of cooccurring L. monocytogenes strains. Our results suggest quasimetagenomic sequencing is a valuable and promising hybrid surveillance tool for the food industry that enables faster identification of L. monocytogenes during early enrichment. Routine application of this approach could lead to more efficient and proactive actions in the food industry that prevent contamination and subsequent product recalls and food destruction, economic and reputational losses, and human listeriosis cases.

Marine algae as efficacious bioresources housing antimicrobial compounds for preserving foods - A review

Certain synthetic chemicals used in global food industries eliminate pathogenic food microbes and prevent spoilage. Nevertheless, their toxicity precludes human consumption. This phenomenon has made scientific fraternity to look for alternative antimicrobial compounds from natural resources. In recent times, marine algae have been illustrated to be potent and rich sources of antimicrobial agents as chemical replacements for applications in food. Identifying novel antimicrobial agents from natural resources have become a worldwide research with immense significance. Marine algae are now considered as one of the most inexhaustible and unexposed sources of antimicrobial agents due to their abundance in seawaters and renewability. This review elaborated on marine algal antimicrobial agents against foodborne pathogens, mode of action and cumulated the prospective use of algal compounds in active food packaging as a natural food preservative. Due to poor solubility, unpleasant odor and ineffectiveness of plant derived antimicrobial agents against Gram-negative bacteria, researchers opted for marine algae, an ideal candidate to be used as natural food preservatives. This article elaborates and summarizes the efficient bioactive molecules in marine algae and their possible application in food preservation to extend shelf life of foods without causing any toxicity. In conclusion, marine algae have potential antimicrobial property against food pathogens and have more advantages than other natural sources of antimicrobial agents.

The Opposing Role of Propionate in Modulating Listeria monocytogenes Intracellular Infections

Listeria monocytogenes is a Gram-positive, intracellular pathogen responsible for the highly fatal foodborne illness listeriosis. Establishing intracellular infections requires the coordinated expressions of a variety of virulence factors, such as the pore-forming toxin listeriolysin O (LLO), in response to various intra- and extracellular signals. For example, we previously reported that L. monocytogenes differentially modulated LLO production in response to exogenous propionate, a short chain fatty acid either used in salt form as a human food ingredient or produced endogenously by gut microbial fermentation. Therefore, propionate is likely a continuously present signal throughout the L. monocytogenes transmission and infection process. However, little is known about the role of propionate in modulating L. monocytogenes-host interactions. Here we investigated the impact of propionate treatment on L. monocytogenes intracellular infections using cell culture infection models. Propionate treatment was performed separately on L. monocytogenes or host cells before or during infections to better distinguish pathogen-versus-host responses to propionate. Intracellular CFU in RAW264.7 macrophages and plaque diameters in L-fibroblasts were measured as proxy for intracellular infection outcomes. Nitrite levels and cellular morphology were also measured to assess host responses to propionate. We found that propionate pretreatment of anaerobic, but not aerobic, L. monocytogenes significantly enhanced subsequent intracellular infections in both cell types and nitrite production by infected macrophages. Propionate treatment of uninfected macrophages significantly altered cell morphology, seen by longer cells and greater migration, and reduced nitrite concentration in activated macrophages. Treatment of macrophages with propionate prior to or during infections significantly inhibited intracellular growth of L. monocytogenes, including those pre-treated with propionate. These results showcased an opposing effect of propionate on L. monocytogenes intracellular infections and strongly support propionate as an important signaling molecule for both the pathogen and the host cell that can potentially alter the outcome of L. monocytogenes-host interactions.

Management of Listeria monocytogenes on Surfaces via Relative Air Humidity: Key Role of Cell Envelope

Although relative air humidity (RH) strongly influences microbial survival, its use for fighting surface pathogens in the food industry has been inadequately considered. We asked whether RH control could destroy Listeria monocytogenes EGDe by envelope damage. The impact of dehydration in phosphate-buffered saline (PBS) at 75%, 68%, 43% and 11% RH on the bacterial envelope was investigated using flow cytometry and atomic force microscopy. Changes after rehydration in the protein secondary structure and peptidoglycan were investigated by infrared spectroscopy. Complementary cultivability measurements were performed by running dehydration–rehydration with combinations of NaCl (3–0.01%), distilled water, city water and PBS. The main results show that cell membrane permeability and cell envelope were greatly altered during dehydration in PBS at 68% RH followed by rapid rehydration. This damage led cells to recover only 67% of their initial volume after rehydration. Moreover, the most efficient way to destroy cells was dehydration and rehydration in city water. Our study indicates that rehydration of dried, sullied foods on surfaces may improve current cleaning procedures in the food industry.

