Implicated Food Products for Listeriosis and Changes in Serovars of Listeria monocytogenes Affecting Humans in Recent Decades

Molecular Characteristics and Virulence Profile of Clinical Listeria monocytogenes Isolates in Northern Taiwan, 2009-2019

Listeria monocytogenes is a critical foodborne pathogen that causes severe invasive and noninvasive diseases and is associated with high mortality. Information on the prevalence of L. monocytogenes infections in Taiwan is very limited. This study aimed to analyze the molecular epidemiological surveillance and virulence gene distribution of 176 human clinical L. monocytogenes isolates collected between 2009 and 2019 in northern Taiwan. Our results showed that the isolates belonged to 4 serogroups (IIa, IIb, IVb, and IIc), with most isolates in serogroups IIa (81/176, 46%) and IIb (71/176, 40.3%). Multilocus sequence typing analysis revealed 18 sequence types (STs) and 13 clonal complexes (CCs). Eighty-four percent of all isolates belonged to six STs: CC87-ST87 (40/176, 22.7%), CC19-ST378 (36/176, 19.9%), CC155-ST155 (28/176, 15.5%), CC1-ST710 (16/176, 8.8%), CC5-ST5 (16/176, 8.8%), and CC101-ST101 (11/176, 6.1%). Furthermore, our analysis showed the distributions of four Listeria pathogenicity islands (LIPI) among all isolates. LIPI-1 and LIPI-2 existed in all isolates, whereas LIPI-3 and LIPI-4 only existed in specific STs and CCs. LIPI-3 existed in the STs, CC1-ST710, CC3-ST3, CC288-ST295, and CC191-ST1458, whereas LIPI-4 could be found in the STs, CC87-ST87 and CC87-ST1459. Strains containing LIPI-3 and LIPI-4 are potentially hypervirulent; thus, 68/176 isolates (39.1%) collected in this study were potentially hypervirulent. Since L. monocytogenes infections are considered highly correlated with diet, molecular epidemiological surveillance of Listeria in food is important; continued surveillance will provide critical information to prevent foodborne diseases.

Controlling of foodborne pathogen biofilms on stainless steel by bacteriophages: A systematic review and meta-analysis

This study investigates the potential of using bacteriophages to control foodborne pathogen biofilms on stainless steel surfaces in the food industry. Biofilm-forming bacteria can attach to stainless steel surfaces, rendering them difficult to eradicate even after a thorough cleaning and sanitizing procedures. Bacteriophages have been proposed as a possible solution, as they can penetrate biofilms and destroy bacterial cells within, reducing the number of viable bacteria and preventing the growth and spread of biofilms. This systematic review and meta-analysis evaluates the potential of bacteriophages against different biofilm-forming foodborne bacteria, including Cronobacter sakazakii, Escherichia coli, Staphylococcus aureus, Pseudomonas fluorescens, Pseudomonas aeruginosa and Listeria monocytogenes. Bacteriophage treatment generally causes a significant average reduction of 38 % in biofilm formation of foodborne pathogens on stainless steel. Subgroup analyses revealed that phages are more efficient in long-duration treatment. Also, applying a cocktail of phages is 1.26-fold more effective than applying individual phages. Phages at concentrations exceeding 107 PFU/ml are significantly more efficacious in eradicating bacteria within a biofilm. The antibacterial phage activity decreases substantially by 3.54-fold when applied at 4 °C compared to temperatures above 25 °C. This analysis suggests that bacteriophages can be a promising solution for controlling biofilms in the food industry.

Whole Genome Sequence Analysis of Listeria monocytogenes Isolates Obtained from the Beef Production Chain in Gauteng Province, South Africa

The study used whole-genome sequencing (WGS) and bioinformatics analysis for the genomic characterization of 60 isolates of Listeria monocytogenes obtained from the beef production chain (cattle farms, abattoirs, and retail outlets) in Gauteng province, South Africa. The sequence types (STs), clonal complexes (CCs), and the lineages of the isolates were determined using in silico multilocus sequence typing (MLST). We used BLAST-based analyses to identify virulence and antimicrobial genes, plasmids, proviruses/prophages, and the CRISPR-Cas system. The study investigated any association of the detected genes to the origin in the beef production chain of the L. monocytogenes isolates. Overall, in 60 isolates of Listeria monocytogenes, there were seven STs, six CCs, forty-four putative virulence factors, two resistance genes, one plasmid with AMR genes, and three with conjugative genes, one CRISPR gene, and all 60 isolates were positive for proviruses/prophages. Among the seven STs detected, ST204 (46.7%) and ST2 (21.7%) were the most prominent, with ST frequency varying significantly (p < 0.001). The predominant CC detected were CC2 (21.7%) and CC204 (46.7%) in lineages I and II, respectively. Of the 44 virulence factors detected, 26 (across Listeria Pathogenicity Islands, LIPIs) were present in all the isolates. The difference in the detection frequency varied significantly (p < 0.001). The two AMR genes (fosX and vga(G)) detected were present in all 60 (100%) isolates of L. monocytogenes. The only plasmid, NF033156, was present in three (5%) isolates. A CRISPR-Cas system was detected in six (10%), and all the isolates carried proviruses/prophages. The source and sample type significantly affected the frequencies of STs and virulence factors in the isolates of L. monocytogenes. The presence of fosX and vga(G) genes in all L. monocytogenes isolates obtained from the three industries of the beef production chain can potentially cause therapeutic implications. Our study, which characterized L. monocytogenes recovered from the three levels in the beef production chain, is the first time genomics was performed on this type of data set in the country, and this provides insights into the health implications of Listeria.

