Persistence Of Listeria Monocytogenes Research Articles

Is the persistence of Listeria monocytogenes in food processing facilities and its resistance to pathogen intervention linked to its phylogeny?

The foodborne pathogen Listeria monocytogenes is differentiated into four distinct lineages which differ in their virulence. It remains unknown, however, whether the four lineages also differ with respect to their ability to persist in food processing facilities, their resistance to high pressure, a preservation method that is used commercially for Listeria control on ready-to-eat meats, and their ability to form biofilms. This study aimed to determine differences in the pressure resistance and biofilm formation of 59 isolates of L. monocytogenes representing lineages I and II. Furthermore, the genetic similarity of 9 isolates of L. monocytogenes that were obtained from a meat processing facility over a period of 1 year and of 20 isolates of L. monocytogenes from food processing facilities was analyzed to assess whether the ability of the lineages of L. monocytogenes to persist in these facilities differs. Analysis of 386 genomes with respect to the source of isolation revealed that genomes of lineage II are over-represented in meat isolates when compared with clinical isolates. Of the 38 strains of Lm. monocytogenes that persisted in food processing facilities (this study or published studies), 31 were assigned to lineage II. Isolates of lineage I were more resistant to treatments at 400 to 600 MPa. The thickness of biofilms did not differ between lineages. In conclusion, strains of lineage II are more likely to persist in food processing facilities while strains of lineage I are more resistant to high pressure.IMPORTANCEListeria monocytogenes substantially contributes to the mortality of foodborne disease in developed countries. The virulence of strains of four lineages of L. monocytogenes differs, indicating that risks associated with the presence of L. monocytogenes are lineage specific. Our study extends the current knowledge by documentation that the lineage-level phylogeny of L. monocytogenes plays a role in the source of isolation, in the persistence in food processing facilities, and in the resistance to pathogen intervention technologies. In short, the control of risks associated with the presence of L. monocytogenes in food is also lineage specific. Understanding the route of contamination L. monocytogenes is an important factor to consider when designing improved control measures.

The Listeria monocytogenes persistence factor ClpL is a potent stand-alone disaggregase.

Heat stress can cause cell death by triggering the aggregation of essential proteins. In bacteria, aggregated proteins are rescued by the canonical Hsp70/AAA+ (ClpB) bi-chaperone disaggregase. Man-made, severe stress conditions applied during, e.g., food processing represent a novel threat for bacteria by exceeding the capacity of the Hsp70/ClpB system. Here, we report on the potent autonomous AAA+ disaggregase ClpL from Listeria monocytogenes that provides enhanced heat resistance to the food-borne pathogen enabling persistence in adverse environments. ClpL shows increased thermal stability and enhanced disaggregation power compared to Hsp70/ClpB, enabling it to withstand severe heat stress and to solubilize tight aggregates. ClpL binds to protein aggregates via aromatic residues present in its N-terminal domain (NTD) that adopts a partially folded and dynamic conformation. Target specificity is achieved by simultaneous interactions of multiple NTDs with the aggregate surface. ClpL shows remarkable structural plasticity by forming diverse higher assembly states through interacting ClpL rings. NTDs become largely sequestered upon ClpL ring interactions. Stabilizing ring assemblies by engineered disulfide bonds strongly reduces disaggregation activity, suggesting that they represent storage states.

The Listeria monocytogenes persistence factor ClpL is a potent stand-alone disaggregase

