Listeria monocytogenes contamination patterns for the smoked fish processing environment and for raw fish

Radiation D10-values on thawed and frozen catfish and tilapia for finfish isolates of Listeria monocytogenes

With the popularity of catfish and tilapia in the healthy diet, the consumption and harvesting of farm-raised finfish have increased. Since 1987 the pathogenic bacterium Listeria monocytogenes has been isolated from seafood, particularly farm-raised catfish in the United States. Seafood isolates of L. monocytogenes are now available. In order to maintain the raw finfish product, nonthermal interventions to remove bacterial pathogens need to be evaluated using these isolates. A nonthermal intervention process, irradiation, was used to determine the destruct values of the L. monocytogenes seafood isolates along with a nonpathogenic Listeria strain and an L. monocytogenes strain previously studied. The irradiation destruct values were obtained for each individual isolate inoculated on raw and frozen catfish or tilapia irradiated at 4 or -10 degrees C. The Dradiation values obtained for L. monocytogenes inoculated on raw or frozen catfish did not differ (P > 0.05) from the values obtained for strains inoculated on the raw or frozen tilapia. The Dradiation-values ranged from 0.48 to 0.85 kGy, with an average of 0.62 +/- 0.09 kGy, which is typical for Listeria. The data obtained have identified a multi-isolate cocktail that can be used for future radiation inactivation studies for L. monocytogenes inoculated on finfish.

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.

Altered hydrophobicity and membrane composition in stress-adapted Listeria innocua

Exposure of Listeria innocua to acid and starvation stress decreases sensitivity to the quaternary ammonium compound cetrimide, whereas exposure to cold and heat stress increases sensitivity to this compound. Changes in membrane lipids occur in response to certain types of stress, and these changes likely impact cell sensitivity to chemical sanitizers. The present study included an assessment of the effects of acid, starvation, cold, and heat stress on net cell hydrophobicity and fatty acid composition in L. innocua. Net cell hydrophobicity was determined by measuring absorbance of stress-adapted cell suspensions after partitioning with the nonpolar solvent n-hexadecane. Free fatty acids extracted from stress-adapted suspensions were analyzed by gas chromatography. Adaptation to acid and starvation increased net cell hydrophobicity and decreased membrane fluidity, which was correlated with reductions in anteiso fatty acids and in ratios of anteiso to iso fatty acids. Conversely, cold-stressed populations exhibited decreased net cell hydrophobicity and increased membrane fluidity with a corresponding increase in C15:C17 and anteiso:iso ratios and in C18 unsaturated fatty acids. No significant changes in net cell hydrophobicity or membrane fluidity were observed in heat-stressed cells, which exhibited increased sensitivity to cetrimide, suggesting another mechanism for altered cell sensitivity. These findings indicate that the efficacy of cetrimide against Listeria is partially dependent on the physiological state of the organism following exposure to various environmental stresses.

Comparative analysis of the sigma B-dependent stress responses in Listeria monocytogenes and Listeria innocua strains exposed to selected stress conditions

The alternative sigma factor sigma(B) contributes to transcription of stress response and virulence genes in diverse gram-positive bacterial species. The composition and functions of the Listeria monocytogenes and Listeria innocua sigma(B) regulons were hypothesized to differ due to virulence differences between these closely related species. Transcript levels in stationary-phase cells and in cells exposed to salt stress were characterized by microarray analyses for both species. In L. monocytogenes, 168 genes were positively regulated by sigma(B); 145 of these genes were preceded by a putative sigma(B) consensus promoter. In L. innocua, 64 genes were positively regulated by sigma(B). sigma(B) contributed to acid stress survival in log-phase cells for both species but to survival in stationary-phase cells only for L. monocytogenes. In summary, (i) the L. monocytogenes sigma(B) regulon includes >140 genes that are both directly and positively regulated by sigma(B), including genes encoding proteins with importance in stress response, virulence, transcriptional regulation, carbohydrate metabolism, and transport; (ii) a number of L. monocytogenes genes encoding flagellar proteins show higher transcript levels in the Delta sigB mutant, and both L. monocytogenes and L. innocua Delta sigB null mutants have increased motility compared to the respective isogenic parent strains, suggesting that sigma(B) affects motility and chemotaxis; and (iii) although L. monocytogenes and L. innocua differ in sigma(B)-dependent acid stress resistance and have species-specific sigma(B)-dependent genes, the L. monocytogenes and L. innocua sigma(B) regulons show considerable conservation, with a common set of at least 49 genes that are sigma(B) dependent in both species.