A comprehensive systematic review and meta-analysis of Listeria monocytogenes prevalence in food products in South Korea

Listeria monocytogenes is a foodborne pathogen that can cause deadly severe listeriosis. While systematic review and meta-analysis are powerful tools for comprehensive analysis by pooling every related study, these approaches to L.monocytogenes contamination food have yet to be studied in South Korea. We aimed to identify high-risk L.monocytogenes foods in South Korea through a prevalence survey of retail food products for the first time. A total of 13,684 samples of 59 articles were used for meta-analysis through the systematic review, and the results were synthesized using a random-effects model considering the heterogeneity. The overall pooled prevalence was 2.26 % (95 % CI: 1.44-3.52 %). Among nine food categories, meat exhibited the highest prevalence at 8.32 % (95 % CI: 4.42-12.14 %) after sample size restriction. Specifically, a post-hoc sensitivity analysis was conducted to identify the prevalence difference among subgroups and the source of heterogeneity. Intriguingly, the analysis revealed chicken as the primary contributor to the elevated prevalence of L.monocytogenes, a key factor deriving the observed heterogeneity. This study carries significant implications for public health and food safety in Korea. Furthermore, knowledge of differences in prevalence levels in various foods will be able to be used as a predictive guideline for foodborne outbreaks.

Detection of Pathogenic Serogroups and Virulence Genes in Listeria monocytogenes Strains Isolated from Beef and Beef Products Retailed in Gauteng Province, South Africa, Using Phenotypic and Polymerase Chain Reaction (PCR)-Based Methods

South Africa recently (2017-18) experienced the largest outbreak of human listeriosis in the world caused by L. monocytogenes following the consumption of "polony," a ready-to-eat meat product. Most (59%) cases originated from Gauteng province, South Africa. As a follow-up study to the outbreak, we used standard bacteriological and molecular methods to determine the prevalence of pathogenic and virulent serogroups of L. monocytogenes in various beef and beef products retailed in Gauteng province, South Africa. The overall prevalence of Listeria spp. was 28% (112/400), comprising Listeria monocytogenes (9.3%), Listeria innocua (16.3%), and Listeria welshimeri (2.5%) (p < 0.001). It is crucial to have detected that the region (p=0.036), type of product (p=0.032), and temperature at storage (p=0.011) significantly affected the occurrence of L. monocytogenes in beef products. It is alarming that pathogenic serogroups 4b-4d-4e (51.4%) and 1/2a-3a (43.2%) were detected among the isolates of L. monocytogenes. Importantly, they were all carriers of seven virulence-associated genes (hlyA, inlB, plcA, iap, inlA, inlC, and inlJ). Our study also demonstrated that 16.7% of "polony" samples investigated were contaminated with L. monocytogenes. Considering that pathogenic and virulent L. monocytogenes contaminated beef and beef products retailed in South Africa, the food safety risk posed to consumers remains and cannot be ignored. Therefore, it is imperative to reduce the contamination of these products with L. monocytogenes during beef production, processing, and retailing to avoid future outbreaks of human listeriosis in the country.

A comparative study on the occurrence, genetic characteristics, and factors associated with the distribution of Listeria species on cattle farms and beef abattoirs in Gauteng Province, South Africa

These cross-sectional studies reported the occurrence, genetic characteristics, and factors associated with the distribution of Listeria species on cattle farms and beef abattoirs in Gauteng Province, South Africa. A total of 328 samples (faeces, feeds, silage, and drinking water) were collected from 23 cattle farms (communal, cow-calf, and feedlot), and 262 samples (faeces, carcass swabs, and effluents) from 8 beef abattoirs (low throughput and high throughput) were processed using standard bacteriological and molecular methods to detect Listeria species. The factors associated with the prevalence of Listeria species were investigated, and multiplex polymerase chain reaction (mPCR) was used to determine Listeria species, the pathogenic serogroups, and the carriage of eight virulence-associated genes by Listeria monocytogenes. The overall prevalence of Listeria species in cattle farms was 14.6%, comprising Listeria innocua (11.3%), Listeria monocytogenes (3.4%), Listeria welshimeri (0.0%) compared with 11.1%, comprising Listeria innocua (5.7%), Listeria monocytogenes (4.6%), Listeria welshimeri (0.8%) for beef abattoirs. Of the three variables (area, type of farm/abattoir, and sample type) investigated, only the sample types at abattoirs had a significant (P < 0.001) effect on the prevalence of L. innocua and L. welshimeri. The frequency of distribution of the serogroups based on 11 L. monocytogenes isolated from farms was 72.7% and 27.3% for the serogroup 1/2a-3a and 4b-4d-4e, respectively, while for the 12 L. monocytogenes isolates recovered from abattoirs, it was 25%, 8.3%, 50% and 16.7% for the serogroup 1/2a-3a, 1/2b-3b, 1/2c-3c, and 4b-4d-4e respectively (P < 0.05). All (100%) isolates of L. monocytogenes from the farms and abattoirs were positive for seven virulence genes (hlyA, inlB, plcA, iap, inlA, inlC, and inlJ). The clinical and food safety significance of the findings cannot be ignored.

Recurring food source-based Listeria outbreaks in the United States: An unsolved puzzle of concern?

