An outbreak of Listeria monocytogenes serotype 3a infections from butter in Finland

Investigation of increased listeriosis revealed two fishery production plants with persistentListeriacontamination in Finland in 2010

In 2010, a marked increase in listeriosis incidence was observed in Finland.Listeria monocytogenesPFGE profile 96 was responsible for one-fifth of the reported cases and a cluster of PFGE profile 62 was also detected. Investigations revealed two fishery production plants with persistentListeriacontamination. It appears likely that the plants were at least partly responsible for the increase of listeriosis. Epidemiological investigation revealed that 57% (31/54) of cases with underlying immunosuppressive condition or medication reported eating gravad or cold-smoked fish. Two public notices were issued by THL and Evira informing which groups were most at risk from the effects of listeriosis and should therefore be cautious in consuming certain products. Systematic sampling of foods and adequate epidemiological investigation methods are required to identify the sources ofListeriainfections. Continuous control measures at fishery production plants producing risk products are essential.

Listeria monocytogenes persistence in food-associated environments: epidemiology, strain characteristics, and implications for public health

Over the last 10 to 15 years, increasing evidence suggests that persistence of Listeria monocytogenes in food processing plants for years or even decades is an important factor in the transmission of this foodborne pathogen and the root cause of a number of human listeriosis outbreaks. L. monocytogenes persistence in other food-associated environments (e.g., farms and retail establishments) may also contribute to food contamination and transmission of the pathogen to humans. Although L. monocytogenes persistence is typically identified through isolation of a specific molecular subtype from samples collected in a given environment over time, formal (statistical) criteria for identification of persistence are undefined. Environmental factors (e.g., facilities and equipment that are difficult to clean) have been identified as key contributors to persistence; however, the mechanisms are less well understood. Although some researchers have reported that persistent strains possess specific characteristics that may facilitate persistence (e.g., biofilm formation and better adaptation to stress conditions), other researchers have not found significant differences between persistent and nonpersistent strains in the phenotypic characteristics that might facilitate persistence. This review includes a discussion of our current knowledge concerning some key issues associated with the persistence of L. monocytogenes, with special focus on (i) persistence in food processing plants and other food-associated environments, (ii) persistence in the general environment, (iii) phenotypic and genetic characteristics of persistent strains, (iv) niches, and (v) public health and economic implications of persistence. Although the available data clearly indicate that L. monocytogenes persistence at various stages of the food chain contributes to contamination of finished products, continued efforts to quantitatively integrate data on L. monocytogenes persistence (e.g., meta-analysis or quantitative microbial risk assessment) will be needed to advance our understanding of persistence of this pathogen and its economic and public health impacts.

New data, strategies, and insights for Listeria monocytogenes dose-response models: summary of an interagency workshop, 2011

Listeria monocytogenes is a leading cause of hospitalization, fetal loss, and death due to foodborne illnesses in the United States. A quantitative assessment of the relative risk of listeriosis associated with the consumption of 23 selected categories of ready-to-eat foods, published by the U.S. Department of Health and Human Services and the U.S. Department of Agriculture in 2003, has been instrumental in identifying the food products and practices that pose the greatest listeriosis risk and has guided the evaluation of potential intervention strategies. Dose-response models, which quantify the relationship between an exposure dose and the probability of adverse health outcomes, were essential components of the risk assessment. However, because of data gaps and limitations in the available data and modeling approaches, considerable uncertainty existed. Since publication of the risk assessment, new data have become available for modeling L. monocytogenes dose-response. At the same time, recent advances in the understanding of L. monocytogenes pathophysiology and strain diversity have warranted a critical reevaluation of the published dose-response models. To discuss strategies for modeling L. monocytogenes dose-response, the Interagency Risk Assessment Consortium (IRAC) and the Joint Institute for Food Safety and Applied Nutrition (JIFSAN) held a scientific workshop in 2011 (details available at http://foodrisk.org/irac/events/). The main findings of the workshop and the most current and relevant data identified during the workshop are summarized and presented in the context of L. monocytogenes dose-response. This article also discusses new insights on dose-response modeling for L. monocytogenes and research opportunities to meet future needs.

Prevalence and characteristics of Listeria monocytogenes in bovine colostrum in Japan

This study was conducted to determine the prevalence and characteristics of Listeria monocytogenes in bovine colostrum in Japan. We collected bovine colostrum samples from 210 dams from 21 dairy farms in Hokkaido prefecture (Japan) between March and June 2009. L. monocytogenes was detected in samples from 6 (28.6%) of the 21 farms. Of the 210 samples, 16 (7.6%) were positive for L. monocytogenes. We recovered 80 L. monocytogenes isolates; 44 (55%) isolates were classified as serotype 1/2b and 36 (45%) were classified as serotype 4b. The isolates were susceptible to penicillin, ampicillin, amoxicillin, gentamicin, kanamycin, streptomycin, erythromycin, vancomycin, tetracycline, chloramphenicol, ciprofloxacin, and trimethoprim-sulfamethoxazole. Pulsed-field gel electrophoresis (PFGE) characterization of the 80 isolates revealed six PFGE types. Two PFGE types corresponded to human listeriosis cases. Most L. monocytogenes isolates possessed virulence-associated genes (actA, hly, iap, inlA, inlC, mpl, plcA, plcB, opuCA, prfA, and clpC). One PFGE type isolate possessed an epidemic clone II marker. Our findings suggest that isolates from bovine colostrum have the potential to cause human and animal listeriosis. This is the first study on the prevalence and characteristics of L. monocytogenes isolated from bovine colostrum obtained from dairy farms. Our results have important implications for improving public health and elucidating the epidemiology of L. monocytogenes in bovine colostrum.

