Achieving continuous improvement in reductions in foodborne listeriosis--a risk-based approach

Improving Foodborne Illness Prevention Among Transplant Recipients

The U.S. Department of Agriculture developed a food safety brochure to educate transplant recipients and their caregivers about the risk of contracting foodborne illnesses and safe food-handling prevention practices. Qualitative research was conducted with transplant recipients and caregivers to collect information on participants’ food safety concerns, knowledge, and practices; changes in food safety knowledge and practices after receiving the food safety brochure; preferred communication channels and dissemination strategies for delivering the brochure; and evaluation of the brochure. Many participants received food safety information from a health care provider, but the content and format varied by institution. Most participants are not following recommended practices to cook and chill food safely, and many participants consume high-risk foods associated with foodborne illnesses. After reading the brochure, many participants made or plan to make at least one food safety recommendation. The research findings were used to refine the brochure before nationwide distribution.

Prevalence of Listeria monocytogenes in ready-to-eat foods sampled from the point of sale in Wales, United Kingdom

A survey of Listeria in ready-to-eat food took place in Wales, United Kingdom, between February 2008 and January 2009. In total, 5,840 samples were taken and examined for the presence of Listeria species, including L. monocytogenes. Samples were tested using detection and enumeration methods, and the results were compared with current United Kingdom guidelines for the microbiological quality of ready-to-eat foods. The majority of samples were negative for Listeria by both direct plating and enriched culture. Seventeen samples (0.29%) had countable levels of Listeria species (other than L. monocytogenes), and another 11 samples (0.19%) had countable levels of L. monocytogenes. Nine samples (0.15%) were unsatisfactory or potentially hazardous when compared with United Kingdom guideline limits; six (0.10%) were in the unsatisfactory category (>100 CFU/g) for Listeria species (other than L. monocytogenes), and three (0.05%) were in the unacceptable or potentially hazardous category (>100 CFU/g) for L. monocytogenes. All three of these samples were from sandwiches (two chicken sandwiches and one ham-and-cheese sandwich). The most commonly isolated serotype of L. monocytogenes was 1/2a. This survey was used to determine the current prevalence of Listeria species and L. monocytogenes in ready-to-eat foods sampled from the point of sale in Wales.

Growth of Listeria spp. in shredded cabbage is enhanced by a mild heat treatment

Mild thermal processing can enhance the shelf life of cut fruits and vegetables by delaying the onset of spoilage and preserving the organoleptic properties of shredded cabbage. However, food safety issues related to this process have not been fully investigated. Therefore, the survival and growth of Listeria spp. on cabbage treated in this manner was examined. Experimentally, 24 strains of Listeria spp. (including L. monocytogenes) were inoculated onto cut and intact cabbage tissues and stored at 5 degrees C. All strains on intact tissues exhibited a moderate decline in numbers (up to 1.0 log CFU/cm(2)) over the 28-day storage period. Conversely, cut tissue supported growth of most strains during the first 7 to 14 days of incubation with maximum increases of 1.2 log CFU/cm(2). Subsequently, the survival or growth on heat-treated (50 degrees C for 3 min) and untreated shredded cabbage of four L. monocytogenes and four nonpathogenic Listeria spp. strains were compared during storage for 21 days at 5 degrees C. Growth on untreated shred for all strains was similar to the results observed on cut tissue with a maximum increase of approximately 1.0 log CFU/g. However, in the heat-treated cabbage shred all strains displayed a rapid increase in growth (up to 2.5 log CFU/g) during the first 7 days of incubation, which may be indicative of the destruction of an endogenous growth-inhibiting compound within the cabbage. In conclusion, this study shows that mild thermal treatments of cut cabbage may promote pathogen growth if other inimical barriers are not implemented downstream of the thermal treatment.

Overview of current meat hygiene and safety risks and summary of recent studies on biofilms, and control of Escherichia coli O157:H7 in nonintact, and Listeria monocytogenes in ready-to-eat, meat products

As meat consumption is increasing around the world, so do concerns and challenges to meat hygiene and safety. These concerns are mostly of a biological nature and include bacterial pathogens, such as Escherichia coli O157:H7, Salmonella and Campylobacter in raw meat and poultry, and Listeria monocytogenes in ready-to-eat processed products, while viral pathogens are of major concern at foodservice. A major goal of scientists, industry, public health and regulatory authorities is to control pathogenic microorganisms and improve meat product hygiene and safety within a country and internationally. This paper is not a comprehensive or critical review of the scientific literature on the broad area of meat hygiene and safety, but it provides an overview of major current meat hygiene and safety issues, and then a summary of studies on biofilm formation by pathogens, control of E. coli O157:H7 in nonintact meat products, and control of L. monocytogenes in ready-to-eat meat products, conducted at the Center for Meat Safety & Quality and Food Safety Cluster of Colorado State University in recent years.

Quantitative Risk Assessment of Listeriosis-Associated Deaths Due to Listeria monocytogenes Contamination of Deli Meats Originating from Manufacture and Retail

The objective of this study was to estimate the relative risk of listeriosis-associated deaths attributable to Listeria monocytogenes contamination in ham and turkey formulated without and with growth inhibitors (GIs). Two contamination scenarios were investigated: (i) prepackaged deli meats with contamination originating solely from manufacture at a frequency of 0.4% (based on reported data) and (ii) retail-sliced deli meats with contamination originating solely from retail at a frequency of 2.3% (based on reported data). Using a manufacture-to-consumption risk assessment with product-specific growth kinetic parameters (i.e., lag phase and exponential growth rate), reformulation with GIs was estimated to reduce human listeriosis deaths linked to ham and turkey by 2.8- and 9-fold, respectively, when contamination originated at manufacture and by 1.9- and 2.8-fold, respectively, for products contaminated at retail. Contamination originating at retail was estimated to account for 76 and 63% of listeriosis deaths caused by ham and turkey, respectively, when all products were formulated without GIs and for 83 and 84% of listeriosis deaths caused by ham and turkey, respectively, when all products were formulated with GIs. Sensitivity analyses indicated that storage temperature was the most important factor affecting the estimation of per annum relative risk. Scenario analyses suggested that reducing storage temperature in home refrigerators to consistently below 7°C would greatly reduce the risk of human listeriosis deaths, whereas reducing storage time appeared to be less effective. Overall, our data indicate a critical need for further development and implementation of effective control strategies to reduce L. monocytogenes contamination at the retail level.

