Risk factors for L. monocytogenes contamination of dairy products in Switzerland, 1990-1999

Whole-herd risk factors associated with wean-to-finish mortality under the conditions of a Midwestern USA swine production system

Swine wean-to-finish (W2F) mortality is a multifactorial, dynamic process and a key performance indicator of commercial swine production. Although swine producers typically capture the relevant data, analysis of W2F mortality risk factors is often hindered by the fact that, even if data is available, they are typically in different formats, non-uniform, and dispersed among multiple unconnected databases. In this study, an automated framework was created to link multiple data streams to specific cohorts of market animals, including sow farm productivity parameters, sow farm and growing pig health factors, facilities, management factors, and closeout data from a Midwestern USA production system. The final dataset (master-table) contained breeding-to-market data for 1,316 cohorts of pigs marketed between July 2018 and June 2019. Following integration into a master-table, continuous explanatory variables were categorized into quartiles averages, and the W2F mortality was log-transformed, reporting geometric mean mortality of 8.69 % for the study population. Further, univariate analyses were performed to identify individual variables associated with W2F mortality (p < 0.10) for further inclusion in a multivariable model, where model selection was applied. The final multivariable model consisted of 13 risk factors and accounted for 68.2 % (R²) of the variability of the W2F mortality, demonstrating that sow farm health and performance are closely linked to downstream W2F mortality. Higher sow farm productivity was associated with lower subsequent W2F mortality and, conversely, lower sow farm productivity with higher W2F mortality e.g., groups weaned in the highest quartiles for pre-weaning mortality and abortion rate had 13.5 %, and 12.5 %, respectively, which was statistically lower than the lowest quartiles for the same variables (10.5 %, and 10.6 %). Moreover, better sow farm health status was also associated with lower subsequent W2F mortality. A significant difference was detected in W2F mortality between epidemic versus negative groups for porcine reproductive and respiratory syndrome virus (15.4 % vs 8.7 %), and Mycoplasma hyopneumoniae epidemic versus negative groups (13.7 % vs 9.9 %). Overall, this study demonstrated the application of a whole-herd analysis by aggregating information of the pre-weaning phase with the post-weaning phase (breeding-to-market) to identify and measure the major risk factors of W2F mortality.

Population Genomic Analysis of Listeria monocytogenes From Food Reveals Substrate-Specific Genome Variation

Listeria monocytogenes is the major causative agent of the foodborne illness listeriosis. Listeriosis presents as flu-like symptoms in healthy individuals, and can be fatal for children, elderly, pregnant women, and immunocompromised individuals. Estimates suggest that L. monocytogenes results in ∼1,600 illnesses and ∼260 deaths annually in the United States. L. monocytogenes can survive and persist in a variety of harsh environments, including conditions encountered in production of fermented dairy products such as cheese. For instance, microbial growth is often limited in soft cheese fermentation because of harsh pH, water content, and salt concentrations. However, L. monocytogenes has caused a number of deadly listeriosis outbreaks through the contamination of cheese. The purpose of this study was to understand if genetically distinct populations of L. monocytogenes are associated with particular foods, including cheese and dairy. To address this goal, we analyzed the population genetic structure of 504 L. monocytogenes strains isolated from food with publicly available genome assemblies. We identified 10 genetically distinct populations spanning L. monocytogenes lineages 1, II, and III and serotypes 1/2a, 1/2b, 1/2c, 4b, and 4c. We observed an overrepresentation of isolates from specific populations with cheese (population 2), fruit/vegetable (population 2), seafood (populations 5, 8 and 9) and meat (population 10). We used the Large Scale Blast Score Ratio pipeline and Roary to identify genes unique to population 1 and population 2 in comparison with all other populations, and screened for the presence of antimicrobial resistance genes and virulence genes across all isolates. We identified > 40 genes that were present at high frequency in population 1 and population 2 and absent in most other isolates. Many of these genes encoded for transcription factors, and cell surface anchored proteins. Additionally, we found that the virulence genes aut and ami were entirely or partially deleted in population 2. These results indicate that some L. monocytogenes populations may exhibit associations with particular foods, including cheese, and that gene content may contribute to this pattern.