Recent developments in molecular sub-typing of Listeria monocytogenes

As a vast majority of the human listeriosis cases are caused by serotypes 1/2a, 1/2b and 4b strains, it is imperative that strains from clinical as well as from food and environment are further characterised so that accurate and timely epidemiological determination of sources of the contamination can be established to minimise the disease burden. Recent developments in the field of genomics provide a great opportunity to use these tools towards the development of molecular sub-typing techniques with a greater degree of discrimination spanning the entire length of the genome. This brief review summarises a few of these DNA-based techniques with an emphasis on DNA microarray and other whole genome sequencing-based approaches and their usefulness in Listeria monocytogenes sub-typing and outbreak investigations.

Genetic diversity and molecular typing of Listeria monocytogenes in China

BACKGROUND

Listeria monocytogenes can cause invasive diseases in humans and farm animals and is frequently isolated from dairy products and poultry. Listeriosis is uncommon in China but L. monocytogenes has been isolated from foods and food processing environments in China. However little is known of genetic diversity of Chinese L. monocytogenes isolates and their relationships with global isolates.

RESULTS

Two hundred and twelve isolates of L. monocytogenes from food sources from 12 provinces/cities in China were analysed by serotyping, Pulsed Field Gel Electrophoresis (PFGE) and Multi-locus Sequence Typing (MLST). The predominant serotypes are 1/2a, 1/2b and 1/2c accounting for 90.1% of the isolates. PFGE divided the isolates into 61 pulse types (PTs). Twenty nine PTs were represented by more than one isolates with PT GX6A16.0004 containing the most number of isolates. MLST differentiated the isolates into 36 STs, among which 15 were novel. The 3 most common STs were ST9 (29.1%), ST8 (10.7%) and ST87 (9.2%), accounting for 49.0% of the isolates.

CONCLUSIONS

STs prevalent in other parts of the world are also prevalent in China including 7 STs (ST1-ST3, ST5, ST6, ST8, ST9) which caused maternal fetal infections or outbreaks, suggesting that these STs potentially can also cause severe human infections or outbreaks in China. Surveillance of these STs will provide important information for prevention of listeriosis. This study also enhances our understanding of genetic diversity of L. monocytogenes in China.

[Nosocomial outbreak due to Listeria monocytogenes in a neonatal unit]

INTRODUCTION

Description of an outbreak of Listeria monocytogenes in a neonatal intensive care unit.

METHODS

A questionnaire, environmental investigation and molecular study were performed.

RESULTS

We identified a nosocomial outbreak of L. monocytogenes, confirmed by the genetic study, in a neonatal intensive care unit. Three infants were affected. Although the transmission mechanism could not be elucidated, cross-infection was strongly suggested.

CONCLUSION

Adherence to universal hygiene standards is necessary to avoid nosocomial outbreaks.

Epidemiology and risk management of listeriosis in India

Listeria monocytogenes is a foodborne pathogen that can cause serious invasive illness, mainly in certain well-defined high-risk groups, including elderly and immunocompromised patients, pregnant women, newborns and infants. In India, this pathogen has been isolated from humans, animals and foods. The incidence of Listeria is generally comparable to those reported elsewhere in the world. In humans, maternal/neonatal listeriosis is the most common clinical form reported. Among animal populations, spontaneous abortions, subclinical mastitis, meningoencephalitis and endometritis were the commonest forms reported. The disease largely remains undiagnosed and under reported. From reported analyses of a variety of foods for Listeria, milk and milk products, meat and meat products, seafood and vegetables have been reported to be contaminated in India. The legal framework for microbiological safety of foods against microbes including L. monocytogenes is summarised. The epidemiological studies would help in understanding of the sources of infection and persistence and their risk assessment, routes of transmission, clinical forms and allow for better management of the infection.