Obstetrician/gynecologists' knowledge, attitudes, and practices regarding prevention of infections in pregnancy

BACKGROUND

Maternal infection during pregnancy is a well-recognized cause of birth defects and developmental disabilities, as well as an important contributor to other adverse pregnancy outcomes. The objective of the present survey was to gain information about the knowledge, attitudes, and practices of obstetrician/gynecologists regarding prevention of infections during pregnancy.

METHODS

A survey was mailed to 606 Collaborative Ambulatory Research Network (CARN) members of the American College of Obstetricians and Gynecologists (ACOG) (approximately 2% of membership). CARN members were sampled to demographically represent ACOG.

RESULTS

Of the 606 eligible respondents, surveys were received from 305 (response rate: 50%). Most obstetrician/gynecologists knew that specific actions by pregnant women could reduce the risk of infection. Seventy-nine to eighty-eight percent reported counseling pregnant women about preventing infection from Toxoplasma gondii, hepatitis B virus, and influenza, 50%-68% about varicella-zoster virus, Listeria monocytogenes, and Parvovirus B19, and <50% about cytomegalovirus, Bordetella pertussis, and lymphocytic choriomeningitis virus. The majority reported time constraints were a barrier to counseling, although most reported educational materials would be helpful.

CONCLUSIONS

Knowledge was accurate and preventive counseling was appropriate for some infections, but for others it could be improved. Further studies are needed to identify strategies to increase preventive counseling.

Prevalence and molecular diversity of Listeria monocytogenes in retail establishments

As our understanding of Listeria monocytogenes transmission in retail and deli operations is limited, we conducted a cross-sectional study of L. monocytogenes contamination patterns in 121 retail establishments, using testing of food and environmental samples and subtype analysis (ribotyping) of L. monocytogenes isolates. Seventy-three (60%) establishments had at least one sample that tested positive for L. monocytogenes; 5 (2.7%) of the 183 food and 151 (13.0%) of the 1,161 environmental samples tested positive for L. monocytogenes, including 125 (16.7%) and 26 (6.3%) of non-food contact and food contact surface samples, respectively. Thirty-two EcoRI ribotypes were identified among the 156 L. monocytogenes isolated. Twenty-seven establishments had two or more L. monocytogenes with the same ribotype within a given establishment, including 9 establishments where isolates from 3 to 5 samples had the same ribotype. In 5 of 7 establishments where follow-up sampling was conducted 8 to 19 months after the initial sampling, isolates with the same ribotype were obtained in both samplings; persistence of a given strain was also confirmed by pulsed-field gel electrophoresis. Our data indicate that (i) L. monocytogenes is regularly found in some retail environments; (ii) L. monocytogenes strains are often widely distributed in retail, indicating cross-contamination and dispersal; (iii) L. monocytogenes can persist in retail environments for more than 1 year; and (iv) a number of L. monocytogenes subtypes isolated at retail are common among human listeriosis cases. We also identified specific contamination patterns in retail establishments, providing critical information for the development of L. monocytogenes control strategies.

Development and validation of an extensive growth and growth boundary model for Listeria monocytogenes in lightly preserved and ready-to-eat shrimp

An existing cardinal parameter growth and growth boundary model for Listeria monocytogenes (O. Mejlholm and P. Dalgaard, J. Food Prot. 70:70-84 and 2485-2497, 2007) was expanded with terms for the effects of acetic, benzoic, citric, and sorbic acids to include a total of 12 environmental parameters and their interactive effects. The new model predicted growth rates (micro(max) values) of L. monocytogenes accurately with bias and accuracy factors of 1.0 and 1.5, respectively, for 16 batches of brined shrimp with benzoic, citric, and sorbic acids. Corresponding values of 0.9 and 1.2, respectively, were obtained for five batches of brined shrimp with acetic and lactic acids. Growth and no-growth responses of L. monocytogenes were also appropriately predicted with 88% correct prediction for 26 experiments with brined shrimp. The new model performed better than existing L. monocytogenes models with a comparable degree of complexity. The high number of environmental parameters, including six organic acids (acetic acid, benzoic acid, citric acid, diacetate, lactic acid, and sorbic acid), allows the new model to predict the effect of substituting one set of preserving parameters for another. The new model also allowed the distance between the growth boundary and specific product characteristics to be quantified by a psi value. This can be of practical importance in the development or reformulation of seafood with preserving parameters that prevent growth of L. monocytogenes and take variability in product characteristics into account.

Listeriosis in Norway 1977–2003

In Europe, the incidence of invasive listeriosis has increased substantially during the last decades. We here present data from 289 listeriosis cases reported in Norway during the period 1977-2003, of which 12 cases were associated with 2 outbreaks and 39 cases were pregnancy-related. Medical records were obtained from 209 cases with listeriosis reported in 1977-2000. While the incidence of pregnancy-related listeriosis has remained stable at an average rate of 34 per million pregnant women per y during the period, the incidence of sporadic, non-pregnancy-related cases has increased from 1.1 to 3.7 per million per y. The present Norwegian incidence of reported cases is lower than in Denmark, but the case fatality rate is higher, indicating a possible under-reporting of mild listeriosis cases in Norway. We discuss how preventive measures, case identification and surveillance may have influenced listeriosis incidence in Norway.