Recurring Listeria outbreaks in the United States is a growing public healthcare concern. Although no associated reported death, 17 were hospitalized out of the 18 reported illnesses in the recent outbreak in 15 US states. The United States has experienced about 30 Listeria outbreaks in the last decade with 524 Listeriosis cases and 80 deaths. The identified origin were ice cream, leafy greens, mushroom, meat slice, dairy products like cheese, packaged salads, cooked chicken, hard-boiled egg, pork product, frozen vegetable, raw milk, packaged caramel apple, bean sprout and soya products. Although rare, Listeria may lead to serious illness (invasive listeriosis) or death. Listeriosis is critically harmful and medically complicated, especially in the pregnant, the old above 65 years and in the immunocompromised. It could cause premature birth, miscarriage or even neonatal death. Hospitalization is often necessary in the geriatric, being fatal at times. Among Listeria sp., Listeria monocytogenes is often human infection-associated. It is a gram-positive, non-sporulating, motile bacillus opportunistic pathogen. Food-borne listeriosis is often associated with frozen foods due to its ability to thrive at low temperatures. Hypervirulent strains of L. monocytogenes with an ability to infect the respiratory system (the lungs) was recently reported in the coronavirus disease-19 patients during the pandemic. L. monocytogenes seemed to have developed antimicrobial resistance to ciprofloxacin and meropenem, possibly acquired through the food chain. An early onset of listeriosis in the newborn is evident in the first 7 days postparturition. As the bacteria colonize the genitourinary tract, majority of such cases result from teratogenic transfer during vaginal delivery. Premature newborns, neonates born outside healthcare facilities and low-birth-weight babies were increasingly predisposed to an early onset of listeriosis. Listeria outbreaks were earlier reported in South Africa, Australia and Europe, with an unclear origin of the outbreaks. Social media updates about such outbreaks, the most likely food source, and measures to self-protect are suggested as preventive measures. The article deals on various such aspects related to listeriosis primarily originating from food, to ensure better public healthcare and human wellness.

Microbiology of street food: understanding risks to improve safety

Street foods play important socioeconomic and cultural roles and are popular worldwide. In addition to providing convenient and low-cost meals for urban populations, street food offers an essential source of income for vendors, especially women, and it can reflect traditional local culture, which is an important attraction for tourists. Despite these benefits, the microbiological safety of street food has become a worldwide concern because it is often prepared and sold under inadequate safety conditions, without legal control and sanitary surveillance. Consequently, high counts of fecal indicator bacteria and several foodborne pathogens have been detected in street foods. This review provides insight into the microbiology of street food, focus on the associated microbiological safety aspects and main pathogens, and the global status of this important economic activity. Furthermore, the need to apply molecular detection rather than traditional culture-based methods is discussed to better understand the actual risks of microbial infection associated with street foods. Recognition is always the first step toward addressing a problem.

Prevalence, Serotypes, Antimicrobial Resistance and Biofilm-Forming Ability of Listeria monocytogenes Isolated from Bulk-Tank Bovine Milk in Northern Greece

The prevalence of Listeria monocytogenes in bovine bulk-tank milk (BTM) in Greece has not been previously investigated. The aim of the study was to estimate the prevalence of L. monocytogenes in bovine BTM in Greece and to characterize the isolates in terms of carriage of genes encoding for pathogenic determinants, assess the isolates' biofilm-forming ability and determine their susceptibility against 12 antimicrobials. Samples (n = 138) of bovine BTM were obtained from farms located throughout Northern Greece and were analyzed qualitatively and quantitatively for L. monocytogenes. Five samples (3.6%) tested positive for L. monocytogenes. The pathogen's populations in these positive samples were below 5 CFU/mL. Most isolates belonged to the molecular serogroup "1/2a, 3a". All isolates carried the virulence genes inlA, inlC, inlJ, iap, plcA and hlyA, but actA was detected in only three isolates. The isolates displayed weak to moderate biofilm-forming ability and distinct antimicrobial resistance profiles. All isolates were characterized as multidrug resistant, with resistance to penicillin and clindamycin being a common feature. Considering that L. monocytogenes constitutes a serious public health threat, the key findings of the study, related to the carriage of virulence genes and multidrug resistance, highlight the importance of continued monitoring of the pathogen in farm animals.

ARGs Detection in Listeria Monocytogenes Strains Isolated from the Atlantic Salmon (Salmo salar) Food Industry: A Retrospective Study

Among bacterial foodborne pathogens, Listeria monocytogenes represents one of the most important public health concerns in seafood industries. This study was designed as a retrospective study which aimed to investigate the trend of antibiotic resistance genes (ARGs) circulation in L. monocytogenes isolates identified (in the last 15 years) from Atlantic salmon (Salmo salar) fresh and smoked fillets and environmental samples. For these purposes, biomolecular assays were performed on 120 L. monocytogenes strains collected in certain years and compared to the contemporary scientific literature. A total of 52.50% (95% CI: 43.57-61.43%) of these samples were resistant to at least one antibiotic class, and 20.83% (95% CI: 13.57-28.09%) were classified as multidrug resistant. Concerning ARGs circulation, tetracycline (tetC, tetD, tetK, tetL, tetS), aminoglycoside (aadA, strA, aacC2, aphA1, aphA2), macrolide (cmlA1, catI, catII), and oxazolidinone (cfr, optrA, poxtA) gene determinants were majorly amplified. This study highlights the consistent ARGs circulation from fresh and processed finfish products and environmental samples, discovering resistance to the so-called critical important antimicrobials (CIA) since 2007. The obtained ARGs circulation data highlight the consistent increase in their diffusion when compared to similar contemporary investigations. This scenario emerges as the result of decades of improper antimicrobial administration in human and veterinary medicine.