Heat stress can cause cell death by triggering the aggregation of essential proteins. In bacteria, aggregated proteins are rescued by the canonical Hsp70/AAA+ (ClpB) bi-chaperone disaggregase. Man-made, severe stress conditions applied during, e.g., food processing represent a novel threat for bacteria by exceeding the capacity of the Hsp70/ClpB system. Here, we report on the potent autonomous AAA+ disaggregase ClpL from Listeria monocytogenes that provides enhanced heat resistance to the food-borne pathogen enabling persistence in adverse environments. ClpL shows increased thermal stability and enhanced disaggregation power compared to Hsp70/ClpB, enabling it to withstand severe heat stress and to solubilize tight aggregates. ClpL binds to protein aggregates via aromatic residues present in its N-terminal domain (NTD) that adopts a partially folded and dynamic conformation. Target specificity is achieved by simultaneous interactions of multiple NTDs with the aggregate surface. ClpL shows remarkable structural plasticity by forming diverse higher assembly states through interacting ClpL rings. NTDs become largely sequestered upon ClpL ring interactions. Stabilizing ring assemblies by engineered disulfide bonds strongly reduces disaggregation activity, suggesting that they represent storage states.

Recurrent, ICD-associated L. monocytogenes bacteraemia with multiple septic pulmonary embolisms over a 2-year period.

Listeria monocytogenes is a bacterial pathogen known for causing listeriosis, a foodborne illness with a wide spectrum of clinical presentations ranging from mild gastroenteritis to severe invasive disease, particularly affecting immunocompromised individuals, pregnant women, newborns, and the elderly. Successful treatment of patients with recurring listeria episodes due to colonised foreign material is often challenging, typically requiring a combination of antimicrobial treatment and surgical removal. Here, we present a particularly complex case of chronic invasive listeriosis with a total of six relapses. After extensive investigations, the patient's ICD device was identified as the focus of infection. The confirmation of relapses through cgMLST analysis highlights the persistence of Listeria monocytogenes and the potential for recurrence even after apparent resolution of symptoms in patients with foreign material. It emphasises the necessity for a comprehensive assessment to identify and mitigate the risk of relapses, thereby ensuring optimal management and outcomes.

Environmental persistence of Listeria monocytogenes and its implications in dairy processing plants.

Listeriosis, an invasive illness with a fatality rate between 20% and 30%, is caused by the ubiquitous bacterium Listeria monocytogenes. Human listeriosis has long been associated with foods. This is because the ubiquitous nature of the bacteria renders it a common food contaminant, posing a significant risk to the food processingsector. Although several sophisticated stress coping mechanisms have been identified as significant contributing factors toward the pathogen's persistence, a complete understanding of the mechanisms underlying persistence across various strains remains limited. Moreover, aside from genetic aspects that promote the ability to cope with stress, various environmental factors that exist in food manufacturing plants could also contribute to the persistence of thepathogen. The objective of this review is to provide insight into the challenges faced by the dairy industry because of the pathogens' environmental persistence. Additionally, it also aims to emphasize the diverse adaptation and response mechanisms utilized by L. monocytogenes in food manufacturing plants to evade environmental stressors. The persistence of L. monocytogenes in the food processing environment poses a serious threat to food safety and public health. The emergence of areas with high levels of L. monocytogenes contamination could facilitate Listeria transmission through aerosols, potentially leading to the recontamination of food, particularly from floors and drains, when sanitation is implemented alongside product manufacturing. Hence, to produce safe dairy products and reduce the frequency of outbreaks of listeriosis, it is crucial to understand the factors that contribute to the persistence of this pathogen and to implement efficient control strategies.

Listeria monocytogenes in food businesses: From persistence strategies to intervention/prevention strategies-A review.

In 2023, Listeria monocytogenes persistence remains a problem in the food business. A profound understanding of how this pathogen persists may lead to better aimed intervention/prevention strategies. The lack of a uniform definition of persistence makes the comparison between studies complex. Harborage sites offer protection against adverse environmental conditions and form the ideal habitat for the formation of biofilms, one of the major persistence strategies. A retarded growth rate, disinfectant resistance/tolerance, desiccation resistance/tolerance, and protozoan protection complete the list of persistence strategies for Listeria monocytogenes and can occur on themselves or in combination with biofilms. Based on the discussed persistence strategies, intervention strategies are proposed. By enhancing the focus on four precaution principles (cleaning and disinfection, infrastructure/hygienic design, technical maintenance, and work methodology) as mentioned in Regulation (EC) No. 852/2004, the risk of persistence can be decreased. All of the intervention strategies result in obtaining and maintaining a good general hygiene status throughout the establishment at all levels ranging from separate equipment to the entire building.

