Listeria monocytogenes in ready-to-eat vacuum and modified atmosphere packaged meat and fish products of Estonian origin at retail level

Prevalence of Listeria monocytogenes and other Listeria species in fish, fish products and fish processing environment: A systematic review and meta-analysis

Fish and seafood products are one of the most common causes of listeriosis in humans. A systematic review and meta-analysis were conducted using scientific literature to summarize available data on the prevalence of Listeria monocytogenes and Listeria sp. in fish, fish products and fish processing environment. Meta-analysis models were used to estimate the mean prevalence of the pathogen and to compare prevalence among the most popular fishes. Data from a total of 14,496 samples were analysed. Pooled prevalence of L. monocytogenes and Listeria sp. in raw fish was 5.8 % (95 % CI: 4.7-6.9) and 12.2 % (95 % CI: 8.9-15.5) respectively. Significantly higher results were observed for ready-to-eat products, occurrence of L. monocytogenes and Listeria sp. was 14.5 % (95 % CI: 11.1-18.0) and 21.7 % (95 % CI: 11.8-31.6) respectively. Moreover, meta-regression was performed to investigate changes in L. monocytogenes and Listeria sp. occurrence over the years and it shows a downward trend for both raw fish and RTE products. The results of the present study provide useful epidemiological information about the contamination level and distribution of Listeria sp. and L. monocytogenes among raw fish, fish products and fish processing environment.

The Effect of Fruit and Berry Pomaces on the Growth Dynamics of Microorganisms and Sensory Properties of Marinated Rainbow Trout

Plant pomaces in suitable forms (powders, extracts) can be used in foods of animal origin to increase the nutritional value and safety of these foods. In the present study, water extracts of apple, black currant, rhubarb and tomato pomaces were used in fish marinade solutions to evaluate their effect on the growth dynamics of microorganisms and the growth potential of Listeria monocytogenes by challenge testing. The results showed that mesophilic aerobic microorganisms, Pseudomonas spp., yeasts and moulds remained at acceptable levels throughout the predetermined storage period. The challenge test results showed that the overall growth potential of L. monocytogenes in all marinated rainbow trout samples remained at ≤0.5 log10 cfu/g during the study period, and none of the marinated fish samples supported the growth of L. monocytogenes. In addition, the effect of fruit and berry pomaces on the sensory properties of marinated rainbow trout samples was evaluated. The results revealed that it is possible to effectively use fruit and berry pomaces in marinated fish products, ensuring food safety, high microbiological quality, acceptable sensory characteristics and a sufficiently long shelf life of the products.

Slight Temperature Deviation during a 56-Day Storage Period Does Not Affect the Microbiota of Fresh Vacuum-Packed Pork Loins

It is profitable to export fresh meat overseas, where it is often regarded as a premium commodity. Meeting this demand for fresh meat, however, necessitates long export times, during which uncontrolled temperature increases can affect the microbiological quality of the meat and thereby, reduce shelf life or compromise food safety. To study the impact of temperature deviations on microbial community composition and diversity, we used 16S rRNA gene sequencing for Listeria monocytogenes and Salmonella spp. detection to describe the surface microbiota of eight batches of vacuum-packed loins stored at -1.5 °C (control) for 56 days and subjected to a 2 °C or 10 °C temperature deviation for a few hours (mimicking problems regularly encountered in the industry) at day 15 or 29. The presence of pathogens was negligible. The applied temperature deviations were not associated with different microbiota. Sequencing analysis showed the presence of Yersinia, an unexpected pathogen, and relative abundance increased in the groups subjected to temperature deviations. Over time, Lactobacillales_unclassified genus became the main constituent of the microbiota of vacuum-packed pork loins. Although the microbiota of the eight batches appeared similar at the beginning of storage, differences were revealed after 56 days, suggesting unequal aging of the microbiota.