Evaluation of the international reference methods NF EN ISO 11290-1 and 11290-2 and an in-house method for the isolation of Listeria monocytogenes from retail seafood products in france

Retail seafood products were analyzed on their use-by date using the international reference methods NF EN ISO 11290-1 and 11290-2 (collectively method R) or an in-house method (method B) for the isolation of Listeria monocytogenes. The sensitivity of the methods was about 78%. Method R detected more positive samples of smoked salmon and herb-flavored slices of smoked salmon than did method B, whereas the reverse was true for samples of carpaccio-like salmon, herb-flavored slices of raw salmon, and smoked trout. Most products produced a positive result after the first of two enrichments, and little difference was observed after changing the isolation medium (Listeria selective agar, L. monocytogenes blood agar, agar for Listeria according to Ottaviani and Agosti, Oxford agar, and Palcam agar). L. monocytogenes was isolated from 151 (27.8%) of the 543 samples, with concentrations mostly below 100 CFU/g. The pathogen prevalence and concentration in these seafood products varied greatly depending on the producer and the nature of the product. In certain cases, these differences could be explained by problems in cleaning and disinfection operations in the food-processing environment. The identities of L. monocytogenes isolates were confirmed by PCR, and isolates were characterized by random amplification of polymorphic DNA and pulsed-field gel electrophoresis (PFGE). PFGE patterns obtained with the enzymes Apal and AscI produced 26 different pulsotypes. In general, different pulsotypes were present in the different categories of seafood products and were not specific to one producer. The genetic diversity observed in the products was not related to the prevalence found at the manufacturing site. It is therefore important for producers to determine the source(s) of contamination of their product so the risks linked to the presence of L. monocytogenes can be reduced.

Pathogen kinetics and heat and mass transfer-based predictive model for Listeria innocua in irregular-shaped poultry products during thermal processing

The increasing demand of ready-to-eat poultry products has led to serious concerns over product safety, and more emphasis has been placed on thorough cooking of products. In this study, processing conditions and thermal inactivation of Listeria innocua in chicken breast meats were evaluated during convection cooking in a pilot-plant scale air-steam impingement oven. A predictive model was developed by integrating heat and mass transfer models with a pathogen kinetics model to predict temperature, water content, product yield, and bacterial inactivation during air-steam impingement cooking. Skinless boneless chicken breasts were cooked at oven air temperatures of 177 and 200 degrees C for 2 to 10 min at a humidity of 70 to 75% (moisture by volume) and an air velocity of 1 m/s at the exit of the nozzles. The reduction in Listeria in chicken breasts after 2 to 5 min of cooking was from 0.3 to 1.4 log CFU/g and from 0.8 to 1.8 log CFU/g at 177 and 200 degrees C, respectively. After cooking for 10 min at both temperatures, no survivors were detected in any of the cooked chicken breasts from an initial bacterial concentration of 10(6) CFU/g. The standard errors of prediction for the endpoint center temperatures after 2 to 10 min of cooking were 2.8 and 3.0 degrees C for air temperatures of 177 and 200 degrees C, respectively. At 177 and 200 degrees C, the median relative errors of prediction for water content were 2.5 and 3.7% and those for product yield were 5.4 and 8.4%, respectively. The developed model can be used as a tool to assist in evaluating thermal processing schedules for poultry products cooked in an air-steam impingement oven.

Ecology of Listeria spp. in a fish farm and molecular typing of Listeria monocytogenes from fish farming and processing companies

This study focused on the ecology of Listeria monocytogenes in a fish farm by following the changes in its occurrence in different types of samples for a three year period. In addition, L. monocytogenes isolates from different seafood industry areas were compared with pulsed field gel electrophoresis (PFGE) typing to discover possible associations between primary production, further processing and final products. Weather conditions were found to have a strong influence on the probability of finding Listeria spp. in a fish farm environment. The number of samples contaminated with Listeria spp. was typically bigger after rainy periods. Brook and river waters as well as other runoff waters seemed to be the main contamination source at the farm studied. The farmed fish originally found to carry L. monocytogenes become gradually Listeria free. The time needed for the purification of the fish was several months. The sea bottom soil samples were the ones that preserved the L. monocytogenes contamination the longest time. It can be stated that the fish and fish farm equipment studied did not spread listeria contamination. On the contrary, they were found to suffer from listeria contamination coming from outside sources like the brook water. There was a wide range of different L. monocytogenes PFGE-pulsotypes (30) found at 15 Finnish fish farms and fish processing factories. L. monocytogenes isolates from the final products often belonged to the same pulsotypes as did the isolates from the processing environment as well as from the raw fish. This suggests that, in addition to the fish processing factory environment, the fish raw materials are important sources of L. monocytogenes contamination in final products.