Quantitative risk assessment for Listeria monocytogenes in selected categories of deli meats: impact of lactate and diacetate on listeriosis cases and deaths

Foodborne disease associated with consumption of ready-to-eat foods contaminated with Listeria monocytogenes represents a considerable pubic health concern. In a risk assessment published in 2003, the U.S. Food and Drug Administration and the U.S. Food Safety and Inspection Service estimated that about 90% of human listeriosis cases in the United States are caused by consumption of contaminated deli meats. In this risk assessment, all deli meats were grouped into one of 23 categories of ready-to-eat foods, and only the postretail growth of L. monocytogenes was considered. To provide an improved risk assessment for L. monocytogenes in deli meats, we developed a revised risk assessment that (i) models risk for three subcategories of deli meats (i.e., ham, turkey, and roast beef) and (ii) models L. monocytogenes contamination and growth from production to consumption while considering subcategory-specific growth kinetics parameters (i.e., lag phase and exponential growth rate). This model also was used to assess how reformulation of the chosen deli meat subcategories with L. monocytogenes growth inhibitors (i.e., lactate and diacetate) would impact the number of human listeriosis cases. Use of product-specific growth parameters demonstrated how certain deli meat categories differ in the relative risk of causing listeriosis; products that support more rapid growth and have reduced lag phases (e.g., turkey) represent a higher risk. Although reformulation of deli meats with growth inhibitors was estimated to reduce by about 2.5- to 7.8-fold the number of human listeriosis cases linked to a given deli meat subcategory and thus would reduce the overall risk of human listeriosis, even with reformulation deli meats would still cause a considerable number of human listeriosis cases. A combination of strategies is thus needed to provide continued reduction of these cases. Risk assessment models such as that described here will be critical for evaluation of different control approaches and to help define the combinations of control strategies that will have the greatest impact on public health.

Evaluation of changes in Listeria monocytogenes populations on frankfurters at different stages from manufacturing to consumption

This study evaluated the fate of inoculated Listeria monocytogenes on frankfurters stored under conditions simulating those that may be encountered between manufacturing and consumption. Frankfurters with or without 1.5% potassium lactate and 0.1% sodium diacetate (PL/SD) were inoculated (1.8 +/- 0.1 log CFU/cm(2)) with a 10-strain composite of L. monocytogenes, vacuum-packaged, and stored under conditions simulating predistribution storage (24 h, 4 degrees C), temperature abuse during transportation (7 h, 7 degrees C followed by 7 h, 12 degrees C), and storage before purchase (60 d, 4 degrees C; SBP). At 0, 20, 40, and 60 d of SBP, samples were exposed to conditions simulating delivery from stores to homes or food establishments (3 h, 23 degrees C), and then opened or held vacuum-packaged at 4 or 7 degrees C for 14 d (SHF). Pathogen counts remained relatively constant on frankfurters with PL/SD regardless of product age and storage conditions; however, they increased on product without antimicrobials. In vacuum-packaged samples, during SHF at 4 degrees C, the pathogen grew faster (P < 0.05) on older product (20 d of SBP) compared to product that was fresh (0 d of SBP); a similar trend was observed in opened packages. At 7 degrees C, the fastest growth (0.35 +/- 0.02 log CFU/cm(2)/d) was observed on fresh product in opened packages; in vacuum-packages, growth rates on fresh and aged products were similar. By day 40 of SBP the pathogen reached high numbers and increased slowly or remained unchanged during SHF. This information may be valuable in L. monocytogenes risk assessments and in development of guidelines for storage of frankfurters between package opening and product consumption.

Different contamination patterns of lineage I and II strains of Listeria monocytogenes in a Spanish broiler abattoir

The purpose of this study was to determine whether genetically similar or diverse strains of Listeria monocytogenes colonize the environment and carcasses in a single Spanish broiler abattoir over time. The study was composed of 5 surveys over a 1.5-yr period and included the monitoring of cleaning and disinfection procedures. Overall, a total of 212 samples were tested for the presence of L. monocytogenes, and 31% of the samples were found to be positive. Listeria monocytogenes was isolated from carcasses and product contact and noncontact sites in the evisceration and carcass classification areas of the abattoir. A total of 132 L. monocytogenes isolates were characterized by PCR-based serotyping and pulsed-field gel electrophoresis (PFGE) restriction analysis with the endonucleases ApaI and AscI. Molecular serotyping showed that L. monocytogenes isolates were of serotypes 1/2a and 1/2b. Isolates of serotype 1/2b (89.4%) were contaminating carcasses as well as environmental product contact and noncontact sites, whereas isolates of serotype 1/2a (10.6%) were recovered only from environmental product noncontact sites. A relatively low genetic diversity was found in this group of L. monocytogenes isolates from the abbatoir; only 14 different PFGE types (A1 to A14) were obtained. Nine pulsotypes belonging to serotype 1/2b (lineage I) were grouped in only one PFGE genetic cluster, whereas 5 pulsotypes belonging to serotype 1/2a (lineage II) were grouped into 4 PFGE genetic clusters. Two genetically related pulsotypes of serotype 1/2b (A1 and A2, 64.4% of the isolates) predominated and persisted in the abattoir. Our study indicated that a few strains of L. monocytogenes lineage I that were genetically very closely related might be specifically adapted to colonizing the evisceration zone of the abattoir and were predominant on carcasses over 1 yr. On the other hand, a genetically diverse group of lineage II strains were present in the abattoir environment, but never contaminated carcasses.

The growth limits of a large number of Listeria monocytogenes strains at combinations of stresses show serotype--and niche-specific traits

AIMS

The aim of this study was to associate the growth limits of Listeria monocytogenes during exposure to combined stresses with specific serotypes or origins of isolation, and identify potential genetic markers.

