Listeria monocytogenes in retail establishments: Contamination routes and control strategies

In the past two decades, serious outbreaks of foodborne disease were caused by Listeria monocytogenes, a pathogen frequently found in delicatessens at retail. Although the prevalence of listeriosis is not high, the severity of disease is significant, with high hospitalization and mortality rates. Potential sources of L. monocytogenes and food contamination routes in retail and food service operations include incoming raw materials, food products, food handlers, customers, vendors, and environmental sources. Risk mitigation strategies for L. monocytogenes should be based on integrated control along the food chain continuum, from farm to retail establishment.     

Emergence of Antibiotic Resistance in Listeria monocytogenes Isolated from Food Products: A Comprehensive Review

Listeria monocytogenes is an opportunistic pathogen that has been involved in several deadly illness outbreaks. Future outbreaks may be more difficult to manage because of the emergence of antibiotic resistance among L. monocytogenes strains isolated from food products. The present review summarizes the available evidence on the emergence of antibiotic resistance among L. monocytogenes strains isolated from food products and the possible ways this resistance has developed. Furthermore, the resistance of food L. monocytogenes isolates to antibiotics currently used in the treatment of human listeriosis such as penicillin, ampicillin, tetracycline, and gentamicin, has been documented. Acquisition of movable genetic elements is considered the major mechanism of antibiotic resistance development in L. monocytogenes. Efflux pumps have also been linked with resistance of L. monocytogenes to some antibiotics including fluoroquinolones. Some L. monocytogenes strains isolated from food products are intrinsically resistant to several antibiotics. However, factors in food processing chains and environments (from farm to table) including extensive or sub‐inhibitory antibiotics use, horizontal gene transfer, exposure to environmental stresses, biofilm formation, and presence of persister cells play crucial roles in the development of antibiotic resistance by L. monocytogenes.     

Longitudinal monitoring of Listeria monocytogenes contamination patterns in a farmstead dairy processing facility

Contamination of dairy products with Listeria monocytogenes is a concern because multiple human listeriosis outbreaks have been linked to contaminated cheese and dairy products. Dairy production on farmstead operations may be a particular concern because L. monocytogenes is also an animal pathogen that can be shed by ruminants with and without clinical symptoms; physical proximity between production animal and dairy processing facilities may thus provide a higher risk for introduction of L. monocytogenes into the dairy production process. To better understand the risks of L. monocytogenes contamination associated with farmstead dairy production, samples from a farmstead dairy processing operation and the milking barn of the directly adjacent dairy sheep operation were tested for L. monocytogenes over a 3-yr period. Prevalence of L. monocytogenes for samples collected on the farm (n = 85) and the dairy production facility (n = 674) was 9.4 and 2.7%, respectively. Molecular subtyping using automated EcoRI ribotyping of L. monocytogenes isolates revealed that distinct subtypes were associated with the dairy production facility and the farm's milking parlor. Although a total of 5 and 4 different ribotypes were identified among isolates obtained from the dairy production facility and the milking parlor, respectively, only 1 ribotype (DUP-1030A) was isolated from both. Different ribotypes were predominant among isolates from the dairy production facility (ribotype DUP-1052A, representing 15 of 18 isolates) and the farm's milking parlor (ribotype DUP-1039A, representing 4 of 8 isolates); each of these ribotypes appeared to persist over time in the respective area. Our data support that i) in farmstead dairy processing facilities, L. monocytogenes present on the farm can largely be prevented from being introduced into the processing facility; and ii) L. monocytogenes can persist on farm and in processing areas, providing a potential high-risk source for contamination. Preventing cross contamination between dairy production and processing facilities and control of persistent L. monocytogenes are thus critical to assuring the microbial safety of farmstead dairy products.     

Listeria monocytogenes in Vacuum‐Packed Smoked Fish Products: Occurrence,Routes of Contamination,and Potential Intervention Measures

The occurrence of Listeria monocytogenes in ready‐to‐eat (RTE) fish products is well documented and represents an important food safety concern. Contamination of this pathogen in vacuum‐packed (VP) smoked fish products at levels greater than the RTE food limit (100 CFU/g) has been traced to factors such as poor sanitary practices, contaminated processing environments, and temperature abuse during prolonged storage in retail outlets. Intervention technologies including physical, biological, and chemical techniques have been studied to control transmission of L. monocytogenes to these products. High‐pressure processing, irradiation, and pulsed UV‐light treatment have shown promising results. Potential antilisterial effects of some sanitizers and combined chemical preservatives have also been demonstrated. Moreover, the concept of biopreservation, use of bioactive packaging, and a combination of different intervention technologies, as in the hurdle concept, are also under consideration. In this review, the prevalence, routes of contamination, and potential intervention technologies to control transmission of L. monocytogenes in VP smoked fish products are discussed.     