Tracing Listeria monocytogenes contamination in artisanal cheese to the processing environments in cheese producers in southern Chile

Artisanal cheese from southern Chile is made primarily by rural families who raise dairy cows and produce cheese as a way to add value to their milk. The most common cheese produced is chanco, a semi-hard cheese that is typically sold in unauthorized markets. The methods of chanco production do not always follow good manufacturing practices; however, the presence of Listeria monocytogenes contamination in this cheese has not been previously documented. To better understand production practices and L. monocytogenes contamination, 39 cheese producers were surveyed with regard to infrastructure, cleaning and sanitation, pest control, personal hygiene, training, raw materials, and manufacturing. During four sampling trips in 2016 (March, May, August, and November), 546 samples were collected (468 cheese samples and 78 milk samples). For producers that tested positive for L. monocytogenes, environmental monitoring was also conducted, for which 130 additional samples were collected. Presumptive L. monocytogenes isolates (N = 94) were further characterized and subtyped using standard techniques and qPCR-based species/subtype verification; a subset of 52 isolates were also subtyped by Pulsed Field Gel Electrophoresis (PFGE). L. monocytogenes was found in 19 cheeses (4.1%) from five producers (12.8%). The most frequent serotypes were 1/2b (48.9%), group 4B (4b, 4d, 4e) (45.7%), and serotype 1/2a (5.4%). Although no milk samples tested positive for L. monocytogenes, all cheese samples from two producers tested positive during two of the samplings. Distinct PFGE types were recovered from each facility, demonstrating persistence of certain subtypes of the pathogen that ultimately caused end-product contamination. Environmental monitoring of the five positive producers revealed a prevalence of L. monocytogenes ranging from 0 to 30%, with food contact surfaces having the highest incidence of this organism. The findings of this study contribute to the understanding of L. monocytogenes incidence in artisanal cheese in the region of southern Chile.

Isolation and Characterization of an Anti-listerial Bacteriocin from Leuconostoc lactis SD501

Although bacteriocins with anti-listerial activity have been isolated from a wide variety of lactic acid bacteria, little is known about those from Leuconostoc lactis, a heterofermentative bacterium that produces diacetyl and exopolysaccharides in dairy foods. In this study, an anti-listerial bacteriocin was isolated from Leuc. lactis SD501 and characterized. It was particularly potent against Listeria monocytogenes and also inhibited Enterococcus faecalis. Anti-listerial activity reached a maximum during the early stationary phase and then decreased gradually. The anti-listerial substance was sensitive to proteinase K and ɑ-chymotrypsin, confirming its proteinaceous nature. Its activity remained stable at pH values ranging from 1 to 10. In addition, it was strongly resistant to high temperatures, retaining its activity even after incubation for 15 min at 121℃. The apparent molecular mass of the partially purified anti-listerial bacteriocin was approximately 7 kDa. The characteristics of the SD501 bacteriocin, including its small molecular size (<10 kDa), strong anti-listerial activity, wide pH stability and good thermostability, indicate its classification as a Class IIa bacteriocin.

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

Listeriosis is a foodborne disease with a high fatality rate, and infection is mostly transmitted through ready-to-eat (RTE) foods contaminated with Listeria monocytogenes, such as gravad/smoked fish, soft cheeses, and sliced processed delicatessen (deli) meat. Food products/dishes stored in vacuum or in modified atmospheres and with extended refrigerator shelf lives provide an opportunity for L. monocytogenes to multiply to large numbers toward the end of the shelf life. Elderly, pregnant women, neonates, and immunocompromised individuals are particularly susceptible to L. monocytogenes. Listeriosis in humans manifests primarily as septicemia, meningitis, encephalitis, gastrointestinal infection, and abortion. In the mid 1990s and early 2000s a shift from L. monocytogenes serovar 4b to serovar 1/2a causing human listeriosis occurred, and serovar 1/2a is becoming more frequently linked to outbreaks of listeriosis, particularly in Europe and Northern America. Consumer lifestyle has changed, and less time is available for food preparation. Modern lifestyle has markedly changed eating habits worldwide, with a consequent increased demand for RTE foods; therefore, more RTE and take away foods are consumed. There is a concern that many Listeria outbreaks are reported from hospitals. Therefore, it is vitally important that foods (especially cooked and chilled) delivered to hospitals and residential homes for senior citizens and elderly people are reheated to at least 72°C: cold food, such as turkey deli meat and cold-smoked and gravad salmon should be free from L. monocytogenes. Several countries have zero tolerance for RTE foods that support the growth of Listeria.