Simulated transmission and decontamination of Listeria monocytogenes biofilms from plastic cutting boards

The presence and persistence of Listeria monocytogenes biofilms on food contact surfaces are often considered as a critical source of cross-contamination of foodstuffs and hence pose a major risk in food safety. This is particularly an issue in numerous countries where knowledge of appropriate food hygiene practices is limited. In this study, the biofilm formation of L. monocytogenes (ATCC 19115) on plastic cutting boards as well as their transmission rates to selected food samples were simulated and evaluated. The effectiveness of the typical household washing practice in biofilm removal from artificially-contaminated plastic cutting boards was also assessed. Crystal violet indicated that L. monocytogenes biofilms on plastic cutting boards mature within 24 h, during which biofilm adhesion was the greatest with 1.62 OD value. The transfer rates of L. monocytogenes from artificially-contaminated plastic cutting board to cabbage (79.27%), chicken fillet (69.88%) and bread (83.25%) during the cutting process were determined. With regards to washing treatment, scrubbing with a commercial dishwashing liquid was shown to be effective at reducing microbial concentrations by 3–6 log CFU/cm2, with virtually all biofilms removed after 5 s of washing. Tap water rinsing on cutting boards was less effective with a mere decrease of 1–2 log CFU/cm2. The findings have provided an insight on the risks associated with the cross-contamination of L. monocytogenes and likely other pathogenic microbes which should be a concern everywhere in the world. Therefore, promoting a better awareness of proper cleaning practices, including the sanitization of cutting boards during food preparations is necessary.

Presence and Persistence of Listeria monocytogenes in the Danish Ready-to-Eat Food Production Environment

Listeria monocytogenes is an ubiquitously occurring foodborne bacterial pathogen known to contaminate foods during the production processes. To assess the presence and persistence of L. monocytogenes in Danish ready-to-eat (RTE) food production companies in response to a Listeria awareness campaign, the production environment of selected companies were sampled in 2016 and in 2020. Whole genome sequencing (WGS) was performed to characterize the isolates (n = 50, plus 35 isolates obtained from the routine surveillance during 2016–2020), including investigation of the presence of virulence, persistence and resistance genes. The number of companies that tested positive by culture was 17/39 (43.6%) in 2016 and 11/34 (32.4%) in 2020, indicating a limited effect of the campaign. WGS analyses of the 85 isolates showed that the most common sequence types (STs) were ST8 and ST121. The single nucleotide polymorphism (SNP) analysis showed that isolates coming from the same company and belonging to the same ST exhibited <10 SNP differences regardless of the sampling year and whether the samples came from the environment or products, indicating the persistence of the specific STs. Several prevalent STs were found in clinical cases concurrently, including genetically similar isolates. This highlights the issue of persistent L. monocytogenes in the food production environment and the need for improved risk communication and mitigation strategies.

Examining Patterns of Persistent Listeria Contamination in Packinghouses Using Agent-Based Models