Ribotype diversity of Listeria monocytogenes isolates from two salmon processing plants in Norway

The purpose of this study was to use automated ribotyping procedure to track Listeria monocytogenes transmission in the cold smoked fish production chain and to characterize L. monocytogenes subtypes associated with the salmon processing industry. A total of 104 isolates, which had previously been obtained from a raw fish slaughter and processing plant (plant B) and an adjacent, downstream, salmon smoking operation (plant A), were characterized. These isolates had been obtained through a longitudinal study on Listeria presence, which covered a 31-week period, in both plants. Isolates had been obtained from samples taken from different machinery used throughout the production process. In addition, six isolates obtained from products produced in plant A two years after the initial study were included, so that a total of 110 isolates were characterized. Automated ribotyping was performed using both the restriction enzymes EcoRI and PvuII to increase the discriminatory power. The 110 L. monocytogenes isolates could be divided into 11 EcoRI ribotypes; PvuII ribotype data yielded multiple subtypes within 7 EcoRI ribotypes for a total of 21 subtypes based on both EcoRI and PvuII ribotyping. A total of three EcoRI ribotypes (DUP-1023C, DUP-1045B, and DUP-1053E) were isolated at multiple sampling times from both plants. In addition, one subtype (DUP-1053B) was isolated at multiple sampling times in only plant A, the salmon smoking operation. These data not only support that L. monocytogenes can persist throughout the salmon production system, but also showed that L. monocytogenes may be transmitted between slaughter and smoking operations or may be unique to smoking operations. While the majority of subtypes isolated have been rarely or never linked to human listeriosis cases, some subtypes have previously caused human listeriosis outbreaks and cases. Molecular subtyping thus is critical to identify L. monocytogenes transmission and niches in order to allow design and implementation of control strategies at the appropriate stage of production and in order to reduce the prevalence of L. monocytogenes linked to human disease.

Potassium lactate combined with sodium diacetate can inhibit growth of Listeria monocytogenes in vacuum-packed cold-smoked salmon and has no adverse sensory effects

Growth of Listeria monocytogenes in ready-to-eat fish products such as cold-smoked salmon is an important food safety issue. The objective of this study was to evaluate the antilisterial activity of potassium lactate (PL) in combination with sodium acetate (SA) or sodium diacetate (SDA) in cold-smoked salmon and to determine whether these compounds could be incorporated easily into the formulations and technology currently used by processors. A commercial brine injector was used to inject salmon filets with either saturated saline brine or saturated saline brine supplemented with combinations of PL and SA (PURASAL Opti. Form PA 4) or PL and SDA (PURASAL Opti. Form PD 4). In the brine-injected cold-smoked salmon, 2.1% (water phase) PL and 0.12% (water phase) SDA delayed the growth of L. monocytogenes for up to 42 days of vacuum-packaged storage at 10 degrees C. Storage at 25 degrees C for 6 h resulted in only a 1-log CFU/g increase in L. monocytogenes. Treatments with lower concentrations of PL and SDA or similar concentrations of PL and SA resulted in an extended lag phase and slower growth of L. monocytogenes. It was not possible to incorporate more than 2% (water phase) PL while ensuring a minimum of 3% (water phase) NaCl in the finished product because PL decreased the solubility of NaCl. Sensory analyses revealed that the preservatives did not negatively affect flavor or odor. The combination of PL and SDA is therefore a viable technology for preventing L. monocytogenes growth on cold-smoked salmon.