METHODS AND RESULTS

The growth of 138 strains was assessed at different temperatures using combinations of low pH, sodium lactate, and high salt concentrations in brain heart infusion broth. None of the strains was able to grow at pH < or = 4.4, a(w) < or = 0.92, or pH < or = 5.0 combined with a(w) < or = 0.94. In addition, none of the strains grew at pH < or = 5.2 and NaLac > or = 2%. At 30 degrees C, the serotype 4b strains showed the highest tolerance to low pH and high NaCl concentrations at both pH neutral (pH 7.4) and mild acidic conditions (pH 5.5). At 7 degrees C, the serotype 1/2b strains showed the highest tolerance to high NaCl concentrations at both pH 7.4 and 5.5. Serotype 1/2b meat isolates showed the highest tolerance to low pH in the presence of 2% sodium lactate at 7 degrees C. ORF2110 and gadD1T1 were identified as potential biomarkers for phenotypic differences.

CONCLUSIONS

Differences in growth limits were identified between specific L. monocytogenes strains and serotypes, which could in some cases be associated with specific genetic markers.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our data confirm the growth limits of L. monocytogenes as set out by the European Union for ready-to-eat foods and provides an additional criterion. The association of L. monocytogenes serotypes with certain stress responses might explain the abundance of certain serotypes in retail foods while others are common in clinical cases.

Consumer assessment of safety and date labeling statements on ready-to-eat meat and poultry products designed to minimize risk of listeriosis

Point-of-purchase safety-based labeling guidance on the proper storage and handling of refrigerated ready-to-eat (RTE) meat and poultry products could help reduce the risk of listeriosis. Seniors and pregnant women are two population groups at increased risk of listeriosis due to suppressed or compromised immune systems. We conducted 11 focus groups with senior-aged women and women of childbearing age in Colorado and Ohio to assess consumer awareness of Listeria, storage practices of RTE meat products, perceptions regarding the acceptability and usefulness of common date and potential food safety labeling statements on RTE meat and poultry products, and food safety information needs. Storage times for opened and unopened RTE products varied widely, with opened products often being stored longer than recommended. Women in both age groups paid attention to date labels on packages but varied highly in their interpretation of the statements. "Use by" statements were considered clearer and more helpful than "Sell by" or "Best if used by" labels. Proposed food safety-based labeling statements listing "antilisterial" agents used in RTE products were not well received. However, labels giving consumers instructions on how long they could keep RTE products and when to discard them after opening were considered helpful and well received. Participants indicated the need for further information about Listeria and its control. Educational information at point-of-purchase and where seniors and pregnant women congregate are suggested. Manufacturers are encouraged to provide more complete information on the safe storage and use of ready-to-eat meat and poultry products on package labels.

Modeling and predicting the growth of lactic acid bacteria in lightly preserved seafood and their inhibiting effect on Listeria monocytogenes

A cardinal parameter model was developed to predict the effect of diacetate, lactate, CO2, smoke components (phenol), pH, NaCl, temperature, and the interactions between all parameters on the growth of lactic acid bacteria (LAB) in lightly preserved seafood. A product-oriented approach based on careful chemical characterization and growth of bacteria in ready-to-eat seafoods was used to develop this new LAB growth model. Initially, cardinal parameter values for the inhibiting effect of diacetate, lactate, CO2, pH, and NaCl-water activity were determined experimentally for a mixture of LAB isolates or were obtained from the literature. Next, these values and a cardinal parameter model were used to model the effect of temperature (T(min)) and smoke components (P(max)). The cardinal parameter model was fitted to data for growth of LAB (mu(max) values) in lightly preserved seafood including cold-smoked and marinated products with different concentrations of naturally occurring and added organic acids. Separate product validation studies of the LAB model resulted in average bias and accuracy factor values of 1.2 and 1.5, respectively, for growth of LAB (mu(max) values) in lightly preserved seafood. Interaction between LAB and Listeria monocytogenes was predicted by combining the developed LAB model and an existing growth and growth boundary model for the pathogen (O. Mejlholm and P. Dalgaard, J. Food Prot. 70:70-84). The performance of the existing L. monocytogenes model was improved by taking into account the effect of microbial interaction with LAB. The observed and predicted maximum population densities of L. monocytogenes in inoculated lightly preserved seafoods were 4.7 and 4.1 log CFU g(-1), respectively, whereas for naturally contaminated vacuum-packed cold-smoked salmon the corresponding values were 0.7 and 0.6 log CFU g(-1) when a relative lag time of 4.5 was used for the pathogen.

Applying the food safety objective and related standards to thermal inactivation of Salmonella in poultry meat

The objective of this study was to investigate the practicality of designing a heat treatment process in a food manufacturing operation for a product governed by a Food Safety Objective (FSO). Salmonella in cooked poultry meat was taken as the working example. Although there is no FSO for this product in current legislation, this may change in the (near) future. Four different process design calculations were explored by means of deterministic and probabilistic approaches to mathematical data handling and modeling. It was found that the probabilistic approach was a more objective, transparent, and quantifiable approach to establish the stringency of food safety management systems. It also allowed the introduction of specific prevalence rates. The key input analyzed in this study was the minimum time required for the heat treatment at a fixed temperature to produce a product that complied with the criterion for product safety, i.e., the FSO. By means of the four alternative process design calculations, the minimum time requirement at 70 degrees C was established and ranged from 0.26 to 0.43 min. This is comparable to the U.S. regulation recommendations and significantly less than that of 2 min at 70 degrees C used, for instance, in the United Kingdom regulation concerning vegetative microorganisms in ready-to-eat foods. However, the objective of this study was not to challenge existing regulations but to provide an illustration of how an FSO established by a competent authority can guide decisions on safe product and process designs in practical operation; it hopefully contributes to the collaborative work between regulators, academia, and industries that need to continue learning and gaining experience from each other in order to translate risk-based concepts such as the FSO into everyday operational practice.