Listeria monocytogenes in cheese and the dairy environment remains a food safety challenge: The role of stress responses

Dairy products, in particular soft cheeses, pose a major concern to the dairy industry and public health authorities as they are the leading source of listeriosis outbreaks, a severe foodborne infection affecting pregnant women, children, elderly and immunocompromised people, with a high (20–30%) mortality rate. Cheeses offer a suitable environment for the survival and growth of Listeria monocytogenes, allowing this pathogen to display tolerance responses that can favour its presence in cheese and persistence in dairy processing plants. Extensive food safety regulations in the EU towards prevention of contamination of dairy products with L. monocytogenes have been implemented. However due to the specific abilities of this pathogen to overcome the processing hurdles, its control remains a challenge. Compliance with the Good Manufacturing Practices, observation of Hazard Analysis Critical Control Points (HACCP) and the surveillance of the pathogen in the cheese processing environment are crucial to provide consumers with a safe product. This review aims to provide an overview on the current knowledge about the potential for the transmission of L. monocytogenes in cheese and its abilities to overcome the challenging processing conditions and implications for the behaviour of the pathogen in the host.     

Prevalence and biofilm-forming ability of Listeria monocytogenes in New Zealand mussel (Perna canaliculus) processing plants

Greenshell™ mussels are New Zealand’s largest seafood export species. Some export markets require compliance with ‘zero’ tolerance legislation for Listeria monocytogenes in 25 g of product. Even though individually quick frozen (IQF) mussel products are labeled ‘to be cooked’, and are not classified as ready-to-eat, some markets still require them to comply with the strict policy. Three mussel processing plants were assessed for the pattern of L. monocytogenes contamination on raw material, environment, food contact surfaces, and in the final product. Cultures (n = 101) obtained from an industrial Listeria monitoring program from August 2007 to June 2009 were characterized by serotyping and pulsed field gel electrophoresis. Using the crystal violet method, isolates were assessed for their ability to form biofilms. This work confirmed the presence of L. monocytogenes in raw and processed product, and the importance of cross-contamination from external and internal environments. Processing plants had L. monocytogenes pulsotypes that were detected more than once over 6 months. No correlation was found between biofilm-forming ability and persistent isolates. Two pulsotypes (including a persistent one), were previously isolated in human cases of listeriosis in New Zealand, but none of the pulsotypes matched those involved in international outbreaks.     

Control of Listeria monocytogenes Contamination in Ready‐to‐Eat Meat Products

The ubiquitous nature of Listeria monocytogenes and its ability to grow at refrigerated temperature makes L. monocytogenes a significant threat to the safety of ready‐to‐eat (RTE) meat products. The contamination by L. monocytogenes in RTE meat primarily occurs during slicing and packaging after cooking. The effectiveness of post‐package decontamination technology such as in‐package thermal pasteurization, irradiation, and high‐pressure processing are discussed. Formulating meat products with antimicrobial additives is another common approach to control L. monocytogenes in RTE meat. Irradiation is an effective technology to eliminate L. monocytogenes but can influence the quality of RTE meat products significantly. The effect of irradiation or the combination of irradiation and antimicrobials on the survival of L. monocytogenes and the quality of RTE meat is discussed.     