Human isolates ofListeria monocytogenesin Sweden during half a century (1958–2010)

Isolates ofListeria monocytogenes(n = 932) isolated in Sweden during 1958–2010 from human patients with invasive listeriosis were characterized by serotyping and pulsed-field gel electrophoresis (PFGE) (AscI). Of the 932 isolates, 183 different PFGE types were identified, of which 83 were each represented by only one isolate. In all, 483 serovar 1/2a isolates were distributed over 114 PFGE types; 90 serovar 1/2b isolates gave 32 PFGE types; 21 serovar 1/2c isolates gave nine PFGE types; three serovar 3b isolates gave one PFGE type; and, 335 serovar 4b isolates gave 31 PFGE types. During the 1980s in Sweden, several serovar 4b cases were associated with the consumption of European raw soft cheese. However, as cheese-production hygiene has improved, the number of 4b cases has decreased. Since 1996, serovar 1/2a has been the dominantL. monocytogenesserovar in human listeriosis in Sweden. Therefore, based on current serovars and PFGE types, an association between human cases of listeriosis and the consumption of vacuum-packed gravad and cold-smoked salmon is suggested.

Room temperature aging to guarantee microbiological safety of Brazilian artisan Canastra cheese

Canastra cheese is one of the oldest and most traditional cheeses made from raw milk in Brazil. However, this type of practice may have severe consequences for human health. According to the current legislation, any cheese made from raw milk must be aged for at least 60 days. Traditionally, Canastra cheese is consumed after different ripening periods, but consumers usually prefer those that are aged less than eight days. This study aimed to evaluate the effects of physicochemical and microbiological parameters, with emphasis on the pathogenic microbiota regulated by law, on cheese aged at room temperature and under refrigeration. Cheese samples were collected from eight different cheese producers located in the Serra da Canastra region twice a year (rainy and dry seasons) and analyzed with 8, 15, 22, 29, 36, and 64 days of ripening. Room temperature aging effectively reduced pathogens, reaching the total count established by law in 22 days, regardless of the season. However, ripening under refrigeration, it was ineffective in reducing the Staphylococcus aureus counts to the legislation limits, even after 64 days. Therefore, Canastra cheese should be ripened for at least 22 days at room temperature in order to fulfill the safety regulatory limits.

Bacteriophage biocontrol of Listeria monocytogenes on soft ripened white mold and red-smear cheeses

Soft-ripened cheeses belong to the type of food most often contaminated with Listeria monocytogenes, and they have been implicated in several outbreaks of listeriosis. Bacteriophages represent an attractive way to combat foodborne pathogens without affecting other properties of the food. We used the broad host range, virulent Listeria phage A511 for control of L. monocytogenes during the production and ripening phases of both types of soft-ripened cheeses, white mold (Camembert-type) cheese, as well as washed-rind cheese with a red-smear surface (Limburger-type). The surfaces of young, unripened cheese were inoculated with 10(1)-10(3) cfu/cm(2)L. monocytogenes strains Scott A (serovar 4b) or CNL 10(3)/2005 (serovar 1/2a). Phage was applied at defined time points thereafter, in single or repeated treatments, at 3 × 10(8) or 1 × 10(9) pfu/cm(2). With Scott A (10(3) cfu/cm(2)) and a single dose of A511 (3 × 10(8) pfu/cm(2)) on camembert-type cheese, viable counts dropped 2.5 logs at the end of the 21 day ripening period. Repeated phage application did not further inhibit the bacteria, whereas a single higher dose (1 × 10(9) pfu/cm(2)) was found to be more effective. On red-smear cheese ripened for 22 days, Listeria counts were down by more than 3 logs. Repeated application of A511 further delayed re-growth of Listeria, but did not affect bacterial counts after 22 days. With lower initial Listeria contamination (10(1)-10(2) cfu/cm(2)), viable counts dropped below the limit of detection, corresponding to more than 6 logs reduction compared to the control. Our data clearly demonstrate the potential of bacteriophage for biocontrol of L. monocytogenes in soft cheese.