Quantitative risk assessment of listeriosis due to consumption of raw milk

The objectives of this study were to estimate the risk of illness for raw milk consumers due to Listeria monocytogenes in raw milk sold by permitted dealers, and the risk for people on farms who consume raw milk. Three scenarios were evaluated for raw milk sold by dealers: raw milk purchased directly from bulk tanks, from on-farm stores, and from retail. To assess the effect of mandatory testing of raw milk by regulatory agencies, the number of listeriosis cases per year was compared where no raw milk testing was done, only a screening test to issue a permit was conducted, and routine testing was conducted and milk was recalled if it was L. monocytogenes positive. The median number of listeriosis cases associated with consumption of raw milk from bulk tanks, farm stores, and retail for an intermediate-age population was 6.6 × 10(-7), 3.8 × 10(-5), and 5.1 × 10(-5) cases per year, respective ly. In populations with high susceptibility, the estimated median number of cases per year was 2.7 × 10(-7) (perinatal, i.e., pregnant women and their fetuses or newborns) and 1.4 × 10(-6) (elderly) for milk purchased from bulk tanks, 1.5 × 10(-5 ) (perinatal) and 7.8 × 10(-5) (elderly) for milk from farm stores, and 2.1 × 10(-5) (perinatal) and 1.0 × 10(-4) (elderly) for milk from retail. For raw milk consumed on farms, the median number of listeriosis cases was 1.4 × 10(-7) cases per year. A greater risk of listeriosis was associated with consumption of raw milk obtained from retail and farm stores as compared with milk obtained from bulk tanks. This was likely due to additional time-temperature combination steps in the retail and farm store models, which increased the chances for growth of L. monocytogenes in raw milk. A close relationship between prevalence of L. monocytogenes in raw milk and the values of disease incidence was observed. Hence, a reduction in the number of cases per year in all populations was observed when a raw milk-testing program was in place, especially when routine testing and recalling of milk was conducted.

Increased in vitro adherence and on-farm persistence of predominant and persistent Listeria monocytogenes strains in the milking system

Dairy farms are a reservoir for Listeria monocytogenes, and the reduction of this pathogen at the farm level is important for reducing human exposure. The objectives of this research were to study the diversity of L. monocytogenes strains on a single dairy farm, assess strain dynamics within the farm, identify potential sources of L. monocytogenes in bulk tank milk and milk filters, and assess the adherence abilities of representative strains. A total of 248 L. monocytogenes isolates were analyzed by pulsed-field gel electrophoresis (PFGE). Combined AscI and ApaI restriction analysis yielded 40 PFGE types (strains). The most predominant strains were T (28.6%), D (22.6%), and F (14.9%). A high level of heterogeneity of strains among isolates from fecal (Simpson's index of diversity [SID] = 0.96) and environmental (SID = 0.96) samples was observed. A higher homogeneity of strains was observed among isolates from milk filters (SID = 0.71) and bulk tank milk (SID = 0.65). Six of 17 L. monocytogenes isolates (35.3%) were classified in an in vitro assay as having a "low adherence ability," 9 (52.9%) were classified as having a "medium adherence ability," and 2 (11.8%) were classified as having a "high adherence ability." The L. monocytogenes strains that were predominant and persistent showed significantly better adherence than did strains that were only sporadic, predominant, or persistent (P = 0.0006). Our results suggest that the milking system was exposed to several L. monocytogenes strains from different sources. Only 3 strains, however, were successful in persisting within the milking system, suggesting that some strains are more suitable to that particular ecological environment than others.

LISTERIA MONOCYTOGENES: OCORRÊNCIA EM PRODUTOS LÁCTEOS E SUAS IMPLICAÇÕES EM SAÚDE PÚBLICA

RESUMO Listeria monocytogenes é o agente causador da listeriose, uma grave doença de origem alimentar que causa severas infecções em humanos com altas taxas de mortalidade. O leite e seus derivados estão entre os produtos alimentícios mais frequentemente envolvidos na transmissão de L. monocytogenes. A listeriose acomete, sobretudo, indivíduos imunodeprimidos, grávidas, recém-nascidos e idosos, o que ressalta o caráter oportunista deste micro-organismo e sua importância para a saúde pública. No presente trabalho, faz-se uma revisão narrativa crítica sobre o risco à saúde humana decorrente da ingestão de leite e derivados contaminados por L. monocytogenes, bem como se discutem os fatores que determinam a contaminação por L. monocytogenes na cadeia de produção e distribuição de leite e derivados. São apresentados e avaliados os dados de ocorrência de L. monocytogenes em leite cru e em produtos lácteos no Brasil, tendo em vista seu potencial de envolvimento em casos de listeriose humana. Adicionalmente, são indicadas as principais áreas de pesquisa e atuação para prevenir a contaminação de L. monocytogenes em produtos lácteos.