Modeling Surface Transfer of Listeria monocytogenes on Salami during Slicing

ABSTRACT: Listeria monocytogenes has been implicated in several listeriosis outbreaks linked to the consumption of presliced ready‐to‐eat (RTE) deli meats, which has drawn considerable attention in regard to possible cross‐contamination during slicing operation at retail and food service environments. Salami with 15% fat (a moderate fat content deli item) was used to investigate the transfer of L. monocytogenes between a meat slicer and salami slices and to understand its impact on food safety. A 6‐strain cocktail of L. monocytogenes was inoculated onto a slicer blade to an initial level of approximately 3, 5, 6, 7, or 9 log CFU/blade (or approximately 2, 4, 5, 6, or 8 log CFU/cm² of the blade edge area), and then the salami was sliced to a thickness of 1 to 2 mm (case I). For another cross‐contamination scenario, a clean blade was first used to slice salami loaf that was previously surface‐inoculated with L. monocytogenes (approximately 3, 5, 6, 7, 8, or 9 log CFU/100 cm² area), followed by slicing the uninoculated salami loaf (case II). The salami slicing rate was maintained at an average of 3 to 4 slices per minute in all the tests. The results showed that the empirical models developed in this study were reasonably accurate in describing the transfer trend/pattern of L. monocytogenes between the blade and salami slices if the inoculum level was > 5 log CFU on the salami or blade. With an initial inoculum at 3 or 4 log CFU, the experimental data seemed to suggest a rather random pattern of bacterial transfer between blade and salami. The currently developed models are microbial load (n), sequential slice index (X), and contamination route dependent, which might limit their applications to certain conditions. However, the models may be further applied to predict the 3 or 4 log CFU level (and below) cross‐contamination of salami slicing process. Considering only few data are available in the literature regarding food pathogen surface transfer, the empirical models may provide a useful tool in building risk assessment procedures.     

Methicillin‐Resistant Staphylococcus aureus in Seafood: Prevalence,Laboratory Detection,Clonal Nature,and Control in Seafood Chain

Methicillin‐resistant Staphylococcus aureus (MRSA), a versatile pathogen bearing multiple virulence determinants, is increasingly being detected in various food‐producing animals, including fish. In addition, it is a potential food poisoning agent. MRSA is not an inherent microbiota of fish; its presence is attributed to pre‐ or postharvest contamination through fish handlers, water, ice, and processing equipment. Several reviews have been written on MRSA in clinical as well as the food animal‐producing sector, but information specific to MRSA in seafood is scant. This review puts forth insights on MRSA detection in seafood, antibiotic resistance, diversity of clones in seafood, and possible control measures in seafood production chain. Emphasis has been given on assessing the variations in the protocols employed for isolation and identification in different food matrices and lay the foundation for researchers to develop optimized procedure.     

Development of an Internal Amplification Control Using Multiplex PCR for the Detection of Listeria monocytogenes in Food Products

The bacterial pathogen Listeria monocytogenes is responsible for listeriosis, a food-borne disease, which may result in severe illness and possible death. Large outbreaks of listeriosis have been associated with food products including soft cheeses and ready to eat food products. Polymerase chain reaction (PCR) is a molecular identification method for food-borne pathogens; however, a drawback of this method is that false-positive or false-negative results may occur. To validate the accuracy of the PCR as a powerful molecular tool for pathogen detection, it is important that false-negative results be distinguishable from true-negative PCR results. The aim of this study was to design and include an internal amplification control (IAC) within the PCR to coamplify with L. monocytogenes in order to identify false-negative results of L. monocytogenes from ostrich meat and camembert cheese samples. The IAC had to be incorporated into the PCR without loss of specificity and sensitivity on the detection limit of L. monocytogenes and was developed and tested for use in a multiplex PCR detection system. A region of the pUC19 plasmid was selected as the IAC for this study. The optimal concentration at which pUC19 would coamplify with L. monocytogenes was determined to be 0.001 pg/μL. Following an enrichment procedure, the minimum number of organisms detected in a spiked food sample by the PCR was 8 CFU/mL L. monocytogenes; the same detection limit was attained when the pUC19 IAC was included in the PCR. An optimal pUC19 IAC concentration increased the reliability of the PCR for food diagnostic purposes.     