Daily variability of Listeria contamination patterns in a cold-smoked salmon processing operation

An understanding of Listeria transmission and contamination patterns in processing environments of ready-to-eat foods is critical for improving control of Listeria monocytogenes. A cold-smoked fish processing operation was the site used to study variability in Listeria contamination in a processing environment associated with a ready-to-eat food product throughout one production week (five consecutive days). Intensive testing was conducted on finished products and environmental samples collected at the beginning, middle, and end of each working day. A total of 20 finished products and 22 to 36 environmental samples were collected at each sampling time, and an additional 12 environmental samples were collected on days 4 and 5. Overall, a total of 782 samples, 300 finished products and 482 environmental samples, were tested. All samples were collected from processing steps after smoking, including skinning, trimming, slicing, staging, and packing. A total of 28 finished and 57 environmental samples (9.3 and 11.8%, respectively) were positive for Listeria spp. (including 1 and 5 samples positive for L. monocytogenes, respectively). DNA sequencing of the sigB gene allowed differentiation of eight Listeria subtypes. Listeria prevalence varied significantly between days, and a high prevalence in both environmental samples and finished products on day 3 was likely associated with a point source contamination event by a single Listeria welshimeri subtype. There were no consistent differences in Listeria prevalence among samples collected from the beginning, middle, and end of the production day, but subtype data often revealed unique contamination patterns for samples collected at different times of a given day. Listeria contamination patterns and prevalences were highly variable between days and within a given day. These findings indicate that chance events play an important role in the contamination of finished products, thus complicating efforts to define Listeria transmission patterns in processing environments associated with ready-to-eat foods.

Effect of x-ray irradiation on reducing the risk of listeriosis in ready-to-eat vacuum-packaged smoked mullet

Listeria monocytogenes can pose a serious threat in several areas of the nation's food supply including ready-to-eat seafood products. Use of irradiation processing can potentially reduce the risk of listeriosis caused by consumption of ready-to-eat seafood products. This study measured the effect of X-ray irradiation on reducing the population of L. monocytogenes on ready-to-eat, vacuum-packaged smoked mullet. Smoked mullet were inoculated with a five-strain mixture of L. monocytogenes (10(4) CFU/g), vacuum packaged, and irradiated (0, 0.5, 1.0, 1.5, and 2.0 kGy). The packaged fish were then stored at 3 and 10 degrees C for 90 and 17 days, respectively. Radiation doses of 0.5, 1.0, and 1.5 kGy reduced the initial population of L. monocytogenes by 1.1, 1.6, and 2.1 log CFU/g, respectively. The 2.0-kGy dose reduced L. monocytogenes to undetectable levels with no recovery growth at either temperature. Compared to the control, irradiation at 1.5 kGy demonstrated 1.0 and 1.7 log CFU/g less growth at 3 degrees C after 60 days and 10 degrees C after 17 days, respectively. Sensory flavor analysis was conducted to determine if a difference existed between irradiated samples. Panelists indicated that there were no differences among treated and untreated samples. An X-ray dose of 2 kGy effectively eliminated 10(4) CFU/g L. monocytogenes on smoked mullet without changing sensory quality.

Survival of Listeria monocytogenes in uncooked Italian dry sausage (salami)

This study was undertaken to supplement existing information on the survival of Listeria monocytogenes in Italian salami. The fact that Italian salami is frequently consumed by a large number of people poses some serious health implications. Some raw materials have been found to be microbiologically contaminated, for their production occurs without any thermic treatment, and these are in circulation throughout Italy all year round. We selected the product for its microbiological, technological, and commercial characteristics. We analyzed 1,020 samples taken during the autumn and winter 2002 and spring and summer 2003 periods and immediately before selling. The samples were collected from 17 plants with an annual production of between 1 and 2000 metric tons and with a distribution of products in over 80% of Italy in geographic terms. To detect and enumerate L. monocytogenes, we followed International Organization for Standardization (ISO) 11290 part 1 and 2: 1996 (modified using chromogenic medium Agar Listeria according to Ottarviani and Agosti [ALOA]). L. monocytogenes was found in 22.7% of samples, but the contamination level was less than 10 CFU/g. Contamination prevalence ranged from 1.6 to 58.3% and was lower than 10% in 5 of the 17 plants checked. The most frequently isolated serotypes were 1/2c, 1/2a, 1/2b, and 4b. Additional studies are necessary to establish if the exposure to a small number of L. monocytogenes cells through the consumption of salami represents a significant health risk and, in light of the future introduction of the SANCO/4198/2001 revision 21 "Commission Regulation on Microbiological Criteria for Foodstuffs," is a necessary investigation.