A review of the incidence and transmission of Listeria monocytogenes in ready-to-eat products in retail and food service environments

Contamination of ready-to-eat products with Listeria monocytogenes may occur at several stages before consumption. Accessibility to the public and relatively limited control interventions at retail and food service establishments (compared with the processing sector of the food industry) and the lack of a specific regulatory framework increase the likelihood of introduction of this pathogen into some foods in these establishments. This review is a compilation of available information on the incidence and transmission of L. monocytogenes through ready-to-eat products at the retail and food service level. The potential transmission of L. monocytogenes within retail and food service operations has been indicated in epidemiological investigations and by survey data. Potential sources of the organism in these operations include the environment, food handlers, and incoming raw ingredients or processed products that have become contaminated after the lethality treatment at the manufacturing facility. L. monocytogenes may be present at retail and food service establishments in various ready-to-eat products, both prepackaged and those packaged in the store, and occasionally at high concentrations. This issue dictates the need for development and application of effective control measures, and potential control approaches are discussed here. Good manufacturing practices, appropriate cleaning, sanitation and hygiene programs, and temperature control required for prevention or inhibition of growth of the pathogen to high levels are critical for control of L. monocytogenes in the retail and food service sector. A comprehensive food safety system designed to be functional in retail and food service operations and based on the philosophy of hazard analysis and critical control point systems and a series of sound prerequisite programs can provide effective control of L. monocytogenes in these environments. However, competent delivery of food safety education and training to retail and food service managers and food handlers must be in place for successful implementation of such a system.

Establishment of procedures provoking sub-lethal injury of Listeria monocytogenes, Campylobacter jejuni and Escherichia coli O157 to serve method performance testing

In this study procedures provoking sub-lethal injury for three different pathogens are described which may be used in determination of accuracy and robustness of methods, comparison studies and or validation of rapid detection methods. Three common food-borne pathogens were used, Listeria monocytogenes, Campylobacter jejuni and Escherichia coli O157. The pathogens were exposed to heat stress, cold stress, freeze stress, acid stress, oxidative stress and "food" stress. Sub-lethal injury was determined by plating in parallel on selective and non-selective media. The statistical significant differences in enumeration were established. The choice of stress to create sub-lethal injury to cells depended on the fact that the procedure must be easy to handle, repeatable and relevant for stress conditions in foods, but also on the micro-organism itself. Oxidative stress (1000 microM H(2)O(2)) was chosen to impose sub-lethal injury on L. monocytogenes and a specific "food" stress for E. coli O157. For C. jejuni a specific "food" stress as well as the oxidative stress (750 microM H(2)O(2)) were capable of creating a standardized procedure of provoking injury.

Microarray-based characterization of the Listeria monocytogenes cold regulon in log- and stationary-phase cells

Whole-genome microarray experiments were performed to define the Listeria monocytogenes cold growth regulon and to identify genes differentially expressed during growth at 4 and 37 degrees C. Microarray analysis using a stringent cutoff (adjusted P < 0.001; >/=2.0-fold change) revealed 105 and 170 genes that showed higher transcript levels in logarithmic- and stationary-phase cells, respectively, at 4 degrees C than in cells grown at 37 degrees C. A total of 74 and 102 genes showed lower transcript levels in logarithmic- and stationary-phase cells, respectively, grown at 4 degrees C. Genes with higher transcript levels at 4 degrees C in both stationary- and log-phase cells included genes encoding a two-component response regulator (lmo0287), a cold shock protein (cspL), and two RNA helicases (lmo0866 and lmo1722), whereas a number of genes encoding virulence factors and heat shock proteins showed lower transcript levels at 4 degrees C. Selected genes that showed higher transcript levels at 4 degrees C during both stationary and log phases were confirmed by quantitative reverse transcriptase PCR. Our data show that (i) a large number of L. monocytogenes genes are differentially expressed at 4 and 37 degrees C, with more genes showing higher transcript levels than lower transcript levels at 4 degrees C, (ii) L. monocytogenes genes with higher transcript levels at 4 degrees C include a number of genes and operons with previously reported or plausible roles in cold adaptation, and (iii) L. monocytogenes genes with lower transcript levels at 4 degrees C include a number of virulence and virulence-associated genes as well as some heat shock genes.

Occurrence of Listeria monocytogenes in sandwiches available to hospital patients in Wales, United Kingdom

A survey for the presence of Listeria monocytogenes in hospital sandwiches was carried out in Wales, United Kingdom, between October 2005 and March 2006. The main aim of the survey was to establish the baseline rate of L. monocytogenes in hospital sandwiches after an outbreak of listeriosis among hospital patients in 2004 was epidemiologically linked to the consumption of contaminated sandwiches. The overall positive rate found in hospital sandwiches was 2.84% for enriched culture and 0.21% for direct counts. The unsatisfactory rate (> 100 CFU/g) for hospital sandwiches was 0.1%. The conclusion was that hospital sandwiches generally presented a low hazard to consumers. In addition to establishing the overall baseline and the unsatisfactory rates in hospital sandwiches in Wales for this period, the study compared the rates found in hospital sandwiches with the rates found in sandwiches simultaneously sampled from general retailers. The aim of this part of the study was to compare the relative rates associated with hospital and retail sandwiches to ascertain if there were any differences in the positive rate. The conclusion of this part of the survey was that there was not a statistically significant difference in rates between sandwiches sampled from hospitals and those sampled from general retailers.