Investigation of Listeria monocytogenes contamination pattern in local Chinese food market and the tracing of two clinical isolates by RAPD analysis

This study was undertaken to investigate the contamination pattern of Listeria monocytogenes in local Chinese food markets and to trace two clinical isolates. Random amplification polymorphic DNA (RAPD) was developed to track the source of L. monocytogenes in ready-to-eat (RTE) food products and from clinical origin. Three random primers, PB1, PB4 and HLWL74, were used to subtype all the L. monocytogenes strains isolated from RTE food products, fresh food products, environmental sewages in the markets and two clinical meningitis patients. It was shown that all the 49 isolates could be classified into 5, 4 and 4 types using these three random primers, PB1, PB4, and HLWL74, respectively. Twenty-seven composite profiles were identified by a combination of the three primers. The same composite profiles of L. monocytogenes could be found both in the fresh food products, environmental sewages and RTE food products, suggesting that the L. monocytogenes in the RTE food products may come from those in the fresh food products or sewage in the same market. The composite profiles of the two clinical isolates were the same as those of strains isolated from RTE food products, indicating that the disease might have resulted from the consumption of the RTE food products contaminated with L. monocytogenes. The results show that RAPD could be a powerful tool for the investigation of contamination pattern of L. monocytogenes in Chinese food markets and also for tracking the source of L. monocytogenes in clinical patients.     

Occurrence of Listeria Monocytogenes in a Serbian Salmon and Seafood Processing Line During 2013

The objective of this study was to examine the occurrence of L. monocytogenes in a selected fish and seafood processing line. Results showed that during 2013, 12.4%, 8.3% and 2.3% of fish, seafood salads and environmental swabs were positive for L. monocytogenes. All positive food samples showed a contamination level below 100 CFU/g. Environmental swabs from surface of slicing and trimming tables, slicing machines, fish filleting and trimming knives, belt glazer and working table were positive for L. monocytogenes. Therefore, strict attention must be paid to cleaning and disinfection to control the level of L. monocytogenes.     

An Ecological Perspective of Listeria monocytogenes Biofilms in Food Processing Facilities

Listeria monocytogenes can enter the food chain at virtually any point. However, food processing environments seem to be of particular importance. From an ecological point of view, food processing facilities are microbial habitats that are constantly disturbed by cleaning and sanitizing procedures. Although L. monocytogenes is considered ubiquitous in nature, it is important to recognize that not all L. monocytogenes strains appear to be equally distributed; the distribution of the organism seems to be related to certain habitats. Currently, no direct evidence exists that L. monocytogenes-associated biofilms have played a role in food contamination or foodborne outbreaks, likely because biofilm isolation and identification are not part of an outbreak investigation, or the definition of biofilm is unclear. Because L. monocytogenes is known to colonize surfaces, we suggest that contamination patterns may be studied in the context of how biofilm formation is influenced by the environment within food processing facilities. In this review, direct and indirect epidemiological and phenotypic evidence of lineage-related biofilm formation capacity to specific ecological niches will be discussed. A critical view on the development of the biofilm concept, focused on the practical implications, strengths, and weaknesses of the current definitions also is discussed. The idea that biofilm formation may be an alternative surrogate for microbial fitness is proposed. Furthermore, current research on the influence of environmental factors on biofilm formation is discussed.     

Listeria monocytogenes: A Target for Bacteriophage Biocontrol

Listeria monocytogenes is a growing concern in the food industry as it is the causative agent of human listeriosis. There are many research articles concerning the growth, survival, and diversity of L. monocytogenes strains isolated from food‐related sources, elucidating the difficulty in controlling these bacteria in a food‐processing facility. Bacteriophage biocontrol of L. monocytogenes strains was introduced in 2006, through the first commercial bacteriophage product targeting L. monocytogenes ListShield^TM. This review focuses on the use of bacteriophage biocontrol to target L. monocytogenes in the food industry, specifically direct application of the bacteriophages to food products. In addition, we discuss characteristics of these bacteria that will have a significant influence on the effective treatment of bacteriophages such as genetic diversity between strains prevalent in one facility. There are many positive results of phage treatments targeting L. monocytogenes in food; however, success of in vitro studies might not be reproducible in practice. Future studies should focus on creating experimental design that will imitate the conditions found in the food industry, such as a stressed state of the targeted bacteria. In situ evaluation of bacteriophage treatment of L. monocytogenes will also be necessary because the presence of these bacteria in a processing facility can vary greatly regarding genetic diversity. The potential use of phages in the food‐processing facility as a biosanitizer for L. monocytogenes, as well as the use of lysins to target these bacteria should also be explored. Despite the exciting research avenues that have to be explored, current research shows that biocontrol of L. monocytogenes is feasible and has potential to positively impact the food industry.     