Pulsed-field gel electrophoresis typing of Listeria monocytogenes isolated in two Finnish fish farms

The aim of this study was to find sources of Listeria monocytogenes contamination in fish products from a fish farm. The occurrence of L. monocytogenes also was compared in two freshwater fish farms with different types of fishponds. Samples collected from chilled rainbow trout (Oncorhynchus mykiss) and the slaughterhouse environment did not contain L. monocytogenes, but Listeria innocua was found in two samples from the slaughterhouses. Ten isolates of L. monocytogenes were discovered in sediment and water samples from farming tanks and earth ponds. Further characterization by serovar revealed the same serovar (1/2a) for all the isolates. Pulsed-field gel electrophoresis was used to divide the isolates into five different pulsotypes, three of which have been identified previously in fish products on the retail market. This finding supports the assumption that the primary production, and probably the raw fish, is a source of Listeria contamination in fish products. Some of the isolates were associated with a certain type of fishpond, indicating the need for hygienic analysis of the suitability of different types of farming ponds.

Contamination of cooked peeled shrimp (Pandalus borealis) by Listeria monocytogenes during processing at two processing plants

Listeria spp. and Listeria monocytogenes contamination was evaluated in cooked peeled shrimp (final or semifinal product, 82 samples) and the shrimp-processing environment (two plants, 613 samples) in eight surveys conducted from 1998 through 2001. Listeria was detected in 12.5% (78) of the 695 samples (11.2% of the samples were positive for L. monocytogenes), but none of the samples of final product contained Listeria. One hundred seventy-two L. monocytogenes isolates were characterized by pulsed-field gel electrophoresis. Cleavage with macrorestriction enzymes AscI and ApaI yielded 14 different pulsotypes in the plants; two types were dominant, one in each plant. Sixty-three of the 106 isolates in plant A and 43 of the 66 isolates in plant B were of the dominant types. Certain strains, mainly of serotypes 1/2c and 4b and pulsotypes 1A and 2H, were persistent for long periods in both plants. Adaptation of good hygienic practices in the processing plants, including strict rules concerning traffic of staff and equipment, and existing hygienic requirements appeared to be effective in preventing contamination between areas within plants and in the final product. The persistence of Listeria strains in these two processing plants indicates the importance of detecting the places in the processing environment (e.g., transporters, equipment, floors, and drains) where L. monocytogenes can survive so that cleaning and disinfection efforts can be directed to such niches.

One group of genetically similar Listeria monocytogenes strains frequently dominates and persists in several fish slaughter- and smokehouses

Contamination of foods with the human pathogen Listeria monocytogenes may occur during processing, and the purpose of this study was to determine whether genetically similar strains colonize different processing plants or whether specific persistent strains are unique to each processing plant. We hypothesized that specific L. monocytogenes strains may be better adapted to specific environmental niches in the processing environment. L. monocytogenes contamination patterns were identified by the collection of 686 and 267 samples from the processing environments: raw fish and products of four fish smokehouses and four fish slaughterhouses, respectively. Samples were collected both during production and after cleaning and disinfection. Typically, these samplings were separated by 1 to 3 months. Sampling sites were targeted toward areas likely to harbor the bacterium. L. monocytogenes was isolated from 213 samples, and one strain from each positive sample was typed by RAPD (random amplified polymorphic DNA) analysis with four different primers. The 213 strains were divided into 37 RAPD types. One RAPD type was predominant; 86 of 213 strains belonged to this type. This type was found in three smokehouses and two slaughterhouses and was predominant in three of these plants. A subset of 35 strains was also analyzed by amplified fragment length polymorphism typing, which confirmed the genetic similarity of the groups. Moreover, strains of the dominant RAPD type were indistinguishable from strains isolated frequently from smoked fish products 10 years ago. One smokehouse was surveyed for a year and a half, and the dominant RAPD type persisted throughout the survey period and accounted for 94 of 118 isolates. Our study indicates that strains of L. monocytogenes that are genetically very closely related may be especially adapted to colonizing the processing equipment or especially resistant to cleaning and disinfection.