Microbiological characterization of randomly selected Portuguese raw milk cheeses with reference to food safety

Seventy raw milk cheeses made in different regions of Portugal, both hard and soft varieties, made with cow's, ewe's, or goat's milk or combinations of these, were sampled within their quoted shelf lives for microbiological safety. On the basis of the presence or numbers of Escherichia coli, E. coli O157, Staphylococcus aureus, Salmonella, and Listeria monocytogenes, cheeses were categorized as satisfactory, acceptable, unsatisfactory, or unacceptable and potentially hazardous. Twenty-two of the 70 cheeses were classified as satisfactory or acceptable. Thirty-seven of the cheeses were considered unsatisfactory because of the presence of E. coli, S. aureus, or both, while 11 of the cheeses were graded as unacceptable and potentially hazardous because of the presence of excessive numbers of S. aureus, E. coli, or L. monocytogenes and the presence of Salmonella in three of these. All cheeses graded as unacceptable and potentially hazardous were soft or semisoft cheeses made with ewe's and goat's milk, with the exception of two hard cheeses made with cow's milk. E. coli O157 was not detected in any of the cheeses. According to the present results, it seems that the presence or counts of pathogenic or indicator organisms in raw milk cheeses cannot be related to the processing conditions, milk type, or region of production.

The onset of rapidly progressive neurologic deterioration after a brief gastrointestinal illness in a renal allograft recipient

Renal transplant recipients are at increased risk for life-threatening complications, most commonly infections. Because of their impaired cell-mediated immunity, these patients are particularly susceptible to organisms that rely on intracellular survival and spread, such as Listeria monocytogenes. Despite being a food-borne pathogen, L. monocytogenes is associated with significant morbidity and mortality. Here we report the case of a renal transplant recipient who developed rapidly progressive neurological symptoms after a brief gastrointestinal illness. Magnetic resonance imaging scan of the brain showed a large mass that was identified as an abscess due to L. monocytogenes. Timely aspiration and antibiotic treatment resulted in complete recovery, as opposed to worse outcomes in the available case reports. We further review the epidemiology, microbiology, clinical presentation, and therapeutic options for listerial brain abscess.

Oxygen restriction increases the infective potential of Listeria monocytogenes in vitro in Caco-2 cells and in vivo in guinea pigs

Background

Listeria monocytogenes has been implicated in several food borne outbreaks as well as sporadic cases of disease. Increased understanding of the biology of this organism is important in the prevention of food borne listeriosis.

The infectivity of Listeria monocytogenes ScottA, cultivated with and without oxygen restriction, was compared in vitro and in vivo. Fluorescent protein labels were applied to allow certain identification of Listeria cells from untagged bacteria in in vivo samples, and to distinguish between cells grown under different conditions in mixed infection experiments.

Results

Infection of Caco-2 cells revealed that Listeria cultivated under oxygen-restricted conditions were approximately 100 fold more invasive than similar cultures grown without oxygen restriction. This was observed for exponentially growing bacteria, as well as for stationary-phase cultures.

Oral dosage of guinea pigs with Listeria resulted in a significantly higher prevalence (p < 0.05) of these bacteria in jejunum, liver and spleen four and seven days after challenge, when the bacterial cultures had been grown under oxygen-restricted conditions prior to dosage. Additionally, a 10–100 fold higher concentration of Listeria in fecal samples was observed after dosage with oxygen-restricted bacteria. These differences were seen after challenge with single Listeria cultures, as well as with a mixture of two cultures grown with and without oxygen restriction.

Conclusion

Our results show for the first time that the environmental conditions to which L. monocytogenes is exposed prior to ingestion are decisive for its in vivo infective potential in the gastrointestinal tract after passage of the gastric barrier. This is highly relevant for safety assessment of this organism in food.

Traceback identification of an ingredient (pork dewlap) as the possible source of Listeria monocytogenes serotype 4b contamination in raw chicken products

In surveys conducted on finished product samples from a single poultry processing plant in Spain, Listeria monocytogenes was found in 14 different uncooked products. To track contamination patterns, 77 L. monocytogenes isolates were characterized by PCR-based serotyping, pulsed-field gel electrophoresis (PFGE) restriction analysis, and PCR-based allelic analysis of the virulence gene actA. Serotyping revealed that 12 isolates (15.6%) were of the L. monocytogenes serotype 4b complex (serotype 4b or the closely related serotypes 4d and 4e). A combination of endonucleases AscI and ApaI PFGE patterns yielded 15 different pulsotypes among all 77 tested isolates. All the serotype 4b isolates belonged to one pulsotype. Sequencing of the actA gene confirmed that all serotype 4b isolates corresponded to the same allelic subtype. The subtype was recovered from five product types, but its presence was not correlated with the production line or the date of isolation, suggesting a possible association of this strain with a common ingredient. This traceback investigation established that pork dewlap, an ingredient common to all the products contaminated with this strain, was the most probable source of L. monocytogenes 4b. The same 4b strain was isolated from four samples of pork dewlap from one specific supplier. After replacement of this contaminated ingredient in the fresh products, this strain of L. monocytogenes serotype 4b was not detected. This study confirms the effectiveness of molecular subtyping to control contamination by specific strains of L. monocytogenes and the importance of testing the different ingredients added to the food products.

Listeria monocytogenes: silage, sandwiches and science

Listeria monocytogenesis amongst the most intriguing and well studied of the pathogenic bacteria. However, the understanding and perspective one has ofL. monocytogenesdepends to a large extent on the microbiological issues with which one is faced as a part of your professional duties. The focus of the veterinary clinician or investigator is likely to be foremost on the neurologic (circling disease) and reproductive diseasesL. monocytogenescauses. To the food microbiologist, the principal concern is to prevent introduction ofL. monocytogenesinto food products, or to identify its presence and prevent its multiplication to numbers of organisms that are likely to pose a substantial risk to humans who ingest the product. To the cellular immunologist, listeriosis represents a robust murine model that helped to elucidate many important concepts in innate and adaptive immunity, andL. monocytogenesis a potential vector for delivery of novel vaccines. To the student of molecular pathogenesis,L. monocytogenesis a powerful and well-characterized model organism for studying the cellular microbiology of an intracellular pathogen. In this brief overview, I will attempt to highlight some of the classical observations, and contemporary insights, onL. monocytogenesand listeriosis, and integrate these perspectives into a common framework. By so doing, I hope to provide those with one perspective on listeriosis with an appreciation of the broad array of problems and issues faced by those who focus on some other aspect ofL. monocytogenesand its pathogenesis.