Prevalence, characterization and sources of Listeria monocytogenes in blue crab (Callinectus sapidus) meat and blue crab processing plants

Seven blue crab processing plants were sampled to determine the prevalence and sources of Listeria spp. and Listeria monocytogenes for two years (2006–2007). A total of 488 raw crabs, 624 cooked crab meat (crab meat) and 624 environmental samples were tested by standard methods. Presumptive Listeria spp. were isolated from 19.5% of raw crabs, 10.8% of crab meat, and 69.5% of environmental samples. L. monocytogenes was isolated from 4.5% of raw crabs, 0.2% of crab meat, and 2.1% of environmental samples. Ninety-seven percent of the isolates were resistant to at least one of the ten antibiotics tested. Eight different serotypes were found among 76 L. monocytogenes isolates tested with the most common being 4b, 1/2b and 1/2a. Automated EcoRI ribotyping differentiated 11 ribotypes among the 106 L. monocytogenes isolates. Based on ribotyping analysis, the distribution of the ribotypes in each processing plant had a unique contamination pattern. A total of 92 ApaI and 88 AscI pulsotypes among the 106 L. monocytogenes isolates were found and distinct pulsotypes were observed in raw crab, crab meat and environmental samples. Ribotypes and serotypes recovered from crab processing plants included subtypes that have been associated with listeriosis cases in other food outbreaks. Our findings suggest that molecular methods may provide critical information about sources of L. monocytogenes in crab processing plants and will augment efforts to improve food safety control strategies such as targeting specific sources of contamination and use of aggressive detergents prior to sanitizing.     

INCIDENCE OF LISTERIA MONOCYTOGENES IN RETAIL CHICKEN MEAT AND ESTABLISHING RELATIONSHIP WITH SOME BACTERIA BY LOGISTIC REGRESSION

ABSTRACT

The objective of this study was to determine the contamination of retail poultry meat in Erzurum (Turkey) by Listeria monocytogenes and to evaluate its relationship with indicator bacteria using logistic regression. The incidence of L. monocytogenes was 32.76%, found in 38 of 116 samples. The application of logistic regression showed Enterobacteriaceae, Pseudomonas spp. and Brochothrix thermosphacta populations have positive, and yeast and mold population have negative relationship with L. monocytogenes presence. The results of this study demonstrate that there is a serious risk in raw poultry meat for consumer health in Erzurum, because of the high incidence of L. monocytogenes in the samples of the present study. Hygienic conditions described in HACCP program should still be enforced in order to minimize L. monocytogenes in poultry meat during the processing.

PRACTICAL APPLICATIONS

Listeria monocytogenes and other Listeria species have been isolated from many different types of raw and processed food, but the main sources and routes of contamination are still not fully understood. There is a need for more knowledge, and data are needed for risk assessment and for improved preventive measures. In order to prevent and control contamination of the environment and food products with this pathogen, it is important to detect the most important sources of contamination and to understand the mechanisms of growth, including relationships with other bacteria. The main concern with raw poultry meat is the incidence of L. monocytogenes in raw chicken because of cross‐contamination with other foods in the home and the possibility of the microorganism surviving in processed chicken. Application of logistic regression was used to identify the main hazards associated with the presence of microorganisms, as well as for applications in predictive food microbiology, modeling binomially distributed data that involve the use of probability models, where the response variable is a “presence/absence” observation of whether or not growth will occur.     

Susceptibility of Listeria monocytogenes to high pressure processing: A review

Listeria monocytogenes has been regarded as an emerging food pathogen responsible for listeriosis, a serious disease given its high mortality rate. The need for better food processing methods has led to an increased interest in high pressure processing (HPP), a novel nonthermal method presented as “producer” of safer food products. This review provides an overview of the effects of HPP on Listeria monocytogenes and on L. innocua, with the latter often used as an amenable surrogate for the pathogenic species. The factors that affect the susceptibility of listeriae to HPP, as well as the long-term implications of postprocessing recovery, are discussed in the perspective of the use of HPP to improve the safety of potential food vehicles.     