Consumer attitudes and perceptions towards chilled ready-to-eat foods: a multi-national study

Understanding consumers' behavior and their handling of high-risk foods at home is essential for reducing the number of foodborne illnesses. This study shows the results of a cross-national analysis of consumers' perception from nine countries, and the identification of customers' clusters and its characteristics in order to understand customers' behavior, and to build safe chilled ready-to-eat (RTE) foods prevention strategies. The cluster analysis resulted in two clusters: (1) "Precautious consumers" characterized by the orientation towards pre-packed RTE foods, with consumers mainly coming from Bosnia and Herzegovina, India, Poland, Portugal, Spain, and Turkey. Their attitudes and self-reported practices may be categorized as less risky in terms of food-borne illnesses connected with the consumption of RTE foods; (2) "Unconcerned consumers" preferred cutting and slicing RTE foods freshly at the point of purchase, usually sold at the delicatessen department in a supermarket or at open markets. Those consumers mostly came from Croatia, Serbia and Slovenia and their attitudes and self-reported practices were riskier. These results allow a better understating of what characterizes consumers of RTE foods in different countries.

Vacuum-Packed Steak Tartare: Prevalence of Listeria monocytogenes and Evaluation of Efficacy of ListexTM P100

Steak tartare is a raw, ready-to-eat meal popular in European countries, the safety of which is often discussed due to the risk of foodborne illness. The aim of this study was to determine the prevalence of Listeria monocytogenes in vacuum-packed steak tartare from retailers in the Czech Republic, characterize the strains obtained by typing methods and to evaluate the efficacy of Listex^TM P100 against L. monocytogenes artificially inoculated into steak tartare samples. The prevalence of L. monocytogenes was 55% and 17 isolates belonging mostly to serotype 1/2a were obtained. Altogether 11 sequence types and 11 clonal complexes were assigned based on the whole genome sequencing (WGS) signifying the high diversity of L. monocytogenes isolates obtained. Core genome multi-locus sequence typing (cgMLST) did not confirm an epidemiological connection with human cases of listeriosis. The efficacy of Listex^TM P100 treatment at concentrations of 10⁸ and 10⁹ PFU/g on artificially inoculated beef steak tartare samples was not efficient. Based on the results of this study, steak tartare from retailers can be considered as a source of L. monocytogenes that remains a challenge to the food industry.

Retrospective Use of Whole-Genome Sequencing Expands the Multicountry Outbreak Cluster of Listeria monocytogenes ST1247

Listeria monocytogenes sequence type 1247 clonal complex 8 caused a prolonged multicountry outbreak in five EU countries: Denmark, Estonia, Finland, France, and Sweden. A total of 22 disease cases were identified with onset of symptoms between July 2014 and February 2019. Five patients died due to, or with, the disease. The retrospective analysis of L. monocytogenes isolate VLTRLM2013 revealed the presence of an outbreak-related strain (cgMLST type L2-SL8-ST1247-CT4158) in ready-to-eat fish product more than a year prior to the first outbreak-related cases. Reference outbreak strain and VLTRLM2013 strain were compared using core genome and whole-genome multilocus sequence typing analyses. Genomic level differences of the persistent L. monocytogenes strains associated with a prolonged multicountry foodborne listeriosis outbreak are described. It was concluded that the persistent nature of the multicountry outbreak-related L. monocytogenes strain VLTRLM2013 together with stress island, virulence, and antibiotic resistance genes could potentially be the determining factors for the extensive and prolonged outbreak affecting five European Union countries. Our results support the systematic application of whole-genome sequencing in food and public health surveillance and further encourages its wide adoption.

Tolerance of Listeria monocytogenes to biocides used in food processing environments

Listeria monocytogenes is a foodborne pathogen that causes a life-threatening disease in humans known as listeriosis. Contamination of food during processing is the main route of transmission of Listeria monocytogenes. Therefore, biocides play a crucial role in food processing environments as they act as the first line of defense in the prevention and control of L. monocytogenes. Residues of biocides may be present at sublethal concentrations after disinfection. This, unfortunately, subjects L. monocytogenes to selection pressure, giving rise to tolerant strains, which pose a threat to food safety and public health. This review will give a brief description of L. monocytogenes, the clinical manifestation, treatment of listeriosis as well as recently recorded outbreaks. The article will then discuss the current literature on the ability of L. monocytogenes strains to tolerate biocides especially quaternary ammonium compounds as well as the mechanisms of tolerance towards biocides including the activation of efflux pump systems.