Molecular typing to trace Listeria monocytogenes isolated from cold-smoked fish to a contamination source in a processing plant

In this study, Listeria monocytogenes contamination in a cold-smoked fish processing plant in Osaka, Japan, was examined from 2002 to 2004. A total of 430 samples were collected and divided into five categories: raw fish, materials during processing, processing equipment, environment, and finished products. A total of 59 finished products were examined throughout this study. L. monocytogenes was isolated from four of these samples during summer and autumn but was not found during winter or spring. During the warmer seasons, L. monocytogenes was more prevalent on processing equipment, especially slicing machines (8 of 54 samples in summer and autumn versus 1 of 50 samples in winter and spring). L. monocytogenes was not detected on whole skins removed from 23 frozen raw fish. L. monocytogenes strains isolated from 56 samples were characterized by serotyping, pulsed-field gel electrophoresis, and three PCR-based methods. Seventy-seven L. monocytogenes strains were recognized as contaminants of the samples: 2 distinguishable strains were identified in each of 13 samples, 3 strains were identified in 2 samples, 5 strains were identified in 1 sample, and the other 40 strains were identified in 40 samples. Combining the results from these techniques, 77 strains were classified into 13 different types. Three of these types prevailed throughout the plant, and two of the three were also isolated from final products. The DNA subtype found in the product was also found on the slicing machines. Our findings suggest that the slicing machines at this plant were the source of the product contamination. Implementing an appropriate cleaning regime for the slicing machines was effective in preventing contamination.

Concentrations and tracking of listeria monocytogenes strains in a seafood-processing environment using a most-probable-number enrichment procedure and randomly amplified polymorphic DNA analysis

Concentrations of environmental microflora and Listeria monocytogenes were monitored at multiple environmental locations within a seafood-processing facility over the course of 6 months. Concentrations of L. monocytogenes were determined using a most-probable-number (MPN) enrichment procedure. Two floor drains had persistent low concentrations of L. monocytogenes (0.03 to >1,100 MPN/cm2). In comparison, concentrations of the other organisms in the drain were much higher (heterotrophic plate count range of 10(5) to 10(8) CFU/cm2). Concentrations of environmental organisms (heterotrophic aerobic plate counts and counts of pseudomonads, Shewanella spp., Aeromonas hydrophila, and coliforms) were not correlated with concentrations of L. monocytogenes. The 178 confirmed L. monocytogenes isolates from the MPN procedure were further characterized by randomly amplified polymorphic DNA analysis. Sixteen different banding patterns were identified, and nine of the patterns were identified from samples collected on two or more collection dates. From all locations, banding type A was observed in 98 confirmed isolates (55%). Although present, L. monocytogenes was a relatively minor component in the ecosystem of the floor drains in this seafood-processing facility.

Discrimination of Listeria monocytogenes contaminated commercial Japanese meats

Discrimination was attempted on 14 Listeria monocytogenes strains isolated from commercially available Japanese pork and chicken. Examination of the isolates was performed by restriction fragment length polymorphism (RFLP) analysis of the chromosomal DNA and amplified products and comparison of the nucleotide sequences of the amplified products. A polymorphism region containing the repeated sequences in the iap gene was amplified by the polymerase chain reaction (PCR). The genetic analyses could discriminate the 14 isolates in combination with traditional serotyping, and some strains isolated from different meats were confirmed to have a genetically close relationship. Genetic analyses used in the present study would be useful for the elucidation of the pathogen tracks from contaminated sources to humans and of the ecological niche in the food environment.

Prevalence and location of Listeria monocytogenes in farmed rainbow trout

A total of 510 rainbow trout originating from fish farms in lakes and sea areas around Finland were studied for the presence of Listeria monocytogenes. Samples were studied as pools from five fish. Gill, viscera, and skin from the pooled samples were analysed separately. The individual samples were analysed later if the pooled sample was found to be Listeria positive. The prevalence of Listeria spp. and L. monocytogenes in pooled unprocessed fresh rainbow trout was on average 35.0% and 14.6%, respectively. On the other hand, the prevalence of Listeria spp. and L. monocytogenes in individual thawed fish was found to be 14.3% and 8.8%, respectively. These numbers tend to overestimate and underestimate the real situation because not all fish in pooled samples were necessarily contaminated and in some of the Listeria positive pooled samples all individual samples turned out to be Listeria free. The prevalence of L. monocytogenes varied greatly between different fish farms from zero to 100% in pooled samples and from zero to 75% according to individually studied fish samples. Some indications of the influence of weather conditions and seasonal variations that strongly affected the Listeria contamination of fish were also noticed. The location of Listeria spp. and L. monocytogenes in different parts of the fish differed with statistical significance in rainbow trout. Up to 95.6% of the L. monocytogenes and 84.5% of Listeria spp. positive samples were gill samples. Only 4.4% (2/45) of the L. monocytogenes positive samples were obtained from skin or viscera. Closer study at one fish farm revealed that there was only one L. monocytogenes ribotype present in the contaminated fish, although water and surfaces were heavily contaminated with six other L. monocytogenes ribotypes.