Effect of Salt Reduction on Growth of Listeria monocytogenes in Meat and Poultry Systems

Abstract: Reducing sodium in food could have an effect on food safety. The objective was to determine differences in growth of Listeria monocytogenes in meat and poultry systems with salt substitutes. For phase 1, fresh ground beef, pork, and turkey with NaCl, KCl, CaCl₂, MgCl₂, sea salt, or replacement salt added at 2.0% were inoculated with L. monocytogenes to determine growth/survival during 5 d at 4 °C to simulate a pre‐blend process. L. monocytogenes populations significantly decreased (0.41 log CFU/g) during the storage time in beef, but no differences (P > 0.05) were observed over time in pork or turkey. Salt type did not affect (P > 0.05) L. monocytogenes populations during pre‐blend storage. MgCl₂ and NaCl allowed significant growth of aerobic populations during storage. For phase 2, emulsified beef and pork products were processed with 2% NaCl, KCl, sea salt, or a NaCl/KCl blend and post‐process surface‐inoculated with L. monocytogenes to determine growth/survival at 4 °C for 28 d. Pork products showed significantly greater L. monocytogenes population growth at all sampling times (0, 7, 14, 21, and 28 d) than beef products, but salt type had no effect on L. monocytogenes populations with sampling times pooled for data analysis. Although salt types had no impact on L. monocytogenes populations in preblend and emulsified meat products, pork and turkey preblends and emulsified pork had greater L. monocytogenes populations compared with beef products. These studies demonstrate that sodium may not affect the safety of preblends and emulsified meat and poultry products. Practical Application: odium reduction in food is an important topic because of sodium's unfavorable health effects. This research shows that reducing sodium in pre‐blends and emulsified meat and poultry products would have no effect on Listeria monocytogenes populations, but replacement of NaCl with MgCl₂ may affect growth of aerobic populations.     

Effect of Filling Type and Heating Method on Prevalence of Listeria species and Listeria monocytogenes in Dumplings Produced in Poland

The count of Listeria monocytogenes was determined, before and after heat treatment, in 200 samples of dumplings of 9 brands and with different types of stuffing. Analyses were conducted according to ISO 11290–1 standard and with real‐time PCR method. The highest count of L. monocytogenes was found in meat dumplings (10² to 10⁴ CFU/g), whereas products with white cheese‐potato stuffing and vegetable‐mushroom stuffing contained significantly less Listeria, 20 to 80 and 5 to 32 CFU/g, respectively. In cooled meat dumplings the extent of contamination depended significantly on the producer. In addition, a significant (P < 0.05) correlation was determined between contamination level and meat content in the stuffing (rho = 0.418), especially in stuffing containing pork meat (0.464), contrary to beef‐containing stuffing (0.284). Heating dumplings in boiling water for 2 min completely eliminated L. monocytogenes in meat dumplings. In contrast, the microwave heating applied for 2 min at 600 W only reduced the count of L. monocytogenes by 1 to 2 logs. Hence, the microwave heating failed to reduce the risk of infection with this pathogen below the level permissible in the EU regulation, especially in the most contaminated samples. In this case, the efficacy of microwave heating was significantly (P < 0.05) affected by the initial count of L. monocytogenes (rho = 0.626), then by meat content in the stuffing (0.476), and to the lowest extent—by the type of meat (0.415 to 0.425). However, no Listeria sp. and L. monocytogenes were isolated from cooked dumplings with fruits (strawberries or blueberries).     

The microbiology of minimally processed fresh fruits and vegetables

Minimally processed fresh (MPF) fruits and vegetables are good media for growth of microorganisms. They have been involved in outbreaks because of the consumption of products contaminated by pathogens. They are also sensitive to various spoilage microorganisms such as pectinolytic bacteria, saprophytic Gram‐negative bacteria, lactic acid bacteria, and yeasts. Contamination of MPF fruits and vegetables occurs at every stage of the food chain, from cultivation to processing. Polluted environments during cultivation or poor hygienic conditions in processing increase the risk of contamination with foodborne pathogens. Although MPF fruits and vegetables may harbor psychrotrophic microorganisms such as fluorescent pseudomonads or Listeria monocytogenes, good control of refrigeration temperature limits growth of spoilage and pathogenic microorganisms. Modified atmospheres are often efficient to maintain or improve visual and organoleptic quality of MPF fruits and vegetables, but their effects on microorganisms are inconsistent. Chemical disinfection can partially reduce the initial bacterial contamination; irradiation seems to be more efficient. The applications of legislations and quality assurance systems to control contamination, survival, and growth of foodborne pathogens in MPF fruits and vegetables are discussed.