Listeria monocytogenes infection in the over-60s in England between 2005 and 2008: a retrospective case-control study utilizing market research panel data

A retrospective case-control study of listeriosis in patients in England aged over 60 years is described. The incidence of listeriosis in patients aged ≥60 years in England has doubled since 2001; hence, the investigation of risk factors for infection in this group is important to inform on prevention and control. Standardized epidemiological information has been sought on cases since 2005, but the value of the data accrued is limited without some perception of exposure prevalence in the population at risk of listeriosis. The exposures of listeriosis cases aged ≥60 years reported in England from 2005 to 2008 were compared to those of market research panel members representing the same population (i.e., residents of England aged ≥60 years) and time period. Exposures were grouped to facilitate comparison. Odds ratios and 95% confidence intervals were calculated. Cases were more likely than panel members to report the consumption of cooked meats (beef and ham/pork, but not poultry), cooked fish (specifically smoked salmon) and shellfish (prawns), dairy products (most noticeably milk but also certain cheeses), and mixed salads. They were less likely to report the consumption of other forms of seafood, dairy spread, other forms of dairy, sandwiches, and fresh vegetables. The diversity of high-risk food exposures reflects the ubiquity of the microorganism in the environment and/or the susceptibility of those at risk, and suggests that a wider variety of foods can give rise to listeriosis. Food safety advice on avoiding listeriosis should be adapted accordingly. While not inexpensive, the application of market research data to infectious disease epidemiology can add value to routine surveillance data.

Contamination patterns of Listeria monocytogenes in cold-smoked pork processing

Contamination patterns of Listeria monocytogenes were studied in a cold-smoked pork processing plant to identify the sources and possible reasons for the contamination. Environmental sampling combined with pulsed-field gel electrophoresis (PFGE) subtyping and serotyping were applied to investigate the genetic diversity of L. monocytogenes in the plant environment and ready-to-eat (RTE) cold-smoked pork products. A total of 183 samples were collected for contamination analyses, including samples of the product at different stages during manufacture (n = 136) and environmental samples (n = 47) in 2009. L. monocytogenes isolates, previously recovered from 73 RTE cold-smoked pork samples and collected from the same meat processing plant in 2004, were included in this study. The brining machine and personnel working with brining procedures were the most contaminated places with L. monocytogenes. The overall prevalence of L. monocytogenes in raw pork (18%) increased to 60% after the brining injections. The brining machine harbored six different PFGE types belonging to serotypes 1/2a, 1/2c, 4b, and 4d, which were found on the feeding teeth, smooth surfaces, and spaces of the machine, thus potentially facilitating dissemination of L. monocytogenes contamination. Two PFGE types (2 and 8) belonging to serotypes 1/2a and 1/2c were recovered from RTE cold-smoked pork collected in 2004, and from surfaces of the brining machine sampled in 2009, and may indicate the presence of persistent L. monocytogenes strains in the plant. Due to poor hygiene design, removal of the brining machine from the production of cold-smoked meat products should be considered to reduce L. monocytogenes contamination in the finished products.

A large outbreak of Listeria monocytogenes infection with short incubation period in a tertiary care hospital

OBJECTIVES

Listeria monocytogenes is a foodborne pathogen with a high mortality rate. We report a large, nosocomial outbreak of Listeria monocytogenes infection.

METHODS

Patients with L. monocytogenes isolated from a sterile site, or from faeces when diarrhoea and fever were present, were included. Clinical data were collected from the patient records. The incubation period was calculated as the time between exposure and start of symptoms.

RESULTS

Seventeen patients (11 women, median age 64 years) were infected of whom 15 patients were at increased risk for listeriosis. Eleven patients received empiric antibiotic treatment, eight of them with cephalosporins. Three patients died with a resulting mortality rate of 18%. The source of the outbreak was a Camembert cheese made from pasteurised milk containing up to 360 million colony forming units per portion. The median incubation period was 3-4 days.

CONCLUSIONS

The incubation period in this outbreak was significantly shorter than previously reported, a fact that may be due to the high number of ingested bacteria. Furthermore, food restrictions in hospitals seem warranted, as do treatment with antibiotics effective against L. monocytogenes in at-risk populations.

Risk of Listeria monocytogenes contamination of raw ready-to-eat seafood products available at retail outlets in Japan

Examination of Listeria monocytogenes prevalence among ready-to-eat foods in Japan revealed frequent (5.7 to 12.1%) contamination of minced tuna and fish roe products, and the isolates had the same virulence levels as clinical isolates in terms of invasion efficiency and infectivity in cell cultures and a murine infection model, respectively. Premature stop codons in inlA were infrequent (1 out of 39 isolates). Cell numbers of L. monocytogenes in minced tuna and salmon roe increased rapidly under inappropriate storage temperatures (from a most probable number [MPN] of 10(0) to 10(1)/g to an MPN of 10(3) to 10(4)/g over the course of 2 days at 10 degrees C). Thus, regulatory guidelines are needed for acceptable levels of L. monocytogenes in these foods.

Recipes for foodborne outbreaks: a scheme for categorizing and grouping implicated foods

BACKGROUND

To better understand the sources of foodborne illness, we propose a scheme for categorizing foods implicated in investigations of outbreaks of foodborne diseases. Because nearly 2000 foods have been reported as causing outbreaks in the United States, foods must be grouped for meaningful analyses.

METHODS

We defined a hierarchy of 17 mutually exclusive food commodities. We defined the following three commodity groups from which nearly all food is derived: aquatic animals, land animals, and plants. We defined three commodities in aquatic animals, six in land animals, and eight in plants. We considered each food as a set of ingredients composed of one or more commodities. We defined a simple food as one made of ingredients that are all in one commodity and a complex food as one containing ingredients in more than one commodity. We determined likely ingredients using a panel of epidemiologists and a web-based search process.