Occurrence of foodborne pathogens and characterization of Staphylococcus aureus in cheese produced on farm-dairies

The objective of this study was to address knowledge gaps identified in an earlier risk assessment of Staphylococcus aureus and raw milk cheese. A survey of fresh and short-time ripened cheeses produced on farm-dairies in Sweden was conducted to investigate the occurrence and levels of S. aureus, Listeria monocytogenes and Escherichia coli, to characterize S. aureus isolates with special emphasis on enterotoxin genes, antibiotic resistance, bio-typing and genetic variation, and to collect information related to production practices. In general, the hygienic quality of farm-dairy cheeses appeared to be of an acceptable microbiological quality, e.g. L. monocytogenes and staphylococcal enterotoxin were not detected in cheese samples. However, E. coli and enterotoxigenic S. aureus were frequently found in raw milk cheeses and sometimes at levels that are of concern, especially in fresh cheese. Interestingly, levels in raw milk fresh cheese were significantly lower when starter cultures were used. Up to five S. aureus colonies per cheese, if possible, were characterized and about 70% of isolates carried one or more enterotoxin genes, most common were sec and sea. The Ovine biotype (73%) was most common among isolates from goat milk cheese and the Human biotype (60%) from cow milk cheese. Of all isolates, 39% showed decreased susceptibility to penicillin, but the proportion of isolates from cows' cheese (66%) compared to isolates from goats' cheese (27%) was significantly higher. S. aureus isolates with different properties were detected in cheese from the same farm and, sometimes even the same cheese. Isolates with the same pulsed-field gel electrophoresis (PFGE)-pattern were detected on geographically distant dairies. This indicates that multiple sources and routes of contamination are important. To improve the safety of these products efforts to raise awareness of the importance of hygiene barriers and raw milk quality as well as improved process control can be suggested, e.g. use of starter cultures and monitoring of fermentation with a pH-meter. For future safety assessments, a better understanding of factors determining toxin production in these cheeses is needed.

Assuring growth inhibition of listerial contamination during processing and storage of traditional Greek Graviera cheese: compliance with the new European Union regulatory criteria for Listeria monocytogenes

The current microbiological regulatory criteria in the European Union specify a maximum Listeria monocytogenes population of 100 CFU/g allowable in ready-to-eat foods provided the product will not exceed this limit throughout its shelf life. The aim of this study was to validate the manufacturing method for traditional Greek Graviera cheese produced from thermized milk. Initial challenge experiments evaluated the fate of inoculated L. monocytogenes (ca. 4 log CFU/ml, three-strain cocktail) in thermized Graviera cheese milk (TGCM; 63 degrees C for 30 s) in the presence and absence of a product-specific starter culture (SC) in vitro. Milk samples were incubated for 6 h at 37 degrees C and then for 66 h at 18 degrees C. Experiments were conducted to evaluate the fate of a cocktail of three nonpathogenic L. monocytogenes and L. innocua indicator strains inoculated (ca. 3 log CFU/g) in Graviera cheeses commercially manufactured from TGCM+SC. Cheeses were brined, ripened at 18 degrees C and 90% relative humidity for 20 days, and stored at 4 degrees C for up to day 60 under vacuum. In TGCM, L. monocytogenes increased by ca. 2 log units, whereas in TGCM+SC L. monocytogenes growth was retarded (P < 0.05) after a ca. 1-log increase within 6 h at 37 degrees C. Populations of Listeria indicator strains did not grow in TGCM+SC cheeses at any stage; they declined 10-fold in fresh cheeses within 5 days and then survived with little death thereafter. Thus, growth inhibition but not inactivation of potent natural Listeria contaminants at levels below 100 CFU/g occurs in the core of traditional Greek Graviera cheese during fermentation, ripening, and storage.

Characterization of human invasive isolates of Listeria monocytogenes in Sweden 1986-2007

Since 1986, 68% of the Listeria monocytogenes isolates from human cases of invasive listeriosis in Sweden are available for retrospective studies. The aim of the present study was to characterize 601 human invasive isolates of L. monocytogenes in Sweden from 1986 to 2007 by using serotyping and pulsed-field gel electrophoresis. Since 1996, serovar 4b was permanently reduced to the second or third most common serovar in human cases in Sweden. During the latter period, 2000-2007, only 13% belonged to serovar 4b and 71% to 1/2a. The dendrogram, based on pulsovars, reveals two clusters with different serovars. Cluster 1 exhibits serovars 4b and 1/2b, whereas cluster 2 consists of serovar 1/2a. Serovar 1/2a seems to be more heterogeneous than serovar 4b.