Biochemical, Antioxidant Properties and Antimicrobial Activity of Epiphytic Leafy Liverwort Frullania dilatata (L.) Dumort

In this study, the biochemical, antioxidant properties, and antimicrobial activity of the epiphytic leafy liverwort Frullania dilatata (L.) Dumort were investigated. Due to the scarcity and difficulty in obtaining liverworts, research on their bioactivity is limited; thus, this study aimed to uncover the potential of F. dilatata. The antimicrobial activity was evaluated against various microorganisms, including food isolates, clinical isolates, multidrug-resistant strains, and standard strains, using the disk diffusion method and determining the minimum inhibitory concentration (MIC) values. This study represents the first antioxidant investigation on F. dilatata and an antimicrobial study using ethanol extract and the disk diffusion method. Notably, susceptibility was observed in Enterococcus faecalis ATCC 29212, Enterococcus faecium FI, Listeria monocytogenes ATCC 7644, Providencia rustigianii MDR, and Staphylococcus aureus ATCC 25923. The antioxidant capacity was assessed using the DPPH method, emphasizing the high scavenging performance. Gas chromatography-mass spectrometry (GC-MS) analysis identified the primary compounds as frullanolide (19.08%), 2,3-Dimethylanisole (15.21%), linoleic acid (11.11%), palmitic acid (9.83%), and valerenic acid (5.3%). The results demonstrated the significant antimicrobial activity of F. dilatata against the tested microorganisms and its potent antioxidant properties. These findings emphasize the potential of F. dilatata as a promising source of natural antimicrobial and antioxidant agents, underscoring the importance of further investigation into its bioactive compounds and elucidating the mechanisms of action in future studies.

Bioprotective cultures and bacteriocins as food preservatives

Food preservation technologies face the challenge of extending product shelf life applying different factors to prevent the microbiological spoilage of food and inhibit/inactivate food borne pathogens maintaining or even enhancing its quality. One such preservation strategy is the application of bacteriocins or bacteriocin-producer cultures as a kind of food biopreservation. Bacteriocins are ribosomally synthesized small polypeptide molecules that exert antagonistic activity against closely related and unrelated bacteria without harming the producing strain by specific immunity proteins. This chapter aims to contribute to current knowledge about innovative natural preservative agents and their application in the food industry. Specifically, its purpose is to analyze the classification of bacteriocins from lactic acid bacteria (LAB), desirable characteristics of bacteriocins that position them in a privileged place in food biopreservation technology, their success story as well as the bacteriocinogenic LAB in various food systems. Finally, challenges and barrier strategies used to enhance the efficiency of the bacteriocins antimicrobial effect are presented in this chapter.

A Comprehensive Virulence and Resistance Characteristics of Listeria monocytogenes Isolated from Fish and the Fish Industry Environment

Listeria monocytogenes is an important pathogen, often associated with fish, that can adapt and survive in products and food processing plants, where it can persist for many years. It is a species characterized by diverse genotypic and phenotypic characteristics. Therefore, in this study, a total of 17 L. monocytogenes strains from fish and fish-processing environments in Poland were characterized for their relatedness, virulence profiles, and resistance genes. The Core Genome Multilocus Sequence Typing (cgMLST) analysis revealed that the most frequent serogroups were IIa and IIb; sequence types (ST) were ST6 and ST121; and clonal complexes (CC) were CC6 and CC121. Core genome multilocus sequence typing (cgMLST) analysis was applied to compare the present isolates with the publicly available genomes of L. monocytogenes strains recovered in Europe from humans with listeriosis. Despite differential genotypic subtypes, most strains had similar antimicrobial resistance profiles; however, some of genes were located on mobile genetic elements that could be transferred to commensal or pathogenic bacteria. The results of this study showed that molecular clones of tested strains were characteristic for L. monocytogenes isolated from similar sources. Nevertheless, it is worth emphasizing that they could present a major public health risk due to their close relation with strains isolated from human listeriosis.

Minimally processed fruits as vehicles for foodborne pathogens

The consumption of minimally processed fruit (MPF) has increased over the last decade due to a novel trend in the food market along with the raising consumers demand for fresh, organic, convenient foods and the search for healthier lifestyles. Although represented by one of the most expanded sectors in recent years, the microbiological safety of MPF and its role as an emergent foodborne vehicle has caused great concern to the food industry and public health authorities. Such food products may expose consumers to a risk of foodborne infection as they are not subjected to prior microbial lethal methods to ensure the removal or destruction of pathogens before consumption. A considerable number of foodborne disease cases linked to MPF have been reported and pathogenic strains of Salmonella enterica, Escherichia coli, Listeria monocytogenes, as well as Norovirus accounted for the majority of cases. Microbial spoilage is also an issue of concern as it may result in huge economic losses among the various stakeholders involved in the manufacturing and commercialization of MPF. Contamination can take place at any step of production/manufacturing and identifying the nature and sources of microbial growth in the farm-to-fork chain is crucial to ensure appropriate handling practices for producers, retailers, and consumers. This review aims to summarize information about the microbiological hazards associated with the consumption of MPF and also highlight the importance of establishing effective control measures and developing coordinated strategies in order to enhance their safety.