RESULTS

We assigned 1709 (95%) of the 1794 foods implicated in outbreaks of foodborne diseases reported to Centers for Disease Control and Prevention from 1973 to 2006. Of those, 987 (57%) were simple foods and 722 (43%) were complex foods.

DISCUSSION

This categorization may serve as an input for modeling the attribution of human illness to specific food commodities and could be used by policy makers, health officials, regulatory agencies, and consumer groups to evaluate the contribution of various food commodities to illness.

Chicken salad as the source of a case of Listeria monocytogenes infection in Connecticut

Listeriosis is a severe infection with high morbidity and mortality. We report a fatal case of listeriosis in a patient with a history of Crohn's disease who consumed chicken salad purchased from a retail food establishment before developing listeriosis. As part of the regulatory testing programs, the U.S. Department of Agriculture Food Safety and Inspection Service and the Florida Department of Agriculture and Consumer Affairs found that chicken products from a single food-production establishment were contaminated with Listeria monocytogenes, resulting in a product recall. The case patient's Listeria isolate was subtyped by pulsed-field gel electrophoresis (PFGE) and matched the Listeria isolates from the recalled chicken products. Identification of the source of Listeria involved collaboration among two state public health laboratories and epidemiologists and state and federal regulatory agencies. PFGE typing can be used to reveal correlations between clusters of human illness and contaminated food products and to rapidly identify sources of Listeria infection to allow implementation of corrective actions at both the state and national levels.

Microbiological safety of food in hospitals and other healthcare settings

Cases and outbreaks of foodborne infection in healthcare settings can result in serious illness, wastage of expensive medical treatments, spread of infection to other patients and staff and disruption of services. Providing nutritious meals for vulnerable people in healthcare settings involves a systematic approach to microbiological safety, as provided by hazard analysis and critical control point (HACCP) principles. The types of food served in healthcare settings should be selected to minimise the risk of foodborne infection.

Reduction of Listeria on ready-to-eat sausages after exposure to a combination of pulsed light and nisin

The risk of listeriosis associated with ready-to-eat foods is a major concern in the United States. Pulsed light (PL) treatment has been effective for killing Listeria. The possibility of enhancing the antilisterial capability of PL treatment by combining PL with an additional hurdle, the natural antimicrobial nisin, was explored in this study. First, the ability of Listeria innocua to mimic the response of Listeria monocytogenes to PL treatment was demonstrated. Subsequently, a series of inoculation studies was performed in which canned sausages were surface inoculated with L. innocua as a surrogate for L. monocytogenes and then treated with a commercial preparation of nisin (Nisaplin), PL, or a combination of the two treatments. The application of a Nisaplin dip alone resulted in an immediate reduction of L. innocua by 2.35 +/- 0.09 log CFU. PL reduced L. innocua by 1.37 +/- 0.30 log CFU after exposure to 9.4 J/cm2. A total reduction of 4.03 +/- 0.15 log CFU was recorded after the combined treatment of Nisaplin and PL for 48 h at 4 degrees C. The long-term survival of L. innocua was evaluated on sausages stored at 4 degrees C. Treatment with Nisaplin and PL resulted in a 4- to 5-log reduction for two replicate studies. The combination treatment resulted in no significant microbial growth during 28 and 48 days of refrigerated storage in the first and second replicates, respectively. These results suggest that this combination treatment can be used as an effective antilisterial step in the production of ready-to-eat foods.

Survival of Listeria monocytogenes in a simulated dynamic gastrointestinal model during storage of inoculated bologna and salami slices in vacuum packages

Listeria monocytogenes counts were determined during storage (82 days, 4 degrees C) in vacuum packages of inoculated bologna and salami slices and after exposure to a simulated dynamic model of the stomach and small intestine. Variables controlled in the model included gastric emptying and gastrointestinal fluid secretion rates, gradual gastric acidification, and intestinal pH maintenance. L. monocytogenes populations increased on bologna and decreased on salami, reaching 8.7 and 1.4 log CFU/g, respectively, on day 82. Inactivation rates (IR) during gastric exposure of bologna and salami ranged from 0.079 (day 14) to 0.158 (day 57) log CFU/g/min and from 0.013 (day 42) to 0.051 (day 1) log CFU/g/min, respectively. On corresponding days, gastric IR for cells on salami were lower than on bologna, suggesting potential protective effects of the former product. However, it is also possible that the low initial L. monocytogenes levels reached with storage of salami (< or = 2.5 log CFU/g after day 27) may have resulted in slower reductions than in the high levels on bologna. Gradual decline of gastric pH allowed survival in the gastric compartment during the initial stages, which resulted in a large fraction of the cells being delivered into the intestinal compartment. Intestinal IR ranged from 0.003 to 0.048 (bologna) and from 0.002 to 0.056 (salami) log CFU/g/min throughout storage. Although findings indicated potential effects of salami against gastric killing of L. monocytogenes, any effects of the food matrix per se on the gastrointestinal survival of the pathogen were overwhelmed by the high and low contamination levels reached on bologna and salami, respectively, during storage.