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.

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.

Production, characterisation and potential application of a novel monoclonal antibody for rapid identification of virulent Listeria monocytogenes

A panel of hybridomas was produced using intact Listeria monocytogenes serotype 1/2a cells as the immunogen. An IgG2a monoclonal antibody (mAb) 'mAb2B3' was isolated that reacted with L. monocytogenes but not with a representative panel of related Listeria spp. and non-Listeria spp. Binding activity was greatest against L. monocytogenes serotype 1/2a and was significantly enhanced when cells were prepared in Listeria enrichment broth (LEB). The reactive epitope was deduced, by immunoblot analysis, to be a surface localised protein of approximately 80 kilodaltons (kDa), putatively assumed to be internalin A (InlA). Recombinant InlA protein was subsequently expressed in Escherischia coli. When crude E. coli cell lysates were subjected to immunoblot analysis, it was demonstrated that the mAb bound specifically to the heterologously expressed recombinant InlA protein, thus confirming the specificity of the mAb. The mAb was further evaluated in a series of enzyme-linked-immunosorbent assay (ELISA)-based formats and in a surface plasmon resonance (SPR)-based biosensor platform. Both configurations were capable of differential identification of virulent L. monocytogenes at concentrations greater than or equal to 1x10(7) cells/ml. Notwithstanding the apparent insensitivity, the results indicate that InlA could be exploited as a marker for highly specific confirmatory identification of pathogenic L. monocytogenes following primary enrichment of suspect food samples, using the anti-InlA antibody 'mAb2B3', described herein.

Characterization of Listeria monocytogenes isolates from 50 small-scale Austrian cheese factories

One hundred eighty-one small-scale cheese factories (annual production < 100,000 kg) were tested for the presence of Listeria monocytogenes in cheese and smear samples from 1997 to 2000. In total, 2615 samples were drawn. Fifty (27.6%) of 181 enterprises yielded L. monocytogenes. From 14 of the cheese-making facilities, we obtained more than four L. monocytogenes isolates. A total of 182 mostly cheese- and smear-borne L. monocytogenes strains were characterized by serotyping and pulsed-field gel electrophoresis. In 12 of 14 cheese factories, over half of the L. monocytogenes isolates were genetically indistinguishable by pulsetype. On average, genetically indistinguishable isolates were recovered for 11.9 months. Regarding serotypes, 27.3% of the isolates were of serovar 4b. Inadequate personal hygiene could explain the high prevalence of serovar 4b isolates in small-scale cheesemaking facilities. Forty-two percent of the serovar 4b isolates recovered from epidemiologically unlinked facilities (in comparison to 40 and 29% of the 1/2a and 1/2b isolates, respectively) were genetically indistinguishable from at least one other isolate. Indistinguishable serovar 1/2a and 1/2b isolates belonged to five and six different pulsetypes, respectively, whereas serovar 4b isolates belonged to only two pulsetypes. This finding suggested a wide distribution of genetically homologous serovar 4b isolates among the facilities tested in our study.

The Hazard Analysis Critical Control Point's (HACCP) concept as applied to some chemical, physical and microbiological contaminants of milk on dairy farms. A prototype

Quality management on dairy farms becomes more and more important regarding the different areas of animal health, animal welfare and food safety. Monitoring animals, farm conditions and farm records can be extended with risk identification and risk management. The hazard analysis critical control point's system is useful as an on farm strategy to control the product as well as the production process on the areas of animal health, animal welfare and food safety. This article deals in detail with the question how to develop a qualitative method where risk can be defined as an interaction between probability and impact. Two parts of the production process (milk harvest and treatment of cows) where used as an example how to apply the hazard analysis critical control point's system on chemical, physical and microbiological contaminants of milk. Not just only by summarizing the different critical checkpoints for each area but also by giving them a precise judgement of probability and impact.