Characterisation of Listeria monocytogenes food-associated isolates to assess environmental fitness and virulence potential

The ability of Listeria monocytogenes isolates to survive within the food production environment (FPE), as well as virulence, varies greatly between strains. There are specific genetic determinants that have been identified which can strongly influence a strains ability to survive in the FPE and/or within human hosts. In this study, we assessed the FPE fitness and virulence potential, including efficacy of selected hygiene or treatment intervention, against 52 L. monocytogenes strains isolated from various food and food environment sources. Phenotypic tests were performed to determine the minimum inhibitory concentration of cadmium chloride and benzalkonium chloride and the sensitivities to five clinically relevant antibiotics. A genomic analysis was also performed to identify resistance genes correlating to the observed phenotypic resistance profiles, along with genetic determinants of interest which may elude to the FPE fitness and virulence potential. A transposon element containing a novel cadmium resistance gene, cadA7, a Tn916 variant insert in the hypervariable Listeria genomic island 1 region and an LGI2 variant were identified. Resistance to cadmium and disinfectants was prevalent among isolates in this study, although no resistance to clinically important antimicrobials was observed. Potential hypervirulent strains containing full length inlA, LIPI-1 and LIPI-3 were also identified in this study. Cumulatively, the results of this study show a vast array of FPE survival and pathogenicity potential among food production-associated isolates, which may be of concern for food processing operators and clinicians regarding L. monocytogenes strains colonising and persisting within the FPE, and subsequently contaminating food products then causing disease in at risk population groups.

Growth and Survival of Attached Listeria on Lettuce and Stainless Steel Varies by Strain and Surface Type

ABSTRACT

The foodborne pathogen Listeria monocytogenes lives as a saprophyte in nature and can adhere to and grow on surfaces as diverse as leaves, sediment, and stainless steel. To discern the mechanisms used by L. monocytogenes for attachment and growth on various surfaces, we studied interactions between the pathogen on lettuce and stainless steel. A panel of 24 strains (23 L. monocytogenes and 1 Listeria innocua) were screened for attachment and growth on lettuce at 4 and 25°C and on stainless steel at 10 and 37°C. Overnight growth of attached cells resulted in a 0- to 3-log increase on lettuce, depending on the strain and the temperature. Among the worst-performing strains on lettuce were two from a large cantaloupe outbreak, indicating that factors important for interactions with cantaloupe may be different from those required on lettuce tissue. Strains that grew the best on lettuce belonged to serotypes 1/2a, 1/2b, and 4b and were from cheese, potatoes, and water-sediment near produce fields. Confocal microscopy of L. monocytogenes tagged with constitutively expressed green fluorescent protein indicated associations with the cut edges and veins of lettuce leaves. On stainless steel coupons, there was a 5- to 7-log increase at 10°C after 7 days and a 4- to 7-log increase at 37°C after 40 h. Statistically, surface growth on stainless steel was better for serotype 1/2a than for serotype 4b strains, even though certain serotype 4b strains grew well on the coupons. The latter included strains that originated from produce and water-sediment. Some strains were fit in both environments, whereas others showed variability between the two different surfaces. Further analysis of these strains should reveal molecular factors needed for adherence and surface growth of L. monocytogenes on different biotic and abiotic surfaces.

HIGHLIGHTS

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.

Mutant and Recombinant Phages Selected from In Vitro Coevolution Conditions Overcome Phage-Resistant Listeria monocytogenes