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.

Comprehensive analysis of polar and apolar constituents of butter and margarine by nuclear magnetic resonance, reflecting quality and production processes

The separation of butter or margarine into polar (soluble in water) and apolar fractions (soluble in chloroform) and subsequent analysis of these fractions by (1)H NMR permits a comprehensive analysis of its constituents. In the polar fraction the preservatives benzoic and sorbic acid, the organic acids citric, lactic, butyric, acetic, and formic acid, and, furthermore, the carbohydrate lactose were quantified. In the apolar fraction the conjugated linoleic acid (CLA) rumenic acid, diglycerides, and linoleic acid were quantified. Rumenic acid is a characteristic component of ruminant fats and was found in all butter samples. The levels varied between 0.50 and 1.08%. Ten brands of Brazilian butter were investigated as was one brand from Norway. Also, two brands of margarine were investigated for comparison. A large variation in especially polar constituents was found between the butter samples, revealing the presence of preservatives in five brands of butter from Brazil, remarkable because these additives are legally not allowed. Furthermore, the levels of organic acids and lactose permitted conclusions about the production process and quality; for example, the presence of higher levels of free butyric acid indicate lipolysis, leading to a lower quality, and low levels of lactose indicate that after churning the residual milk fluids have been removed by an additional washing step in the production process.

Impact of dilution ratios on Listeria monocytogenes growth during University of Vermont medium enrichment of deli meats

In the U.S. Department of Agriculture (USDA) method for Listeria detection, a 25-g composite food sample is enriched in 225 ml of University of Vermont medium (UVM), giving a detection limit of 0.04 CFU/g. However, in a recent large-scale four-state deli meat survey for L. monocytogenes, 125-g samples enriched in 1,125 ml of UVM were requested to increase the detection limit to 0.008 CFU/g. To circumvent problems associated with large volumes of UVM, the impact on L. monocytogenes growth of lower dilution ratios used for enrichment and most-probable-number (MPN) detection was compared with the results obtained using the conventional 1:10 dilution. In this study, 125-g samples of cured turkey, uncured turkey, ham, and roast beef were inoculated with a six-strain L. monocytogenes cocktail to contain approximately 1 x 10(3) CFU/g. This cocktail was then diluted 1:3, 1:5, or 1:10 in UVM, homogenized, enriched at 30 degrees C, and periodically plated on modified Oxford agar to determine generation times during 24 h of incubation. The same enrichment protocol was also assessed in a three-tube MPN assay using 125-g samples inoculated with L. monocytogenes to contain approximately 1 CFU/g. The effects of two homogenization methods, stomaching and pulsifying, on Listeria growth were compared using oven-roasted turkey breast diluted 1:3, 1:5, and 1:10 in UVM. Overall, the growth rates, generation times, and MPN values for each of the four selected deli meats were similar (P > 0.05) using UVM enrichment ratios of 1:3, 1:5, and 1:10, with no significant (P > 0.05) differences in L. monocytogenes growth rate or generation time between experiments using pulsifying and stomaching. These findings indicate that lower volumes of UVM can be used in the USDA procedure when examining deli meats without compromising Listeria recovery.

Antimicrobial susceptibility of Listeria monocytogenes from food products

This study was conducted to determine the susceptibility of Listeria monocytogenes isolated from food products to antimicrobial agents commonly used for treatment of infections with gram-positive bacteria, and to disinfectants. A total of 114 L. monocytogenes retail isolates were tested for susceptibility to ceftiofur, chloramphenicol, ciprofloxacin, erythromycin, florfenicol, penicillin, spectinomycin, streptomycin, tetracycline, tiamulin, trimethoprim, and co-trimoxazole, and the disinfectants benzalkonium chloride and triclosan, by determination of minimum inhibitory concentrations (MICs). All isolates were resistant to ceftiofur, but susceptible to the other antibiotics. A single isolate had a MIC of 4 mg/L for ciprofloxacin. For tiamulin. the MIC values were around the breakpoint used. Most isolates had MICs for triclosan at 16 mg/L. The MICs for benzalkonium chloride formed a bimodal distribution, with 105 isolates having a MIC of 4 mg/L and 9 isolates MICs of 16 and 32 mg/L. This study showed that Danish isolates of L. monocytogenes have not developed or acquired resistance to antimicrobial agents used for treatment or disinfection, except for benzalkonium chloride. The MICs for triclosan was high compared to other gram-positive bacteria, suggesting that triclosan might not be useful against L. monocytogenes if incorporated in materials in the food industry.

The epidemiology of human listeriosis

Listeriosis is a serious invasive disease that primarily afflicts pregnant women, neonates and immunocompromised adults. The causative organism, Listeria monocytogenes, is primarily transmitted to humans through contaminated foods. Outbreaks of listeriosis have been reported in North America, Europe and Japan. Soft cheeses made from raw milk and ready-to-eat meats are high risk foods for susceptible individuals. Efforts by food processors and food regulatory agencies to aggressively control L. monocytogenes in the high risk foods have resulted in significant decreases in the incidence of sporadic listeriosis.