Invasive listeriosis outbreaks and salmon products: a genomic, epidemiological study

Invasive listeriosis, caused by Listeria (L.) monocytogenes, is a severe foodborne infection, especially for immunocompromised individuals. The aim of our investigation was the identification and analysis of listeriosis outbreaks in Germany with smoked and graved salmon products as the most likely source of infection using whole-genome sequencing (WGS) and patient interviews. In a national surveillance programme, WGS was used for subtyping and core genome multi locus sequence typing (cgMLST) for cluster detection of L. monocytogenes isolates from listeriosis cases as well as food and environmental samples in Germany. Patient interviews were conducted to complement the molecular typing. We identified 22 independent listeriosis outbreaks occurring between 2010 and 2021 that were most likely associated with the consumption of smoked and graved salmon products. In Germany, 228 cases were identified, of 50 deaths (22%) reported 17 were confirmed to have died from listeriosis. Many of these 22 outbreaks were cross-border outbreaks with further cases in other countries. This report shows that smoked and graved salmon products contaminated with L. monocytogenes pose a serious risk for listeriosis infection in Germany. Interdisciplinary efforts including WGS and epidemiological investigations were essential to identifying the source of infection. Uncooked salmon products are high-risk foods frequently contaminated with L. monocytogenes. In order to minimize the risk of infection for consumers, food producers need to improve hygiene measures and reduce the entry of pathogens into food processing. Furthermore, susceptible individuals should be better informed of the risk of acquiring listeriosis from consuming smoked and graved salmon products.

A Machine Learning Model for Food Source Attribution of Listeria monocytogenes

Despite its low morbidity, listeriosis has a high mortality rate due to the severity of its clinical manifestations. The source of human listeriosis is often unclear. In this study, we investigate the ability of machine learning to predict the food source from which clinical Listeria monocytogenes isolates originated. Four machine learning classification algorithms were trained on core genome multilocus sequence typing data of 1212 L. monocytogenes isolates from various food sources. The average accuracies of random forest, support vector machine radial kernel, stochastic gradient boosting, and logit boost were found to be 0.72, 0.61, 0.7, and 0.73, respectively. Logit boost showed the best performance and was used in model testing on 154 L. monocytogenes clinical isolates. The model attributed 17.5 % of human clinical cases to dairy, 32.5% to fruits, 14.3% to leafy greens, 9.7% to meat, 4.6% to poultry, and 18.8% to vegetables. The final model also provided us with genetic features that were predictive of specific sources. Thus, this combination of genomic data and machine learning-based models can greatly enhance our ability to track L. monocytogenes from different food sources.

WGS analysis of Listeria monocytogenes from rural, urban, and farm environments in Norway: Genetic diversity, persistence, and relation to clinical and food isolates

Listeria monocytogenes is a ubiquitous environmental bacterium associated with a wide variety of natural and human-made environments, such as soil, vegetation, livestock, food processing environments, and urban areas. It is also among the deadliest foodborne pathogens, and knowledge about its presence and diversity in potential sources is crucial to effectively track and control it in the food chain. Isolation of L. monocytogenes from various rural and urban environments showed higher prevalence in agricultural and urban developments than in forest or mountain areas, and that detection was positively associated with rainfall. Whole-genome sequencing (WGS) was performed for the collected isolates and for L. monocytogenes from Norwegian dairy farms and slugs (218 isolates in total). The data were compared to available data sets from clinical and food-associated sources in Norway collected within the last decade. Multiple examples of clusters of isolates with 0 to 8 whole-genome multilocus sequence typing (wgMLST) allelic differences were collected over time in the same location, demonstrating persistence of L. monocytogenes in natural, urban, and farm environments. Furthermore, several clusters with 6 to 20 wgMLST allelic differences containing isolates collected across different locations, times, and habitats were identified, including nine clusters harboring clinical isolates. The most ubiquitous clones found in soil and other natural and animal ecosystems (CC91, CC11, and CC37) were distinct from clones predominating among both clinical (CC7, CC121, and CC1) and food (CC9, CC121, CC7, and CC8) isolates. The analyses indicated that ST91 was more prevalent in Norway than other countries and revealed a high proportion of the hypovirulent ST121 among Norwegian clinical cases.

IMPORTANCEListeria monocytogenes is a deadly foodborne pathogen that is widespread in the environment. For effective management, both public health authorities and food producers need reliable tools for source tracking, surveillance, and risk assessment. For this, whole-genome sequencing (WGS) is regarded as the present and future gold standard. In the current study, we use WGS to show that L. monocytogenes can persist for months and years in natural, urban, and dairy farm environments. Notably, clusters of almost identical isolates, with genetic distances within the thresholds often suggested for defining an outbreak cluster, can be collected from geographically and temporally unrelated sources. The work highlights the need for a greater knowledge of the genetic relationships between clinical isolates and isolates of L. monocytogenes from a wide range of environments, including natural, urban, agricultural, livestock, food production, and food processing environments, to correctly interpret and use results from WGS analyses.

Exogenous fatty acids affect membrane properties and cold adaptation of Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen that can grow at very low temperatures close to the freezing point of food and other matrices. Maintaining cytoplasmic membrane fluidity by changing its lipid composition is indispensable for growth at low temperatures. Its dominant adaptation is to shorten the fatty acid chain length and, in some strains, increase in addition the menaquinone content. To date, incorporation of exogenous fatty acid was not reported for Listeria monocytogenes. In this study, the membrane fluidity grown under low-temperature conditions was affected by exogenous fatty acids incorporated into the membrane phospholipids of the bacterium. Listeria monocytogenes incorporated exogenous fatty acids due to their availability irrespective of their melting points. Incorporation was demonstrated by supplementation of the growth medium with polysorbate 60, polysorbate 80, and food lipid extracts, resulting in a corresponding modification of the membrane fatty acid profile. Incorporated exogenous fatty acids had a clear impact on the fitness of the Listeria monocytogenes strains, which was demonstrated by analyses of the membrane fluidity, resistance to freeze-thaw stress, and growth rates. The fatty acid content of the growth medium or the food matrix affects the membrane fluidity and thus proliferation and persistence of Listeria monocytogenes in food under low-temperature conditions.