Anti-listerial activity of microalgal fatty acid methyl esters and their possible applications as chicken marinade

Fatty acid methyl esters (FAMEs) from marine microalgae have been reported to possess antimicrobial activities against several Gram positive and Gram negative bacteria, but a majority of them needs to be explored. The objective of this study was to investigate the antibacterial activity, mechanism of FAMEs from selected marine microalgae against Listeria monocytogenes, and to elucidate its efficacy in food model. The minimum inhibitory concentration of FAMEs was calculated to be 155 μg/mL for Chromulina sp. and 162 μg/mL for Nannochloropsis sp. against L. monocytogenes. Time-killing kinetics showed that FAMEs efficiently inhibited the growth of L. monocytogenes in a time and concentration dependent manner. The mechanism of action of FAMEs was studied by analysing its effects at a MIC on the cellular metabolism, membrane permeability, and membrane integrity of L. monocytogenes. Transmission Electron Microscopy (TEM) results showed that cells exposed to FAMEs showed damaged cell membrane structure with leakage of the internal contents in the cells of L. monocytogenes. Fluorescence microscopy images showed that L. monocytogenes cells treated with FAMEs showed high dead cell population corresponding with propidium iodide positive cells. Furthermore, FAMEs significantly down regulated quorum sensing and biofilm related genes (DegU, FlaE, and FlaD). In vivo therapeutic potential of FAMEs revealed improved Caenorhabditis elegans survival and reduced intestinal colonization during L. monocytogenes infection. Growth of listeria was abolished in chicken meat during the cold storage of 9 days when the samples were pre-treated with FAMEs. These results suggest anti-L. monocytogenes activity of FAMEs and elucidated its use in food control of chicken meat at refrigerated conditions.

Growth Potential of Listeria monocytogenes in Three Different Salmon Products

Cold smoked salmon and sushi salmon have been implicated in outbreaks of listeriosis. We performed challenge tests and a durability study with Listeria monocytogenes on different salmon products to determine the growth potential of this important food-borne pathogen. Data from the challenge test showed a significant growth potential of L. monocytogenes on all of the tested salmon products, with faster growth in sushi salmon than in cold smoked salmon. In identical products that were naturally contaminated at low levels, the durability study did not confirm a high growth potential, possibly due to interactions with competing microflora. The injection of sodium lactate (NaL) at a high concentration (30%) into cold smoked salmon significantly reduced the growth potential of L. monocytogenes. In addition to good manufacturing practices, the injection of higher concentrations of NaL may therefore be a useful additional hurdle to prevent growth of L. monocytogenes to high numbers in the tested salmon products.

Purification, characterization and mode of action of enterocin, a novel bacteriocin produced by Enterococcus faecium TJUQ1

Enterococcus faecium TJUQ1 with high bacteriocin-producing ability was isolated from pickled Chinese celery. In this study, enterocin TJUQ1 was purified by ammonium sulfate precipitation, reversed-phase chromatography (Sep-Pak C₈) and cation-exchange chromatography. The activity of the purified bacteriocin was 44,566.41 ± 874.69 AU/mg, which corresponds to a purification fold of 35.89 ± 2.34. The molecular mass was 5520 Da by MALDI-TOF MS and Tris-Tricine SDS-PAGE. The result of LC-MS/MS showed that the bacteriocin shared 59.15% identity with enterocin produced by E. faecium GN (accession no. O34071). PCR amplification revealed that E. faecium TJUQ1 possesses a gene encoding enterocin B with 60% identity to enterocin B. Circular dichroism (CD) spectroscopy showed that the molecular conformation was 32.6% helix, 19.5% beta, 12.9% turn and 35.0% random. The stability of enterocin TJUQ1 was measured. After exposure at 121 °C for 15 min, the residual antimicrobial activity of enterocin TJUQ1 was 85.95 ± 1.32%. The antimicrobial activity of enterocin TJUQ1 was still active over a pH range of 3-11. Enterocin TJUQ1 was inactivated after exposure to proteolytic enzymes but was not inactivated by lipase or amylase. These results showed that enterocin TJUQ1 was a novel class II bacteriocin. Enterocin TJUQ1 showed wide antibacterial activity against food-borne gram-negative and gram-positive pathogens, such as Staphylococcus aureus, Listeria monocytogenes, Escherichia coli and Salmonella enterica. The MIC was 5.26 ± 0.24 μg/mL against L. monocytogenes CMCC 1595. SEM and TEM were used to observe the changes in the morphological and intracellular organization of L. monocytogenes CMCC 1595 cells treated with enterocin TJUQ1. The results demonstrated that enterocin TJUQ1 increased extracellular electrical conductivity, facilitated pore formation, triggered the release of UV-absorbing materials, ATP and LDH, and even caused cell lysis in L. monocytogenes CMCC 1595 cells. Based on the characterization, the wide inhibitory spectrum and mode of action determined so far, enterocin TJUQ1 is a potential preservative for the food industry.