Persistent Listeria monocytogenes contamination may occur in a packinghouse if the pathogen successfully infiltrates the facility and reaches a harborage site, where it may be difficult to remove and may contaminate produce within the facility. There is a need for simulation-based decision support tools that can predict which equipment sites are more likely to undergo persistent contamination and simulate potential corrective actions to prevent this contamination. Thus, we adapted for longer term simulation two existing applications of an agent-based model of Listeria spp. hourly contamination dynamics in produce packinghouses. Next, we developed a novel approach to identify and analyze persistent and transient Listeria contamination patterns on simulated agents representing equipment sites and employees. Testing of corrective actions showed that methods that involved targeted, facility-specific, risk-based sanitation were the most effective in reducing both the likelihood and duration of persistent contamination. Generic approaches to controlling Listeria (e.g., more concentrated sanitizers) are unlikely to be successful and suggest that use of sanitation schedules produced through facility-specific root cause analysis and hygienic design are key in reducing persistence. Hourly Listeria contamination patterns also suggest that transient contamination may be mistaken for persistent contamination, depending on the frequency of environmental sampling. Likewise, as concentrations of Listeria on most contaminated agents were predicted to be very low, there is also a possibility to mistake persistence for transient contamination of sites, or even miss it outright, due to false-negative environmental Listeria monitoring results. These findings support that agent-based models may be valuable decision support tools, aiding in the identification of contamination patterns within packinghouses and assessing the viability of specific corrective actions.

Molecular typing and genome sequencing allow the identification of persistent Listeria monocytogenes strains and the tracking of the contamination source in food environments

The presence of Listeria monocytogenes (Lm) in the food processing environment (facilities and products) is a challenging problem in food safety management. Lm is one of the main causes of mortality in foodborne infections, and the trend is continuously increasing. In this study, a collection of 323 Lm strain isolates recovered from food matrices and food industry environments (surfaces and equipment) over four years from 80 food processing facilities was screened using a restriction site-associated tag sequencing (2b-RAD) typing approach developed for Lm. Thirty-six different restriction site-associated DNA (RAD) types (RTs) were identified, most of which correspond to lineage II. RT1, the most represented genotype in our collection and already reported as one of the most prevalent genotypes in the food environment, was significantly associated with meat processing facilities. The sequencing of the genomes of strains belonging to the same RT and isolated in the same facility in different years revealed several clusters of persistence. The definition of the persistent strains (PSs) allowed the identification of the potential source of contamination in the incoming raw meat that is introduced in the facility to be processed. The slaughterhouses, which, according to the European Union (EU) regulation, are not inspected for the presence of Lm could be hotspots for the persistence of Lm PSs.

Impact of temperature, soil type and compost amendment on the survival, growth and persistence of Listeria monocytogenes of non-environmental (food-source associated) origin in soil

Listeria monocytogenes of varied sources including food-related sources may reach the soil. Associated food safety and environmental health risks of such contamination depend significantly on the capacity of L. monocytogenes to survive in the soil. This study assessed the survival of 13 L. monocytogenes strains isolated from food and food processing environments and a cocktail of three of the strains in two types of soils (loam and sandy) under controlled temperature conditions: 5, 10, 20, 25, 30℃ and ‘uncontrolled’ ambient temperature conditions in a tropical region. The impact of compost amendment on the survival of L. monocytogenes in the two different types of soils was also assessed. Soil type, temperature and compost amendment significantly (P <0.001) impacted the survival of L. monocytogenes in soil. Temperature variations affected the survival of L. monocytogenes in soil, where some strains such as strain 732, a L. monocytogenes 1/2a strain survived better at lower temperature (5°C), for which counts of up to 10.47 ± 0.005 log CFU/g were recovered in compost-amended sandy soil, 60 days post-inoculation. Some other strains such as strain 441, a L. monocytogenes 1/2a survived best at intermediate temperature (25 and 30 °C), while others such as 2739 (L. monocytogenes 1/2b) thrived at higher temperature (between 30 °C - 37 °C). There were significant correlations between the influence of temperature and soil type, where lower temperature conditions (5°C - 20°C) were generally more suitable for survival in sandy soil compared to higher temperature conditions. For some of the strains that thrived better in sandy soil at lower temperature, Pearson correlation analysis found significant correlations between temperature and soil type. Steady, controlled temperature generally favored the survival of the strains compared to uncontrolled ambient temperature conditions, except for the cocktail. The cocktail persisted until the last day of post-inoculation storage (60th day) in all test soils and under all incubation temperature conditions. Loam soil was more favorable for the survival of L. monocytogenes and compost amendment improved the survival of the strains, especially in compost-amended sandy soil. Listeria monocytogenes may exhibit variable survival capacity in soil, depending on conditions such as soil type, compost amendment and temperature.