Pulsed-field gel electrophoresis (PFGE) analysis of temporally matched Listeria monocytogenes isolates from human clinical cases, foods, ruminant farms, and urban and natural environments reveals source-associated as well as widely distributed PFGE types

A total of 495 temporally and geographically matched Listeria monocytogenes isolates from human clinical cases, foods, ruminant farms, and urban and natural environments were used to investigate L. monocytogenes pulsed-field gel electrophoresis (PFGE) type diversity. Two-enzyme (AscI and ApaI) PFGE discriminated 310 PFGE types and exhibited higher overall discriminatory power (Simpson's index of discrimination [D] = 0.995) than either EcoRI ribotyping (D = 0.950) or AscI or ApaI single-enzyme PFGE (D = 0.992 for both). Seven PFGE types showed significant associations with specific sources, including one and four PFGE types, respectively, associated with human clinical cases and foods. Spatial analysis of 13 PFGE types occurring >5 times showed that two PFGE types were specific to a single processing facility each, where they appear to have persisted over time. Nine PFGE types were geographically widespread and occurred among isolates from multiple sources. For example, a PFGE type that matched isolates from listeriosis outbreaks in Los Angeles and Switzerland occurred among isolates from farms (n = 7), human clinical cases (n = 4), environmental sources (n = 3), and foods (n = 1). Our data indicate that (i) PFGE is highly discriminatory for the subtyping of L. monocytogenes, (ii) some L. monocytogenes PFGE types are associated with specific sources, and (iii) some L. monocytogenes PFGE types are widely distributed and appear to be stable and pandemic. Large PFGE type databases representing isolates from different sources are thus needed to appropriately interpret subtype data in epidemiological investigations and to identify common as well as source-specific PFGE types.

Evaluation of BBL CHROMagar Listeria agar for the isolation and identification of Listeria monocytogenes from food and environmental samples

The performance of BBL CHROMagar Listeria chromogenic agar for the detection of Listeria monocytogenes was evaluated for its ability to isolate and identify L. monocytogenes from food and environmental samples. The medium was compared to non-chromogenic selective agars commonly used for Listeria isolation: Oxford, Modified Oxford, and PALCAM. BBL CHROMagar Listeria had a sensitivity of 99% and 100% for the detection of L. monocytogenes from 200 natural and artificially inoculated food samples, respectively, with a colony confirmation rate of 100%. The sensitivity of non-chromogenic selective media for the detection of L. monocytogenes from these same samples was 97-99% with colony confirmation rates of 65-67.5%. From 93 environmental samples, BBL CHROMagar Listeria agar results correlated 100% with a Listeria spp. visual immunoassay (TECRA) performed on these same samples and the USDA-FSIS standard culture method for the isolation of L. monocytogenes. From environmental samples, the L. monocytogenes confirmation rate was 100% for BBL CHROMagar Listeria as compared to 50% for conventional agars tested. On BBL CHROMagar Listeria, L. monocytogenes forms a translucent white precipitation zone (halo) surrounding blue-pigmented colonies of 2-3 mm in diameter, with an entire border. BBL CHROMagar Listeria offers a high degree of specificity for the confirmation of suspect L. monocytogenes colonies, whereas non-chromogenic selective agars evaluated were not differential for L. monocytogenes from other Listeria species.

Identification of IspC, an 86-kilodalton protein target of humoral immune response to infection with Listeria monocytogenes serotype 4b, as a novel surface autolysin

We identified and biochemically characterized a novel surface-localized autolysin from Listeria monocytogenes serotype 4b, an 86-kDa protein consisting of 774 amino acids and known from our previous studies as the target (designated IspC) of the humoral immune response to listerial infection. Recombinant IspC, expressed in Escherichia coli, was purified and used to raise specific rabbit polyclonal antibodies for protein characterization. The native IspC was detected in all growth phases at a relatively stable low level during a 22-h in vitro culture, although its gene was transiently transcribed only in the early exponential growth phase. This and our previous findings suggest that IspC is upregulated in vivo during infection. The protein was unevenly distributed in clusters on the cell surface, as shown by immunofluorescence and immunogold electron microscopy. The recombinant IspC was capable of hydrolyzing not only the cell walls of the gram-positive bacterium Micrococcus lysodeikticus and the gram-negative bacterium E. coli but also that of the IspC-producing strain of L. monocytogenes serotype 4b, indicating that it was an autolysin. The IspC autolysin exhibited peptidoglycan hydrolase activity over a broad pH range of between 3 and 9, with a pH optimum of 7.5 to 9. Analysis of various truncated forms of IspC for cell wall-hydrolyzing or -binding activity has defined two separate functional domains: the N-terminal catalytic domain (amino acids [aa] 1 to 197) responsible for the hydrolytic activity and the C-terminal domain (aa 198 to 774) made up of seven GW modules responsible for anchoring the protein to the cell wall. In contrast to the full-length IspC, the N-terminal catalytic domain showed hydrolytic activity at acidic pHs, with a pH optimum of between 4 and 6 and negligible activity at alkaline pHs. This suggests that the cell wall binding domain may be of importance in modulating the activity of the N-terminal hydrolase domain. Elucidation of the biochemical properties of IspC may have provided new insights into its biological function(s) and its role in pathogenesis.