Reverse transcription – loop-mediated isothermal amplification assay for the rapid detection of pathogenic Listeria monocytogenes in meat products

This study reports the use of reverse transcription – loop-mediated isothermal amplification (RT–LAMP) to detect Listeria monocytogenes in meat. The assay was designed to target the iap gene of L. monocytogenes, to which four primers, recognizing six distinct iap sites, were designed. We optimized the RT–LAMP conditions and established the following optimal systems: 60 min, 63 °C, 2.0 mmol/L MgSO4, 1.0 mol/L betaine, 2.0 mmol/L dNTPs, 320 U/mL Bst DNA polymerase, 0.4 μmol/L outer primers, and 0.8 μmol/L inner primers. The RT–LAMP amplification products were identified by a visible white Mg2P2O7 precipitate or electrophoresis on a 2% agarose gel. RT–LAMP has a sensitivity of 7.3 × 101 CFU/mL, which is 2-fold higher than that of LAMP. When commercially available raw meat samples (including beef, pork, mutton, and rabbit) were analyzed simultaneously with RT–LAMP and the Chinese National Standard GB 4789.30-2016, their abilities to detect L. monocytogenes were the same. Samples containing L. monocytogenes killed by 15 psi at 121 °C for 15 min were used to confirm the specificity of RT–LAMP for live microorganisms. Thus, we used RT–LAMP to efficiently detect L. monocytogenes in meat products.

Distribution and Persistence of Listeria monocytogenes in a Heavily Contaminated Poultry Processing Facility

We studied the colonization and distribution of Listeria monocytogenes in a heavily contaminated poultry processing plant over a 1-year period. A total of 180 nonfood contact surfaces, 70 food contact surfaces, 29 personnel, and 40 food samples were analyzed. L. monocytogenes isolates were subtyped by PCR serotyping, pulsed-field gel electrophoresis (PFGE), and multilocus sequence typing. L. monocytogenes was detected in samples collected at every visit to the plant, and 43.8% (visit 4) to 65.6% (visit 7) of samples were positive, for an overall prevalence of 55.2%. The deboning area had the highest prevalence of positive samples (83.3%), and the processing area had the highest diversity of PFGE types. Ninety percent of the final products were positive for L. monocytogenes. Most of the isolates belonged to well-known persistent L. monocytogenes sequence types (ST9 and ST121). This study illustrates a well-established L. monocytogenes contamination problem in a poultry processing plant associated with a generalized failure of the food safety system as a whole. These findings reflect the potential for L. monocytogenes contamination when the food safety and quality management system is unsatisfactory, as described in the present study. It is essential to revise food safety and quality management systems to eliminate L. monocytogenes from food processing facilities, to control the entrance of sporadic sequence types, and to prevent L. monocytogenes spread within such facilities, especially in those premises with higher L. monocytogenes prevalence in the environment and final food products.

Prevalence and Numbers of Listeria monocytogenes in Various Ready-to-Eat Foods over a 5-Year Period in Estonia

HIGHLIGHTS

This study is a significant contribution to L. monocytogenes risk analysis. L. monocytogenes was found in 3.6% of 30,016 analyzed RTE food samples. A food safety criterion of 100 CFU/g was exceeded for 0.3% of the RTE food samples. Salted and cold-smoked fish products were found to be potentially high-risk foods.