Diversity of Listeria monocytogenes Strains Isolated from Food Products in the Central European Part of Russia in 2000-2005 and 2019-2020

Totally, 45 L. monocytogenes strains isolated from meat, poultry, dairy, and fish products in the Central European part of Russia in 2001–2005 and 2019–2020 were typed using a combined MLST and internalin profile (IP) scheme. Strains belonged to 14 clonal complexes (CCs) of the phylogenetic lineages I and II. Almost half of the strains (20 of 45) belonged to six CCs previously recognized as epidemic clones (ECs). ECI and ECV strains were isolated during both studied periods, and ECII, ECIV, ECVI, and ECVII strains were isolated in 2001–2005, but not in 2019–2020. ECI, ECIV, ECV, and ECVII strains were isolated from products of animal origin. ECII and ECVI were isolated from fish. Testing of invasion efficiencies of 10 strains isolated in different years and from different sources and belonging to distinct CCs revealed a statistically significant difference between phylogenetic lineage I and II strains but not between ECs and non-EC CCs or strains differing by year and source of isolation. Strains isolated in 2001–2005 were characterized by higher phylogenetic diversity and greater presentation of ECs and CCs non-typical for natural and anthropogenic environments of the European part of Russia comparatively to isolates obtained in 2019–2020.Closing of the Russian market in 2019–2020 for imported food might be responsible for these differences.

Healthcare-associated foodborne outbreaks in high-income countries: a literature review and surveillance study, 16 OECD countries, 2001 to 2019

BackgroundHealthcare-associated foodborne outbreaks (HA-FBO) may have severe consequences, especially in vulnerable groups.AimThe aim was to describe the current state of HA-FBO and propose public health recommendations for prevention.MethodsWe searched PubMed, the Outbreak Database (Charité, University Medicine Berlin), and hand-searched reference lists for HA-FBO with outbreak onset between 2001 and 2018 from Organisation for Economic Co-operation and Development (OECD) countries and HA-FBO (2012-2018) from the German surveillance system. Additionally, data from the European Food Safety Authority were analysed.ResultsThe literature search retrieved 57 HA-FBO from 16 OECD countries, primarily in the US (n = 11), Germany (n = 11) and the United Kingdom (n = 9). In addition, 28 HA-FBO were retrieved from the German surveillance system. Based on the number of outbreaks, the top three pathogens associated with the overall 85 HA-FBO were Salmonella (n = 24), norovirus (n = 22) and Listeria monocytogenes (n = 19). Based on the number of deaths, L. monocytogenes was the main pathogen causing HA-FBO. Frequently reported implicated foods were 'mixed foods' (n = 16), 'vegetables and fruits' (n = 15) and 'meat and meat products' (n = 10). Consumption of high-risk food by vulnerable patients, inadequate time-temperature control, insufficient kitchen hygiene and food hygiene and carriers of pathogens among food handlers were reported as reasons for HA-FBO.ConclusionTo prevent HA-FBO, the supply of high-risk food to vulnerable people should be avoided. Well working outbreak surveillance facilitates early detection and requires close interdisciplinary collaboration and exchange of information between hospitals, food safety and public health authorities.

Identification and serotyping of Listeria monocytogenes, isolated from various salmon products, sold in retail market in Lithuania

Cold smoked salmon products (belly flaps, pieces, fillet, and loin) obtained from the retail market in Lithuania were tested for the presence of L. monocytogenes. It was found that contamination of the cold smoked fish products with Listeria spp. depends on the type of the product. Contamination with listeria in salmon belly flaps was 7.5 times higher than in the loin (P<0.05), 1.8 times higher than in the pieces (P<0.05) and 30 times higher than in the fillet (P<0.05). Microbiological analysis showed that 32.5% (P<0.05) of the fish product samples were infected with L. monocytogenes, while multiplex PCR confirmed 31.25% positive samples (P<0.01). According to the study results, L. monocytogenes strains were divided into two serotypes: 4b (94.6%) and 1/2a (5.4%). High contamination of the products with Listeria spp. showed that cold smoked salmon products, sold in local market, can be a reason of human listeriosis in Lithuania.

Menaquinone-mediated regulation of membrane fluidity is relevant for fitness of Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen with the ability to grow at low temperatures down to − 0.4 °C. Maintaining cytoplasmic membrane fluidity by changing the lipid membrane composition is important during growth at low temperatures. In Listeria monocytogenes, the dominant adaptation effect is the fluidization of the membrane by shortening of fatty acid chain length. In some strains, however, an additional response is the increase in menaquinone content during growth at low temperatures. The increase of this neutral lipid leads to fluidization of the membrane and thus represents a mechanism that is complementary to the fatty acid-mediated modification of membrane fluidity. This study demonstrated that the reduction of menaquinone content for Listeria monocytogenes strains resulted in significantly lower resistance to temperature stress and lower growth rates compared to unaffected control cultures after growth at 6 °C. Menaquinone content was reduced by supplementation with aromatic amino acids, which led to a feedback inhibition of the menaquinone synthesis. Menaquinone-reduced Listeria monocytogenes strains showed reduced bacterial cell fitness. This confirmed the adaptive function of menaquinones for growth at low temperatures of this pathogen.