Management of microbiological safety of ready-to-eat meat products by mathematical modelling: Listeria monocytogenes as an example

The recent Commission Regulation (EC) No 2073/2005 establishes microbiological criteria in foods. For the pathogen Listeria monocytogenes in the category ready-to-eat foods able to support its growth, other than those intended for infants and for special medical purposes, two different microbiological criteria are proposed: (i) L. monocytogenes levels should be <100 cfu/g throughout the shelf-life of the product, (ii) absence in 25 g of the product at the stage before the food has left the immediate control of the food business operator, who has produced it. The application of either the first or the second of these criteria depends on whether or not the manufacturer is able to demonstrate that the level of L. monocytogenes in the food product will not exceed 100 cfu/g throughout its shelf-life. This demonstration should be based on physico-chemical characteristics of the target product and consultation of scientific literature, and, when necessary, on quantitative models and/or challenge tests. Once the characteristics of the product as well as scientific literature show that the pathogen has potential to grow on a specific food commodity, it seems adequate to use quantitative models and/or perform challenge tests to study the extent to which L. monocytogenes could grow. In this study, we aim to illustrate with an example in cooked ham the application of quantitative models as a tool to manage the compliance with these criteria. Two approaches were considered: deterministic and probabilistic, in three different commercial brands (A, B, and C). The deterministic approach showed that the limit 100 cfu/g was exceeded largely at the end of the shelf-life of all three; however, when reducing the storage time, the level of L. monocytogenes remained below 100 cfu/g in B. The probabilistic approach demonstrated very low percentiles corresponding to 100 cfu/g; when reducing the storage time, percentiles for three products increased, especially in products B and C (from 4.92% to 75.90%, and from 0.90% to 73.90%, respectively). This study shows how different storage times influence the level of L. monocytogenes at the end of the shelf-life of cooked ham, and, depending on the level reached, the microbiological criterion applied should be different, as stated above. Beside this, the choice of either point-estimate or probabilistic approach should be determined by the competent sanitary authority, and, in case of selecting the second approach, a certain percentile for the level 100 cfu/g should be established.

Fate of Listeria monocytogenes in commercial ham, formulated with or without antimicrobials, under conditions simulating contamination in the processing or retail environment and during home storage

Commercial cured ham formulated with or without potassium lactate and sodium diacetate was inoculated with Listeria monocytogenes and stored to simulate conditions of processing, retail, and home storage. The ham was sliced, inoculated with a 10-strain composite of L. monocytogenes (1 to 2 log CFU/cm2), vacuum packaged, and stored at 4 degrees C to simulate contamination following lethality treatment at processing (first shelf life). After 10, 20, 35, and 60 days of storage, packages were opened, samples were tested, and bags with remaining slices were reclosed with rubber bands. At the same times, portions of original product (stored at 4 degrees C in original processing bags) were sliced, inoculated, and packaged in delicatessen bags to simulate contamination during slicing at retail (second shelf life). Aerobic storage of both sets of packages at 7 degrees C for 12 days was used to reflect domestic storage conditions (home storage). L. monocytogenes populations were lower (P < 0.05) during storage in ham formulated with lactate-diacetate than in product without antimicrobials under both contamination scenarios. Inoculation of ham without lactate-diacetate allowed prolific growth of L. monocytogenes in vacuum packages during the first shelf life and was the worst case contamination scenario with respect to pathogen numbers encountered during home storage. Under the second shelf life contamination scenario, mean growth rates of the organism during home storage ranged from 0.32 to 0.45 and from 0.18 to 0.25 log CFU/cm2/day for ham without and with lactate-diacetate, respectively, and significant increases in pathogen numbers (P < 0.05) were generally observed after 4 and 8 days of storage, respectively. Regardless of contamination scenario, 12-day home storage of product without lactate-diacetate resulted in similar pathogen populations (6.0 to 6.9 log CFU/cm2) (P > 0.05). In ham containing lactate-diacetate, similar counts were found during the home storage experiment under both contamination scenarios, and only in 60-day-old product did samples from the first shelf life have higher (P < 0.05) pathogen numbers than those found in samples from the second shelf life. These results should be useful in risk assessments and for the establishment of "sell by" and "consume by" date labels for refrigerated ready-to-eat meat products.

Modeling and predicting the growth boundary of Listeria monocytogenes in lightly preserved seafood