Processing plant and machinery sanitation and hygiene practices associate with Listeria monocytogenes occurrence in ready-to-eat fish products

Listeria monocytogenes causes the foodborne illness listeriosis, which exhibits high fatality among people in risk groups. The incidence of listeriosis has increased in Europe, which raises concerns about L. monocytogenes occurrence in foodstuffs. Ready-to-eat seafood products are considered particularly risky vehicles. Poor hygiene at processing facilities predisposes them to L. monocytogenes contamination, which can be controlled by stringent self-checking system measures. We examined the association of fish-processing plant operational and hygiene practices with the occurrence of L. monocytogenes in vacuum-packaged gravad (cold-salted) and cold-smoked salmon and rainbow trout products. Product sampling of 21 fish-processing plants was carried out, and operational procedures relating to L. monocytogenes control were surveyed using an in-depth risk assessment questionnaire. L. monocytogenes occurred only in sliced and mainly in gravad products of seven fish-processing plants. Shortages in preventive measures were discovered predominantly among the L. monocytogenes positive fish-processing plants. Using generalized linear modeling, we identified the following features associated with L. monocytogenes product contamination: the number of processing machines, deficiencies in the processing environment and machinery sanitation, and staff movement from areas of low toward high hygiene. Furthermore, performing frequent periodic thorough sanitation alongside everyday sanitation practices associated with a decreased risk of product contamination.

Assessment of Microbial Populations in the Manufacture of Vacuum-Packaged Ready-to-Eat Roast Beef and in a Related Production Plant

Some microbiological criteria were monitored for 6 months in vacuum-packaged roast beef (15 production batches), raw beef (10 batches), and other meat products (12 batches) produced in an Italian small to medium-size enterprise. Fifty-five environmental swab samples also were analyzed. The main bacterial groups were identified by cultural methods according to International Organization for Standardization standards. Listeria monocytogenes was enumerated with the most-probable-number protocol, and species identification was confirmed with a specific PCR assay. Immediately after vacuum packaging, all ready-to-eat (RTE) products had low mean aerobic colony counts (<10² to 2.4 × 10² CFU g^-1), anaerobic colony counts (1.6 to 6.5 × 10¹ CFU g^-1), Enterobacteriaceae counts (1.1 to 1.4 × 10¹ CFU g^-1), and Escherichia coli counts (generally below the detection limit). Nevertheless, the prevalence of L. monocytogenes in these samples was 3.7%. In roast beef samples, the aerobic and anaerobic colony counts reached unacceptable levels (>10⁶ CFU g^-1) after 14 days of refrigerated storage. Because the prevalence of L. monocytogenes increased to 13.3% during storage, a substantial reduction in the shelf life of these products is recommended. Surfaces without direct contact with food (floors and drains) had the highest mean counts for aerobic colonies (8.0 × 10³ to 9.5 × 10⁵ CFU/cm²), anaerobic colonies (2.9 × 10³ to 3.2 × 10⁴ CFU/cm²), Enterobacteriaceae (1.5 × 10¹ to 8.4 × 10¹ CFU/cm²), and E. coli (6.0 to 7.7 CFU/cm²). The levels of L. monocytogenes on direct food contact surfaces were below the detection limit, but more than 25% of floor samples were contaminated. These results reveal the persistence of L. monocytogenes in food processing environments, although at very low levels, posing a high risk of postcooking recontamination for RTE products. To improve hygienic conditions and reduce cross-contamination, an increase in operator awareness and a reassessment of surface sanitization protocols are needed.

Prevalence and Antibiotic Resistance of Listeria monocytogenes Isolated from Ready-to-Eat Foods in Turkey

The aim of the present study was the determination of the prevalence and antibiotic resistance of L. monocytogenes in ready-to-eat (RTE) foods in Ankara, Turkey. In order to detect and isolate L. monocytogenes from 201 RTE food samples, the EN ISO 11290:1 method was used. All isolates were identified using the polymerase chain reaction. The strains were also confirmed by the detection of the hemolysin gene (hlyA). The overall prevalence of L. monocytogenes was 8.5% among the food samples. Seventeen L. monocytogenes strains were examined by the disk diffusion assay for their resistance to 23 antibiotics. All strains were susceptible to erythromycin, clarithromycin, streptomycin, gentamicin, vancomycin, imipenem, trimethoprim, and chloramphenicol, while all strains were resistant to nalidixic acid, ampicillin, penicillin G, linezolid, and clindamycin. The higher resistance was found against oxacillin (94.1%), kanamycin (76.5%), levofloxacin (70.6%), and teicoplanin (64.7%), followed by amoxicillin/clavulanic acid (53.0%), rifampicin (47.1%), and ciprofloxacin (35.3%). A lower incidence of resistance was observed against tetracycline (5.9%), meropenem (5.9%), and trimethoprim/sulfamethoxazole (17.7%). All isolates were multidrug resistant showing resistance to at least three antibiotic classes. High L. monocytogenes prevalence among analyzed RTE foods represents a high risk for public health. Our findings show a high prevalence of L. monocytogenes in RTE foods in Turkey. More effective control strategies for L. monocytogenes are needed to reduce both prevalence and resistance of L. monocytogenes in Turkish RTE foods.