Antilisterial activity of a Carnobacterium piscicola isolated from Brazilian smoked fish (surubim [Pseudoplatystoma sp.]) and its activity against a persistent strain of Listeria monocytogenes isolated from surubim

Data on the prevalence and growth of Listeria monocytogenes in lightly preserved fish products from subtropical and tropical regions are very scarce. Our research describes L. monocytogenes that was detected in 5% of the packages of cold-smoked surubim, a native Brazilian freshwater fish that we analyzed, and shows that the strains isolated were of the same random amplified polymorphic DNA subtype as the strains that were isolated from the same factory 4 years earlier. A bacteriocinogenic strain of Carnobacterium piscicola (strain C2), isolated from vacuum-packed cold-smoked surubim, and two C. piscicola strains, isolated from vacuum-packed, cold-smoked salmon, were capable of limiting or completely inhibiting the growth of an L. monocytogenes (strain V2) isolated from surubim in fish peptone model systems incubated at 10 degrees C. Monocultures of L. monocytogenes reached 108 CFU/ml (g), whereas the growth of L. monocytogenes was completely inhibited by C. piscicola C2. The bacteriocinogenic C. piscicola A9b+ and its nonbacteriocinogenic mutant A9b- reduced maximum Listeria levels by 2 to 3 log units. Both bacteriocinogenic C. piscicola strains prevented listerial growth in cold-smoked fish juices (surubim and salmon). Although the carnobacteria grew poorly on cold-smoked surubim at 10 degrees C, the strains were able to reduce maximum Listeria counts by 1 to 3 log units in an artificially inoculated product (surubim). We conclude that Brazilian smoked fish products harbor L. monocytogenes and should be stabilized against the growth of the organism. C. piscicola C2 has the potential for use as a bioprotective culture in surubim and other lightly preserved fish, but further studies are required to optimize its effect.

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

Listeria monocytogenes is a foodborne pathogen that can cause listeriosis, a severe disease that can lead to septicemia, meningitis, and spontaneous abortion. Ongoing efforts are needed to further reduce the incidence of listeriosis, due to its high mortality rate. The focus of this report is the use of a risk-based approach to identify strategies that will have the greatest impact on reducing foodborne listeriosis. A continuum of risk for listeriosis is observed in the human population, ranging from exquisitely sensitive groups, who are highly immunocompromised and at very high risk of listeriosis, through the normal healthy population younger than 65 years of age, who appear to have a minimal risk for listeriosis. In addition, unique subpopulations may exist; for example, pregnant Latina women appear to have a higher risk of listeriosis than pregnant women of other ethnic groups, most likely due to consumption of contaminated soft cheeses such as queso fresco and queso blanco. The International Life Sciences Institute Risk Science Institute Expert Panel concluded that certain foods pose a high risk for causing listeriosis. High-risk foods have all of the following properties: (1) have the potential for contamination with L. monocytogenes; (2) support the growth of L. monocytogenes to high numbers; (3) are ready to eat; (4) require refrigeration; and (5) are stored for an extended period of time. Control strategies are needed in the food chain from preharvest through consumption to minimize the likelihood that food will become contaminated by L. monocytogenes and to prevent the growth of the organism to high numbers. The Expert Panel identified three main strategies for ensuring continuous improvement in reducing foodborne listeriosis: (1) preventing contamination of foods with L. monocytogenes; (2) preventing growth of L. monocytogenes to high numbers in foods; and (3) science-based education messages targeted to susceptible populations and their caregivers. Of these strategies, the Expert Panel concluded that preventing growth of L. monocytogenes to high numbers would have the greatest impact in reducing cases of listeriosis. Dose-response models predict that the risk of listeriosis increases as the number of organisms in a food increases and can be used as a scientific basis for a target level below which the organism should be reduced to minimize the likelihood of listeriosis in high-risk populations. This requires implementation of effective food safety control measures and ensuring that these control strategies are consistently met. Most effective strategies to control L. monocytogenes in high-risk foods include (1) good manufacturing practices, sanitation standard operating procedures, and hazard analysis critical control point programs to minimize environmental L. monocytogenes contamination and to prevent cross-contamination in processing plants and at retail; (2) an intensive environmental sampling program in plants processing high-risk foods and an effective corrective action plan to reduce the likelihood of contamination of high-risk foods; (3) time and temperature controls throughout the entire distribution and storage period, including establishing acceptable storage times of foods that support growth of L. monocytogenes to high numbers; (4) reformulating foods to prevent or retard the growth of L. monocytogenes; and (5) using postpackaging treatments to destroy L. monocytogenes on products. Science-based education and risk communication strategies aimed at susceptible populations and focused on high-risk foods should be delivered through health care providers or other credible sources of information. Exquisitely sensitive consumers may become ill when exposed to low numbers of L. monocytogenes or other opportunistic pathogens, so reducing the risk to this population could be achieved by maintaining them on restricted low-microbe diets during those periods when they are most severely immunocompromised. High-risk individuals (i.e., the elderly, pregnant women, and most immunocompromised individuals) should be provided with guidance on healthy eating, including specific information on high-risk foods that they should avoid, and strategies to reduce their risk, such as thorough cooking, avoidance of cross-contamination, and short-term refrigerated storage of cooked perishable foods. Those at low risk for listeriosis should receive information on safe food handling practices, preferably starting at a preschool age.