Bacteriophages (phages) are currently available for use by the food industry to control the foodborne pathogen Listeria monocytogenes Although phage biocontrols are effective under specific conditions, their use can select for phage-resistant bacteria that repopulate phage-treated environments. Here, we performed short-term coevolution experiments to investigate the impact of single phages and a two-phage cocktail on the regrowth of phage-resistant L. monocytogenes and the adaptation of the phages to overcome this resistance. We used whole-genome sequencing to identify mutations in the target host that confer phage resistance and in the phages that alter host range. We found that infections with Listeria phages LP-048, LP-125, or a combination of both select for different populations of phage-resistant L. monocytogenes bacteria with different regrowth times. Phages isolated from the end of the coevolution experiments were found to have gained the ability to infect phage-resistant mutants of L. monocytogenes and L. monocytogenes strains previously found to be broadly resistant to phage infection. Phages isolated from coinfected cultures were identified as recombinants of LP-048 and LP-125. Interestingly, recombination events occurred twice independently in a locus encoding two proteins putatively involved in DNA binding. We show that short-term coevolution of phages and their hosts can be utilized to obtain mutant and recombinant phages with adapted host ranges. These laboratory-evolved phages may be useful for limiting the emergence of phage resistance and for targeting strains that show general resistance to wild-type (WT) phages.IMPORTANCE Listeria monocytogenes is a life-threatening bacterial foodborne pathogen that can persist in food processing facilities for years. Phages can be used to control L. monocytogenes in food production, but phage-resistant bacterial subpopulations can regrow in phage-treated environments. Coevolution experiments were conducted on a Listeria phage-host system to provide insight into the genetic variation that emerges in both the phage and bacterial host under reciprocal selective pressure. As expected, mutations were identified in both phage and host, but additionally, recombination events were shown to have repeatedly occurred between closely related phages that coinfected L. monocytogenes This study demonstrates that in vitro evolution of phages can be utilized to expand the host range and improve the long-term efficacy of phage-based control of L. monocytogenes This approach may also be applied to other phage-host systems for applications in biocontrol, detection, and phage therapy.

Listeria monocytogenes environmental sampling program in ready-to-eat processing facilities: A practical approach

Listeria monocytogenes is a foodborne pathogen that is frequently found in the environment. It can easily enter food processing environments and contaminate food, potentially causing public health issues. Food business operators (FBOs) are responsible for the control of L. monocytogenes in the food processing environment, particularly in facilities producing ready-to-eat food. The design and implementation of an effective environmental monitoring program (EMP) for L. monocytogenes is an integral part of controlling L. monocytogenes. An effective EMP, including all aspects from sampling, to analysis, to data interpretation, to implementation of corrective actions (including food disposition), is a tool that will help with identification and control of L. monocytogenes contamination. It should be used in conjunction with end product testing, not as a replacement for it. An EMP should be specifically designed for a particular facility on a case-by-case risk-based approach, by a food safety team within the facility. It should be reviewed regularly (at least every 6 months) and verified for its effectiveness. The control of L. monocytogenes in the food industry involves the full commitment of management and of all personnel involved with the safety of foods placed on the market, thus reducing the risk of listeriosis to consumers. Several regulatory and guidance documents provide recommendations for designing aspects of an effective L. monocytogenes EMP. However, a comprehensive review of the key components of an EMP in a single document is lacking. The objective of the present review is to provide FBOs with a practical guide to design, implementation, and verification of an EMP tailored by the food safety team for each food business.

The public health risk posed by Listeria monocytogenes in frozen fruit and vegetables including herbs, blanched during processing

A multi-country outbreak of Listeria monocytogenes ST6 linked to blanched frozen vegetables (bfV) took place in the EU (2015-2018). Evidence of food-borne outbreaks shows that L. monocytogenes is the most relevant pathogen associated with bfV. The probability of illness per serving of uncooked bfV, for the elderly (65-74 years old) population, is up to 3,600 times greater than cooked bfV and very likely lower than any of the evaluated ready-to-eat food categories. The main factors affecting contamination and growth of L. monocytogenes in bfV during processing are the hygiene of the raw materials and process water; the hygienic conditions of the food processing environment (FPE); and the time/Temperature (t/T) combinations used for storage and processing (e.g. blanching, cooling). Relevant factors after processing are the intrinsic characteristics of the bfV, the t/T combinations used for thawing and storage and subsequent cooking conditions, unless eaten uncooked. Analysis of the possible control options suggests that application of a complete HACCP plan is either not possible or would not further enhance food safety. Instead, specific prerequisite programmes (PRP) and operational PRP activities should be applied such as cleaning and disinfection of the FPE, water control, t/T control and product information and consumer awareness. The occurrence of low levels of L. monocytogenes at the end of the production process (e.g. < 10 CFU/g) would be compatible with the limit of 100 CFU/g at the moment of consumption if any labelling recommendations are strictly followed (i.e. 24 h at 5°C). Under reasonably foreseeable conditions of use (i.e. 48 h at 12°C), L. monocytogenes levels need to be considerably lower (not detected in 25 g). Routine monitoring programmes for L. monocytogenes should be designed following a risk-based approach and regularly revised based on trend analysis, being FPE monitoring a key activity in the frozen vegetable industry.