The Distribution of Listeria in Pasture-Raised Broiler Farm Soils Is Potentially Related to University of Vermont Medium Enrichment Bias toward Listeria innocua over Listeria monocytogenes

The occurrence of Listeria monocytogenes has been widely investigated in the poultry production chain from the processing plant to the final product. However, limited data are available on Listeria species, including Listeria monocytogenes, in the poultry farm environment. Therefore, fecal and soil samples from 37 pastured poultry flocks from 10 all-natural farms over 3 years were assessed to determine the prevalence and diversity of Listeria within these alternative poultry farm environments using standard cultural and molecular methods. Listeria species were isolated in 15% of poultry farm samples and included Listeria innocua (65.7%), L. monocytogenes (17.4%), and Listeria welshimeri (15.1%). Additional multiplex PCR serotyping showed group 1/2a-3a to be the most dominant L. monocytogenes serovar group. Based on these results, monoculture growth experiments were conducted on four Listeria soil isolates (three L. monocytogenes isolates representing the three recovered serovar groups and one L. innocua isolate) to determine if culture medium [tripticase soy broth (TSB) and University of Vermont modified Listeria enrichment broth (UVM)], inoculum concentration (102 or 105 CFU/ml), or incubation temperature (20, 30, and 42°C) differentially affected these Listeria species. Overall, very few significant growth differences were observed between the behavior of the three L. monocytogenes isolates (representing the three recovered serovar groups) under the growth conditions tested. Alternatively, at 30°C in UVM with the lower inoculum concentration, the L. innocua isolate had a significantly shorter lag phase than the L. monocytogenes isolates. In coculture growth studies under these same incubation conditions, the lag phase of L. innocua and L. monocytogenes was similar, but the final concentration of L. innocua was significantly higher than L. monocytogenes. However, cocultures in UVM for high inoculum concentration did not show preferential growth of L. innocua over L. monocytogenes. These results indicate that the use of UVM as an enrichment medium may preferentially allow L. innocua to outcompete L. monocytogenes at low concentrations, biasing the Listeria prevalence from these farm samples toward L. innocua and potentially underreporting the presence of L. monocytogenes in these environments.

Prevalence, genetic diversity and antimicrobial resistance of Listeria monocytogenes isolated from fresh and smoked fish in Poland

A total of 57 out of 301 (18.9%) fresh and smoked fish samples in Poland were positive for Listeria monocytotgenes. The bacteria were most frequently identified in fresh and smoked salmon (32.0% and 33.8% respectively) as well as in fresh cod (31.8%). Only three samples of smoked salmon were contaminated with the bacteria above 100 CFU/g. Four molecular serogroups were identified and the most prevalent, 1/2a-3a (40 isolates; 70.2%), was present in samples from both marine (33 strains; 71.7%) and freshwater fish (7 isolates; 63.6%). Similar duality of prevalence was observed only for L. monocytogenes of 1/2b-3b-7 serogroup (14 strains; 24.6%), which was identified in 11 (23.9%) marine and 3 (27.3%) freshwater fish. All isolates harboured 10 virulence-associated genes (inlA, inlB, inlC, inlJ, lmo2672, plcA, plcB, hlyA, actA, and mpl) and most of them (56; 98.2%) also possessed the flaA marker. Several strains displayed resistance to oxacillin (33; 57.9%), ceftriaxone (18; 31.6%), or clindamycin (5; 8.8%), and two isolates of serogroup 1/2a-3a showed multiresistance to all three. Genetic subtyping showed the presence of different pulsotypes belonging to six PFGE clusters. The obtained results provide useful information regarding fish contamination with L. monocytogenes which may have implications for public health.