Incidence of Listeria monocytogenes in raw seafood products in Japanese retail stores

The incidence of Listeria monocytogenes in raw fish, shellfish, and fish roe was investigated in seafood products collected from randomly selected retail stores in and around Tokyo, Japan. Of the 10 samples of 208 examined found positive for L. monocytogenes by mini-VIDAS LMO, seven were fish roe (cod, salmon) and three were minced tuna. Three serotypes (1/2a, 1/2b, 3b) were detected among the isolated strains; serotype 1/2a was predominant (8 of 10).

Application of molecular genetic methods in diagnostics and epidemiology of food-borne bacterial pathogens

Salmonella enterica, Campylobacter and Yersinia species, Shiga toxin-producing Escherichia coli (STEC), Listeria monocytogenes and Clostridium perfringens are the bacterial pathogens constituting the greatest burden of food-borne disease in Finland. Several molecular genetic methods have been applied to diagnose, discriminate and survey these bacteria. PCR, PCR-RFLP and PFGE are the most widely and successfully used. However, these methods are unable to replace conventional and internationally standardised phenotyping. Electronic database libraries of the different genomic profiles will enable continuous surveillance of infections and detection of possible infection clusters at an early stage. Furthermore, whole-genome sequence data have opened up new insights into epidemiological surveillance. Laboratory-based surveillance performed in a timely manner and exploiting adequate methods, and co-operation at local, national and international levels are among the key elements in preventing food-borne diseases. This paper reviews different applications of molecular genetic methods for investigating enteric bacterial pathogens and gives examples of the methods successfully used in diagnostics and epidemiological studies in Finland.

Molecular fingerprinting of dairy microbial ecosystems by use of temporal temperature and denaturing gradient gel electrophoresis

Numerous microorganisms, including bacteria, yeasts, and molds, constitute the complex ecosystem present in milk and fermented dairy products. Our aim was to describe the bacterial ecosystem of various cheeses that differ by production technology and therefore by their bacterial content. For this purpose, we developed a rapid, semisystematic approach based on genetic profiling by temporal temperature gradient electrophoresis (TTGE) for bacteria with low-G+C-content genomes and denaturing gradient gel electrophoresis (DGGE) for those with medium- and high-G+C-content genomes. Bacteria in the unknown ecosystems were assigned an identity by comparison with a comprehensive bacterial reference database of approximately 150 species that included useful dairy microorganisms (lactic acid bacteria), spoilage bacteria (e.g., Pseudomonas and Enterobacteriaceae), and pathogenic bacteria (e.g., Listeria monocytogenes and Staphylococcus aureus). Our analyses provide a high resolution of bacteria comprising the ecosystems of different commercial cheeses and identify species that could not be discerned by conventional methods; at least two species, belonging to the Halomonas and Pseudoalteromonas genera, are identified for the first time in a dairy ecosystem. Our analyses also reveal a surprising difference in ecosystems of the cheese surface versus those of the interior; the aerobic surface bacteria are generally G+C rich and represent diverse species, while the cheese interior comprises fewer species that are generally low in G+C content. TTGE and DGGE have proven here to be powerful methods to rapidly identify a broad range of bacterial species within dairy products.

Listeria monocytogenes isolates from invasive infections: variation of sero- and genotypes during an 11-year period in Finland

Listeria monocytogenes strains that were isolated from 314 human listeriosis cases in Finland during an 11-year period were analyzed by O:H serotyping and pulsed-field gel electrophoresis (PFGE). Serotyping divided the isolates into five serotypes, the most common being 1/2a (53%) and 4b (27%). During the study period, the number of cases caused by serotype 1/2a increased from 22% in 1990 to 67% in 2001, and those caused by serotype 4b decreased from 61 to 27%, respectively. PFGE with restriction enzyme AscI divided the strains into 81 PFGE genotypes; among strains of serotypes 1/2a and 4b, 49 and 18 PFGE types were seen, respectively. PFGE type 1 (serotype 1/2a) was the most prevalent single type (37 strains). Together with six other, closely related PFGE types, PFGE type 1 formed a group of 71 strains, representing 23% of all 314 strains. Strains of PFGE type 1 have also been isolated from cold smoked fish, suggesting a source of human infections caused by this type. Moreover, PFGE type 24 (serotype 1/2c) was significantly associated with gender: 5% of 180 male subjects but none of 132 female subjects (P = 0.012). An electronic database library was created from the PFGE profiles to make possible the prompt detection of new emerging profiles and the tracing of potential infection clusters in the future.