A Large-Scale Sequencing-Based Survey of Plasmids in Listeria monocytogenes Reveals Global Dissemination of Plasmids

The food-borne pathogen Listeria monocytogenes is known for its capacity to cope with multiple stress conditions occurring in food and food production environments (FPEs). Plasmids can provide benefits to their host strains, and it is known that various Listeria strains contain plasmids. However, the current understanding of plasmid frequency and function in L. monocytogenes strains remains rather limited. To determine the presence of plasmids among L. monocytogenes strains and their potential contribution to stress survival, a comprehensive dataset was established based on 1,921 published genomes from strains representing 14 L. monocytogenes sequence types (STs). Our results show that an average of 54% of all L. monocytogenes strains in the dataset contained a putative plasmid. The presence of plasmids was highly variable between different STs. While some STs, such as ST1, ST2, and ST4, contained few plasmid-bearing strains (<15% of the strains per ST), other STs, such as ST121, ST5, ST8, ST3, and ST204, possessed a higher proportion of plasmid-bearing strains with plasmids found in >71% of the strains within each ST. Overall, the sizes of plasmids analyzed in this study ranged from 4 to 170 kbp with a median plasmid size of 61 kbp. We also identified two novel groups of putative Listeria plasmids based on the amino acid sequences of the plasmid replication protein, RepA. We show that highly conserved plasmids are shared among Listeria strains which have been isolated from around the world over the last few decades. To investigate the potential roles of plasmids, nine genes related to stress-response were selected for an assessment of their abundance and conservation among L. monocytogenes plasmids. The results demonstrated that these plasmid genes exhibited high sequence conservation but that their presence in plasmids was highly variable. Additionally, we identified a novel transposon, Tn7075, predicted to be involved in mercury-resistance. Here, we provide the largest plasmid survey of L. monocytogenes to date with a comprehensive examination of the distribution of plasmids among L. monocytogenes strains. Our results significantly increase our knowledge about the distribution, composition, and conservation of L. monocytogenes plasmids and suggest that plasmids are likely important for the survival of L. monocytogenes in food and FPEs.

Biodiversity of meatborne Listeria spp. in Himachal Pradesh and their interaction with indigenous probiotics

This study determined the anti-listerial activity of indigenous probiotics from traditional fermented foods of Western Himalaya against meat borne Listera monocytogens isolates from Himachal Pradesh. One hundred samples of meat and meat products like chicken (n = 25), chevon (goat meat, n = 20), fish (n = 20) and pork (n = 30) were collected and were analyzed for the presence of Listeria spp. by recommended culture and biochemical methods. L. monocytogens isolates were confirmed by PCR targeting the virulence gene hlyA (haemolysin A) and by16S rRNA sequencing. Anti-listerial activity of probiotic bacteria isolated from indigenous fermented foods of Himachal Pradesh was determined by well diffusion method using Lactobacillus rhamnosus GG (ATCC 53103) as the reference strain. Five percent of tested samples were found positive for L. monocytogens with incidence of 8.0% in chicken (2/25), 10.0% in fish (2/20) and 4.0% in chevon meat (1/25). None of the tested pork samples were found contaminated with L. monocytogenes. Among 11 indigenous probiotics used in this study, highest antagonistic activity was exhibited by Lactobacillus plantarum (ADF 10) and Enterococcus faecium (ADF1) which was equivalent to the reference strain.

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.

Listeriosis outbreak likely due to contaminated liver pâté consumed in a tavern, Austria, December 2018

In late December 2018, an outbreak of listeriosis occurred after a group of 32 individuals celebrated in a tavern in Styria, Austria; traditional Austrian food (e.g. meat, meat products and cheese) was served. After the celebration, 11 individuals developed gastrointestinal symptoms, including one case with severe sepsis. Cases had consumed mixed platters with several meat products and pâtés originating from a local production facility (company X). Human, food and environmental samples taken from the tavern and company X were tested for L. monocytogenes. Whole genome sequence-based typing detected a novel L. monocytogenes strain of serotype IVb, sequence type 4 and CT7652 in 15 samples; 12 human, two food and one environmental sample from company X with an allelic difference of 0 to 1. Active case finding identified two further cases who had not visited the tavern but tested positive for the outbreak strain. In total, 13 cases (seven females and six males; age range: 4–84 years) were identified. Liver pâté produced by company X was identified as the likely source of the outbreak. Control measures were implemented and since the end of December 2018, no more cases were detected.

Susceptibility (re)-testing of a large collection of Listeria monocytogenes from foods in China from 2012 to 2015 and WGS characterization of resistant isolates

OBJECTIVES

Our aim was to determine the antimicrobial susceptibilities of 2862 Listeria monocytogenes cultured from various foods in China and to use WGS to characterize the antimicrobial resistance and virulence genotypes of those expressing a resistance phenotype.

METHODS

The susceptibilities of 2862 L. monocytogenes were determined by broth microdilution. Twenty-eight L. monocytogenes were found to be resistant to one to four antibiotics. All 28 resistant isolates were subsequently sequenced using short-read high accuracy protocols. The corresponding genomes were assembled and further analysis was carried out using appropriate bioinformatics pipelines.

RESULTS

All 28 resistant L. monocytogenes were classified into five STs (ST3, ST8, ST9, ST155 and ST515). Both ST9 and ST155 were dominant and their genotypes correlated with their resistance phenotypes. All ST9 isolates were MDR and could be phylogenetically classified into two clusters. One was relatively close to clinical origins and one to food. Downstream analysis of the genetic contexts in which these resistance genotypes were found suggested that these may have been acquired from other bacteria by horizontal transfer or insertion into the chromosome. All isolates harboured Listeria pathogenicity island (LIPI)-1 and LIPI-2, and only two harboured LIPI-3.

CONCLUSIONS

This study reported on the antimicrobial susceptibilities of 2862 foodborne L. monocytogenes along with the genomic characterization of 28 resistant isolates, 11 of which expressed an MDR phenotype. These data showed that this bacterium can acquire resistance by horizontal gene transfer in and between species. This study may necessitate a re-evaluation of risk to public health, associated with this bacterial species.