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.

ECOLOGY OF LISTERIA MONOCYTOGENES

Food safety criteria for Listeria monocytogenes in the ready-to-eat food category have been applied since 2006. In the territory of the European Union in 2019, there were 2 621 confirmed cases of invasive listeriosis in humans, with a high mortality rate (17.6%). A broader understanding of the ecology of Listeria monocytogenes is crucial for the successful control of this pathogen in the food chain continuum. The paper presents new knowledge on reservoirs/sources of contamination, distribution, dynamics and transmission routes, survival and persistence of Listeria monocytogenes, both in natural habitat and in farm or processing environment.

Distribution, contamination routes, and seasonal influence of persistent Listeria monocytogenes in a commercial fresh Hypsizigus marmoreus production facility

Listeria monocytogenes is an important foodborne pathogen worldwide, with a high mortality rate of 20%–30% for vulnerable persons. The aim of this study was to determine the distribution, contamination routes, and persistence of L. monocytogenes in a mushroom plant. A total of 204 samples collected during four sampling periods within a one-year time interval, with 27.0% (55/204) were positive for L. monocytogenes. A total of 60 representative L. monocytogenes strains were selected for further analysis based on serogrouping and enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR) fingerprints. Multiplex PCR showed serogroups I.1 (1/2a-3a) and II.2 (1/2b-3b-7) accounted for 68.3% (41/60) and 31.7% (19/60), respectively, of the samples. ST8 strains were isolated from the mycelium-scraping phase to the harvest phase by multilocus sequence typing. These strains had identical ERIC-PCR fingerprints, suggesting that the mycelium-scraping machine was the main contamination source. Sequence types (ST) 8 (61.7%, 37/60) and 87 (23.3%, 14/60) were predominant and repeatedly isolated within a one-year seasonal investigation, indicating that ST8 and ST87 persist in this plant. The minimum inhibitory concentrations of commonly used quaternary ammonium compounds for the 60 isolates were ≥4 μg/mL, indicating that the persistence we observed may be associated with biocide tolerance. In addition, all strains harbored nine virulent genes (prfA, hly, mpl, actA, iap, plcA, plcB, inlA, and inlB). Only ST87 strains carried ptsA (representing LIPI-4), but none of the strains harbored llsX (representing LIPI-3). These results indicate that ST87 strains pose a potential public risk to consumers. All isolates were susceptible to 13 antimicrobials, except for levofloxacin (1.7%, 1/60), erythromycin (3.3%, 2/60), and clindamycin (88.3%, 53/60). This study is the first to explore the contamination routes and persistence of L. monocytogenes in a Hypsizigus marmoreus production facility. Critical disinfection measures should be adopted to eliminate its persistence throughout processing.

Study of the transfer of Listeria monocytogenes during the slaughter of cattle using molecular typing

The introduction, transmission, and persistence of Listeria monocytogenes in Belgian beef slaughterhouses was investigated using genetic characterization. During slaughter, samples were taken of the hide, carcass, and environment to detect the pathogen. Remarkably, L. monocytogenes was massively present on the hide of incoming animals (93%; 112/120), regardless of their visual cleanliness, which implies high contamination pressure levels entering the slaughterhouses. Pathogen transfer via cross-contamination was conclusively confirmed in this study, with the same pulsotypes isolated from the hide, carcass, and environmental samples. Despite the important bacterial presence on the hide of incoming animals, most slaughterhouses succeeded in limiting the transfer as cause of carcass contamination. Persistence along the slaughter line seemed to be a more significant problem, as it was clearly linked to most of the L. monocytogenes positive carcasses. In one slaughterhouse, whole genome sequencing (WGS) revealed that the carcass splitter had been contaminating carcasses with the same strain belonging to CC9 for more than one year.