International Life Sciences Institute North America Listeria monocytogenes strain collection: development of standard Listeria monocytogenes strain sets for research and validation studies

Research and development efforts on bacterial foodborne pathogens, including the development of novel detection and subtyping methods, as well as validation studies for intervention strategies can greatly be enhanced through the availability and use of standardized strain collections. These types of strain collections are available for some foodborne pathogens, such as Salmonella and Escherichia coli. We have developed a standard Listeria monocytogenes strain collection that has not been previously available. The strain collection includes (i) a diversity set of 25 isolates chosen to represent a genetically diverse set of L. monocytogenes isolates as well as a single hemolytic Listeria innocua strain and (ii) an outbreak set, which includes 21 human and food isolates from nine major human listeriosis outbreaks that occurred between 1981 and 2002. The diversity set represents all three genetic L. monocytogenes lineages (I, n = 9; II, n = 9; and III, n = 6) as well as nine different serotypes. Molecular subtyping by EcoRI automated ribotyping and pulsed-field gel electrophoresis (PFGE) with AscI and ApaI separated the 25 isolates in the diversity set into 23 ribotypes and 25 PFGE types, confirming that this isolate set represents considerable genetic diversity. Molecular subtyping of isolates in the outbreak set confirmed that human and food isolates were identical by ribotype and PFGE, except for human and food isolates for two outbreaks, which displayed related but distinct PFGE patterns. Subtype and source data for all isolates in this strain collection are available on the Internet and are linked to the PathogenTracker database (www.pathogentracker.com), which allows the addition of new, relevant information on these isolates, including links to publications that have used isolates from this collection. We have thus developed a core L. monocytogenes strain collection, which will provide a resource for L. monocytogenes research and development efforts with centralized Internet-based data curation and integration.

Invasive Listeria monocytogenes infections in the Netherlands, 1995-2003

In order to add to the limited data available about the incidence of invasive Listeria monocytogenes infection in the Netherlands, two studies were conducted. In the first study, data on hospital patients with listeriosis in the period 1995-2003 were obtained from the National Medical Registration (study 1). In the second study, hospital discharge letters for patients whose Listeria isolates were received by the Netherlands Reference Laboratory for Bacterial Meningitis (NRLBM) in the period 1999-2003 were retrieved (study 2). Serotyping and pulsed-field gel electrophoresis (PFGE) were used to subtype the various strains of Listeria. These reviews revealed 283 hospital patients and 159 patients with Listeria isolates. Discharge letters were received for 107 (67%) patients. The mean annual incidence of listeriosis in both studies was 2.0 per million inhabitants. The main clinical manifestations were meningitis (incidence: 0.9 and 1.0 per million in studies 1 and 2, respectively) and septicaemia (incidence: 0.08 and 1.0 per million, respectively). Listeriosis in pregnancy was rare (incidence: 1.3 and 2.4 per 100,000 pregnancies over 24 weeks of gestation, respectively). Predisposing conditions were present in 47 and 71% of the patients in studies 1 and 2, respectively. The mortality due to listeriosis was 18%. Serotypes 4b, 1/2a, and 1/2b were responsible for 96% of the cases of human listeriosis. Listeriosis is rare in the Netherlands, but its clinical course is severe and the resulting mortality is high. Therefore, the current recommendations for pregnant women to avoid high-risk foods should be continued. These dietary recommendations should also be given to individuals with predisposing conditions, since they, too, are at risk of Listeria infection.

Comparison of Listeria monocytogenes strain types in Irish smoked salmon and other foods

An investigation of Listeria monocytogenes in Irish retail smoked salmon products and other unrelated food products was undertaken. Serotyping and genotyping methods were applied. Twenty-six L. monocytogenes isolates cultured from ready-to-eat smoked salmon and an additional 20 L. monocytogenes isolates from various commercially available food products (other than smoked salmon) were compared. Four serotypes, 12 ribotypes, 12 amplified fragment length polymorphism (AFLP) types and 17 pulsed-field gel electrophoresis (PFGE) types were identified among the 46 isolates studied. Genotyping identified a single dominant strain that accounted for 65% of those cultured from smoked salmon and this strain was present in product obtained from three out of five of the manufacturers surveyed. When compared to the food products obtained from a variety of sources, those from smoked salmon appeared to cluster as a single group. In Irish smoked salmon this strain may have adapted, and be capable of persisting in this food product. All isolates were grouped into genetic lineages based on their EcoR1 ribotypes. The attendant risk to public health following consumption of these foods is discussed.