Attribution of Listeria monocytogenes human infections to food and animal sources in Northern Italy

Listeriosis is a foodborne illness characterized by a relatively low morbidity, but a large disease burden due to the severity of clinical manifestations and the high case fatality rate. Increased listeriosis notifications have been observed in Europe since the 2000s. However, the reasons for this increase are largely unknown, with the sources of sporadic human listerioris often remaining elusive. Here we inferred the relative contributions of several putative sources of Listeria monocytogenes strains from listerioris patients in Northern Italy (Piedmont and Lombardy regions), using two established source attribution models (i.e. 'Dutch' and 'STRUCTURE') in comparative fashion. We compared the Multi-Locus Sequence Typing and Multi-Virulence-Locus Sequence Typing profiles of strains collected from beef, dairy, fish, game, mixed foods, mixed meat, pork, and poultry. Overall, 634 L. monocytogenes isolates were collected from 2005 to 2016. In total, 40 clonal complexes and 51 virulence types were identified, with 36% of the isolates belonging to possible epidemic clones (i.e. genetically related strains from unrelated outbreaks). Source attribution analysis showed that 50% of human listerioris cases (95% Confidence Interval 44-55%) could be attributed to dairy products, followed by poultry and pork (15% each), and mixed foods (15%). Since the contamination of dairy, poultry and pork products are closely linked to primary production, expanding actions currently limited to ready-to-eat products to the reservoir level may help reducing the risk of cross-contamination at the consumer level.

Two perspectives of Listeria monocytogenes hazards in dairy products: the prevalence and the antibiotic resistance

Listeria monocytogenes is among the most food-borne pathogens. It has the ability to grow over a range of temperature, including refrigeration temperature. Foods kept in refrigerator more than the prescribed period of time create an opportunity for the occurrence of Listeria monocytogenes. As this review shows, the prevalence of L. monocytogenes has more likely evident in pasteurized milk than other dairy products, such as raw milk. Inadequate temperature and faults in technology during pasteurization can be the disposing factors for the presence of the organism in dairy products. The organism, on the other hand, has been found to be resistant to those commonly known antibiotics that have human and veterinary importance, namely, ampicillin, Tetracycline, and chloramphenicol, streptomycin, erytromycin, penicillin G., and others. Resistance ability of the organism can be mediated by different natural and acquired resistance mechanisms, such as self-transferrable plasmids, mobilizable plasmids, and conjugative transposons. The emergence and spread of antibiotic resistance of L. monocytogenes has serious public health and economic impacts at large. This paper has reviewed the prevalence and the antibiotic resistance of L. monocytogenes isolates of dairy products and the strategic mechanisms of the organism develop resistance against the antibiotics.

Case report: whole genome sequencing based investigation of maternal-neonatal listeriosis in Sichuan, China

BACKGROUND

Neonatal listeriosis is a rare but severe disease manifesting as septicemia and central nervous system (CNS) infections with a high fatality rate of around 20 to 30%. Whole genome sequencing (WGS) is a promising technique for pathogen identification and infection source tracing with its high resolution.

CASE PRESENTATION

A case of neonatal sepsis with listeriosis was reported with positive blood culture for Listeria monocytogenes. The case was investigated to confirm the vertical transmission of the infection and identify the potential food source of the maternal L. monocytogenes infection using WGS. L. monocytogenes was isolated from the neonate's blood sample the day after caesarean delivery and from the mother's genital and pudenda swab samples 5 days and 13 days after caesarean delivery. WGS showed that the isolate from the neonate was identical to the genome type of the isolates from the mother, with only one of the 4 isolates from the mother differing by one single nucleotide polymorphism (SNP). By WGS, one L. monocytogenes isolate from a ready-to-eat (RTE) meat sample in the patients' community market shared the same sequence type but was ruled out as the cause of infection, with 57 SNP differences to the strain causing the maternal-neonatal infection. The food isolate also carried a novel plasmid pLM1686 that harbored heavy metal resistance genes. After caesarean section, the mother was treated with a third generation cephalosporin which L. monocytogenes is naturally resistant to, which may explain why genital and pudenda swabs were still culture-positive for L. monocytogenes 13 days after delivery.

CONCLUSIONS

Genital swab culture for L. monocytogenes had been informative in the diagnosis of maternal listeriosis in this case. The high resolution of WGS confirmed the maternal-neonatal transmission of L. monocytogenes infection and ruled out the L. monocytogenes contaminated RTE meat from the local market as the direct source of the mother's infection.

The benzalkonium chloride resistant or sensitive phenotype of Listeria monocytogenes planktonic cells did not dictate the susceptibility of its biofilm counterparts

The main goal of this work was to approach food industry conditions in the comparison of the susceptibility of biofilms of Listeria monocytogenes to the biocides benzalkonium chloride (BAC) and peracetic acid (PAA). Twelve isolates of L. monocytogenes, including nine well characterized BAC resistant strains were used. Biofilms were produced on stainless steel coupons (SSC), at 11 °C (refrigeration temperature) or at 25 °C (room temperature), in culture media simulating clean (nutrient limiting) or soiled (nutrient rich) growth conditions. Neither different nutrient availability nor growth temperature showed significant effect (p > .05) on biofilm formation. PAA confirmed to be more effective than BAC in biofilm elimination. Biofilms formed under nutritional stress tended to differentiate more the response to BAC of the resistant or sensitive strains, but the resistant or sensitive phenotype of the planktonic cells did not dictate biofilm susceptibility.