Genomic and Transcriptomic Analysis of Biofilm Formation in Persistent and Transient Listeria monocytogenes Isolates from the Retail Deli Environment Does Not Yield Insight into Persistence Mechanisms.

Persistence of Listeria monocytogenes in retail deli environments is a serious food safety issue, potentially leading to cross-contamination of ready-to-eat foods such as deli meats, salads, and cheeses. We previously discovered strong evidence of L. monocytogenes persistence in delis across multiple states. We hypothesized that this was correlated with isolates' innate characteristics, such as biofilm-forming capacity or gene differences. To test this hypothesis, we sequenced the genomes of 21 L. monocytogenes isolates previously collected longitudinally from the retail deli environment. Isolates were chosen to represent varying attachment capacity and sanitizer tolerance as well as persistence or transience. We used single-nucleotide polymorphism analysis to characterize the isolates' genetic relationships and used BLAST to search the isolates' genomes for antibiotic resistance elements, quaternary ammonium tolerance genes, and stress survival islets. We further chose four isolates for RNA-sequencing analysis and compared their global biofilm transcriptome with their global planktonic transcriptome. We did not find genetic content that explained persistence. The presence of stress survival islet-1 correlated to increased attachment capacity (p < 0.05), but not persistence. Further, the presence of sanitizer tolerance elements was not significantly correlated with phenotypic sanitizer tolerance. Analysis of biofilm versus planktonic gene expression did not show the expected differences in gene expression patterns. Overall, L. monocytogenes persistence in the deli environment is likely a matter of poor sanitation and/or facility design, rather than isolates' biofilm-forming capacity, sanitizer tolerance, or genomic content.

Distribution, diversity and persistence of Listeria monocytogenes in swine slaughterhouses and their association with food and human listeriosis strains.

Listeria monocytogenes is the etiological agent of listeriosis, a major foodborne disease and an important public health concern. Contamination of meat with L. monocytogenes occurs frequently at the slaughterhouse. Our aims were; 1) to investigate the distribution of L. monocytogenes in the processing areas of four swine slaughterhouses; 2) to describe the diversity of L. monocytogenes strains by pulsed-field gel electrophoresis; 3) to identify persistent L. monocytogenes strains and describe their distribution; 4) to investigate the associations between persistence of strains and their following characteristics: detection in food isolates, detection in human clinical isolates, and the presence of benzalkonium chloride (BAC) resistance genes. Various operation areas within the four swine slaughterhouses were sampled on four occasions. A total of 2496 samples were analyzed, and L. monocytogenes was successfully isolated from 243 samples. The proportion of positive samples ranged from 32 to 58% in each slaughterhouse and from 24 to 68% in each operation area. Fifty-eight different pulsotypes were identified and eight pulsotypes, present in samples collected during 4 visits, were considered persistent. The persistent pulsotypes were significantly more likely to be detected in food (P < 0.01, exact χ²) and human clinical cases (P < 0.01, exact χ²), respectively. Among pulsotypes harboring the BAC bcrABC resistance cassette or the emrE multidrug transporter gene, 42.8% were persistent compared to 4.5% for pulsotypes without these resistance genes (P < 0.01, exact χ²). Our study highlights the importance of persistent L. monocytogenes strains in the environmental contamination of slaughterhouses, which may lead to repeated contamination of meat products. It also shows that the presence of disinfectants resistance genes is an important contributing factor.