Predictive Modeling of Microbial Behavior in Food

Microorganisms can contaminate food, thus causing food spoilage and health risks when the food is consumed. Foods are not sterile; they have a natural flora and a transient flora reflecting their environment. To ensure food is safe, we must destroy these microorganisms or prevent their growth. Recurring hazards due to lapses in the handling, processing, and distribution of foods cannot be solved by obsolete methods and inadequate proposals. They require positive approach and resolution through the pooling of accumulated knowledge. As the industrial domain evolves rapidly and we are faced with pressures to continually improve both products and processes, a considerable competitive advantage can be gained by the introduction of predictive modeling in the food industry. Research and development capital concerns of the industry have been preserved by investigating the plethora of factors able to react on the final product. The presence of microorganisms in foods is critical for the quality of the food. However, microbial behavior is closely related to the properties of food itself such as water activity, pH, storage conditions, temperature, and relative humidity. The effect of these factors together contributing to permitting growth of microorganisms in foods can be predicted by mathematical modeling issued from quantitative studies on microbial populations. The use of predictive models permits us to evaluate shifts in microbial numbers in foods from harvesting to production, thus having a permanent and objective evaluation of the involving parameters. In this vein, predictive microbiology is the study of the microbial behavior in relation to certain environmental conditions, which assure food quality and safety. Microbial responses are evaluated through developed mathematical models, which must be validated for the specific case. As a result, predictive microbiology modeling is a useful tool to be applied for quantitative risk assessment. Herein, we review the predictive models that have been adapted for improvement of the food industry chain through a built virtual prototype of the final product or a process reflecting real-world conditions. It is then expected that predictive models are, nowadays, a useful and valuable tool in research as well as in industrial food conservation processes.

Inhibition of L. monocytogenes Biofilm Formation by the Amidase Domain of the Phage vB_LmoS_293 Endolysin

Listeria monocytogenes is a ubiquitous Gram-positive bacterium that is a major concern for food business operators because of its pathogenicity and ability to form biofilms in food production environments. Bacteriophages (phages) have been evaluated as biocontrol agents for L. monocytogenes in a number of studies and, indeed, certain phages have been approved for use as anti-listerial agents in food processing environments (ListShield and PhageGuard Listex). Endolysins are proteins produced by phages in the host cell. They cleave the peptidoglycan cell wall, thus allowing release of progeny phage into the environment. In this study, the amidase domain of the phage vB_LmoS_293 endolysin (293-amidase) was cloned and expressed in Escherichia. coli (E. coli). Muralytic activity at different concentrations, pH and temperature values, lytic spectrum and activity against biofilms was determined for the purified 293-amidase protein. The results showed activity on autoclaved cells at three different temperatures (20 °C, 37 °C and 50 °C), with a wider specificity (L. monocytogenes 473 and 3099, a serotype 4b and serogroup 1/2b-3b-7, respectively) compared to the phage itself, which targets only L. monocytogenes serotypes 4b and 4e. The protein also inhibits biofilm formation on abiotic surfaces. These results show the potential of using recombinant antimicrobial proteins against pathogens in the food production environment.

Use of bacteriophages as biological control agents in horticulture

Bacterial diseases in horticultural settings or infestation of fresh produce with human pathogenic bacteria can constitute a serious public health risk. To control horticultural bacterial diseases, chemical control strategies have traditionally been used, such as the application of bactericides and copper-based products, which resulted in development of resistance in bacteria against these agents. Moreover, the use of such chemical preventative measures on fresh produce can detrimentally affect human, animal and ecosystem health. Bacteriophages have been used to control pathogenic bacteria since the 1920s due to their specificity against host bacteria, as well as their ability to survive and infect their host without detrimental effects to the surrounding environments. As a result, their targeted host specific applications in horticultural settings can be of interest to growers as well as to the consumers. In this laboratory report, the efficacy of a bacteriophage cocktail when applied to fresh herbs inoculated with Escherichia coli was determined. Significant (P ≤ 0.001) reductions in E. coli colony forming units were observed in phage treated herb samples compared to counts in the control. These findings suggest that bacteriophage present as an alternative biocontrol for E. coli in horticulture.