The antimicrobial effect of diacetate and lactate against Listeria monocytogenes was evaluated in challenge tests with vacuum-packaged or modified atmosphere packaged (MAP) cold-smoked salmon, marinated salmon, cold-smoked Greenland halibut, marinated Greenland halibut, and gravad salmon. MAP cold-smoked salmon with the addition of 0.15% (wt/wt) diacetate prevented the growth of L. monocytogenes for more than 40 days at 8 degrees C, whereas the addition of 0.15% (wt/wt) diacetate reduced the growth rate of the pathogen in MAP cold-smoked Greenland halibut. This difference between the two types of products was explained by a higher content of naturally occurring lactate in cold-smoked salmon (0.77 to 0.98%, wt/wt) than in cold-smoked Greenland halibut (0.10 to 0.15%, wt/wt). In fact, the addition of 0.15% (wt/wt) diacetate and 0.75% (wt/wt) lactate to MAP cold-smoked Greenland halibut prevented the growth of L. monocytogenes for more than 45 days at 8 degrees C. A mathematical model that included the effect of diacetate, lactate, CO2, smoke components, nitrite, pH, NaCl, temperature, and interactions between all these parameters was developed to predict the growth boundary of L. monocytogenes in lightly preserved seafood. The developed growth boundary model accurately predicted growth and no-growth responses in 68 of 71 examined experiments from the present study as well as from literature data. Growth was predicted for three batches of naturally contaminated cold-smoked salmon when a no-growth response was actually observed, indicating that the model is fail-safe. The developed model predicts both the growth boundary and growth rate of L. monocytogenes and seems useful for the risk management of lightly preserved seafood. Particularly, the model facilitates the identification of product characteristics required to prevent the growth of L. monocytogenes, thereby making it possible to identify critical control points, and is useful for compliance with the new European Union regulation on ready-to-eat foods (EC 2073/2005).

Methodological Quality Assessment of Review Articles Evaluating Interventions to Improve Microbial Food Safety

Review articles are a means of summarizing the potentially vast volume of research on a topic. However, the methodological quality of review articles varies, and reviews on the same topic may reach different conclusions. We evaluated 65 review articles published between 2000 and 2005 that addressed the effectiveness of microbial food safety interventions, using criteria for methodological soundness developed in the medical field. Overall, the methodological quality of the review articles was poor, with none of the reviews providing information on the method of locating primary research studies or the inclusion/exclusion criteria for selecting primary studies. None of the reviews included a critical appraisal of the methodological quality of the primary studies. Less than half of the reviews stated a focused research question, explored possible reasons for differences in the results of primary studies, discussed the generalizability of results, or proposed directions for future research. There is a need to improve the methodological quality of review articles on microbial food safety interventions if they are to be of use in policy and decision making.

Listeriosis in Portugal: an existing but under reported infection

Background

Listeriosis is a rare disease caused by the bacterium Listeria monocytogenes, the normal vehicle of which is food. The disease, which is largely confined to its risk groups of pregnant women, the elderly and immunocompromised individuals, has increased in incidence in recent years.

In Portugal, listeriosis is not a notifiable infection and available data are scarce.

The objective of this work was to collate the available information concerning listeriosis in Portugal by compiling a retrospective study of cases recorded over a decade.

Methods

Requests for case data on clinically confirmed listeriosis, recorded over the previous decade, were replied to by 23 hospitals and a National Institute of Health delegation.

Results

35 cases of listeriosis were identified for the period between 1994 and 2003 inclusive, the mortality rate being greater than 17%. According to the data collected in this study for the year 2003, the incidence of this disease in Portugal was at least 1.4 cases per million inhabitants in that year.

Conclusion

The study demonstrates, for the first time in the widely available literature, that despite their being no cases of listeriosis in Portugal recorded in official reports, the threat of L. monocytogenes to public health is of a similar dimension to that in other countries.

Effect of high-pressure treatment on the survival of Listeria monocytogenes Scott A in sliced vacuum-packaged Iberian and Serrano cured hams

High-pressure treatment is useful for increasing the microbiological safety of ready-to-eat foods. With dry-cured hams, this treatment can be applied to the finished product after slicing and vacuum packaging. The effect of high-pressure treatment on the survival of inoculated Listeria monocytogenes Scott A and on the sensory characteristics of two Spanish dry-cured hams, Iberian and Serrano, was investigated. Ham slices were inoculated with L. monocytogenes at 6 x 10(6) CFU/g and held at 4 degrees C for 20 h before high-pressure treatment. During this holding period, the population of the pathogen declined by 0.44 and 0.51 log CFU/g in Iberian and Serrano hams, respectively. Treatment at 450 MPa for 10 min at 12 degrees C reduced L. monocytogenes populations by 1.50 and 1.16 log CFU/g in Iberian and Serrano hams, respectively. During the first week of storage at 4 or 8 degrees C, L. monocytogenes populations declined by an average 0.89 log CFU/g in pressurized Iberian ham and 2.09 log CFU/g in pressurized Serrano ham. After 60 days at 4 or 8 degrees C, the respective populations in pressurized and control hams were 3.24 and 4.70 log CFU/g for Iberian ham and 2.73 and 5.07 log CFU/g for Serrano ham. The color parameters L* and a* were not influenced by high-pressure treatment, and parameter b* was increased only in Iberian ham. Sensory characteristics of hams were not affected by high-pressure treatment. Treatment of Iberian and Serrano hams at 450 MPa for 10 min significantly reduced the population of L. monocytogenes Scott A without a detrimental effect on the sensory characteristics of the hams.

Detection of Listeria monocytogenes in food using a combined enrichment/real-time PCR method targeting the prfA gene

A combined enrichment/real-time PCR method for the detection of Listeria monocytogenes is presented. The method is based on a conventional PCR assay targeting the prfA gene, which has been validated and suggested as an international standard PCR method for identifying L. monocytogenes in food. This real-time PCR assay includes an internal amplification control. Inclusivity and exclusivity were 100% each when testing 100 L. monocytogenes isolates, 30 Listeria spp. isolates other than L. monocytogenes, and 29 non-Listeria isolates. The theoretical detection limit was one copy of the target gene per PCR reaction and the practical detection limit was about 5 copies per PCR. Using the combined enrichment/real-time PCR method, 7.5 CFU/25 ml of artificially contaminated raw milk, and 9, 1, and 1 CFU/15 g of artificially contaminated salmon, pâté, and green-veined cheese, respectively, were detected. When analyzing 76 naturally contaminated food samples of various types and comparing the results with the ISO 11290-1 standard method, the relative accuracy was 96%, the relative specificity 100%, and the relative sensitivity, 76.9%.