Persistence of Listeria monocytogenes in food commodities: foodborne pathogenesis, virulence factors, and implications for public health

Listeria monocytogenes is a ubiquitous pathogen found commonly in vast environment conditions and most commonly found in fresh-cut salads, raw fruits raw vegetables, sausages, cured meat, shellfish, refrigerated ready to eat food, soft cheeses, raw and under pasteurized milk. L. monocytogenes is a member of genus Listeria, along with five other members, such as L. innocua, L. ivanovii, L. seeligeri, L. grayi and L. murrayi. However, L. monocytogenes is the widely known species found in human cases, although a few cases with L. ivanovii have also reported. The infection caused by L. monocytogenes is known as listeriosis, and most affected populations are neonates, pregnant women, immune-suppressed and older people. Listeriosis is a severe condition with a higher level of hospitalization (94%) and around 16% fatality. Listeria can tolerate wide ranges of temperature, pH values, ions including Chlorides and Nitrites, different Oxygen levels. Therefore, L. monocytogenes is widely identified and survived easily on harsh environmental conditions. The ability of biofilm formation makes Listeria even harder to eliminate through food processing and associated surfaces. Although outbreaks are not very common, several large outbreaks have been documented in the literature. The extensive incubation period of this pathogen makes it challenging to identify the cause in most of the outbreaks, and most of the Listeria associated cases found to be sporadic. L. monocytogenes can cause substantial economic and public health impacts due to its effect on humans. Once contaminated, Listeria is hard to eliminate due to widespread presence in the environment, intrinsic physiological resistance, ability to adapt to external stresses, and the ability to grow at a wide range of temperatures. The objective of this review was to understand foodborne Listeria infections in humans and animals, the occurrence of infections and outbreaks, the growth and survival of L. monocytogenes in food-related industries, and risk reduction strategies.

Characterization of persistent Listeria monocytogenes strains from ten dry-cured ham processing facilities

The majority of cases of listeriosis are associated with the consumption of contaminated food. Some strains of Listeria monocytogenes can persist over months or years in meat processing plants increasing the risk of product contamination. The presence of L. monocytogenes was examined in 10 dry-cured ham processing facilities. A total of 1801 samples were collected from environment and equipment, during processing (1095) and after cleaning and disinfection (706). These samples were taken from non-food contact (736) and food contact (1065) surfaces. In addition, 204 samples from ham surfaces were also analysed. Prevalence varied from 6% to 34% among facilities, and was higher during processing than after cleaning and disinfection (24.8% vs 11.0%) and from non-food than from food contact surfaces (22.6% vs 17.4%). L. monocytogenes serotype 1/2a was predominant (53.9%), followed by 1/2c (26.0%) and 1/2b (15.3%) and less frequently 4b (4.8%). A total of 142 different pulsotypes were registered. Potential persistent L.monocytogenes strains were isolated in 9 out the 10 facilities, with no more than 6 pulsotypes in a given plant. Two pulsotypes were common in different installations, detected before and after cleaning and disinfection, highlighting the importance of monitoring the presence of this pathogen in dry-cured ham processing environments.

Control of Growth and Persistence of Listeria monocytogenes and β-Lactam-Resistant Escherichia coli by Thymol in Food Processing Settings

The main objective of this study was to evaluate the efficacy of thymol in controlling environmental contamination in food processing facilities. The effect of thymol was tested as an agent to prevent planktonic and bacterial biofilm growth of twenty-five Listeria monocytogenes isolates from a variety of foods and five Escherichia coli isolates from a farm. The E. coli isolates were positive for extended spectrum β-lactamase (ESBL) genes. All isolates and reference strains were susceptible to thymol at Minimum inhibitory concentration (MIC) values ranging from 250 to 800 μg/mL. An interesting activity of interference with biofilm formation of L. monocytogenes and E. coli was found for thymol at sub-MIC concentrations of 200, 100, 75, and 50 μg/mL. Anti-biofilm activity ranging from 59.71% to 66.90% against pre-formed 24-h-old L. monocytogenes biofilms at concentrations of 500 or 800 µg/mL, corresponding to 2× MIC, was determined against free-living forms of six isolates chosen as the best or moderate biofilm producers among the tested strains. The property of thymol to attack L. monocytogenes biofilm formation was also observed at a concentration of 100 µg/mL, corresponding to 1/4 MIC, by using a stainless-steel model to simulate the surfaces in food industries. This study gives information on the use of thymol in food processing setting.