Raw and processed fish show identical Listeria monocytogenes genotypes with pulsed-field gel electrophoresis

A total of 257 raw fish samples at two different sites were examined for the presence of Listeria monocytogenes. The prevalence of L. monocytogenes was 4%. From 11 positive samples, nine different L. monocytogenes pulsed-field gel electrophoresis genotypes were recovered. From nine pulsotypes recovered from raw fish and 32 pulsotypes shown by 101 fish product isolates, two raw fish and fish product pulsotypes were indistinguishable from each other. Although the prevalence of L. monocytogenes in raw fish is low, the range of L. monocytogenes strains entering the processing plant in large amounts of raw material is wide. This indicates that the raw material is an important initial contamination source of L. monocytogenes in fish processing plants. This postulation is supported by the identical pulsotypes recovered from both raw and processed fish. Some L. monocytogenes strains entering a plant may thus contaminate and persist in the processing environment, causing recurrent contamination of the final products via processing machines.

Methods for the isolation and identification of Listeria spp. and Listeria monocytogenes: a review

Listeria monocytogenes is an important food-borne pathogen and is widely tested for in food, environmental and clinical samples. Identification traditionally involved culture methods based on selective enrichment and plating followed by the characterization of Listeria spp. based on colony morphology, sugar fermentation and haemolytic properties. These methods are the gold standard; but they are lengthy and may not be suitable for testing of foods with short shelf lives. As a result more rapid tests were developed based on antibodies (ELISA) or molecular techniques (PCR or DNA hybridization). While these tests possess equal sensitivity, they are rapid and allow testing to be completed within 48 h. More recently, molecular methods were developed that target RNA rather than DNA, such as RT-PCR, real time PCR or nucleic acid based sequence amplification (NASBA). These tests not only provide a measure of cell viability but they can also be used for quantitative analysis. In addition, a variety of tests are available for sub-species characterization, which are particularly useful in epidemiological investigations. Early typing methods differentiated isolates based on phenotypic markers, such as multilocus enzyme electrophoresis, phage typing and serotyping. These phenotypic typing methods are being replaced by molecular tests, which reflect genetic relationships between isolates and are more accurate. These new methods are currently mainly used in research but their considerable potential for routine testing in the future cannot be overlooked.

Molecular Subtyping and Tracking of Listeria monocytogenes in Latin-Style Fresh-Cheese Processing Plants

Latin-style fresh cheeses, which have been linked to at least 2 human listeriosis outbreaks in the United States, are considered to be high-risk foods for Listeria monocytogenes contamination. We evaluated L. monocytogenes contamination patterns in 3 Latin-style fresh-cheese processing plants to gain a better understanding of L. monocytogenes contamination sources in the manufacture of these cheeses. Over a 6-mo period, 246 environmental samples were collected and analyzed for L. monocytogenes using both the Food and Drug Administration (FDA) method and the Biosynth L. monocytogenes detection system (LMDS). Finished cheese samples from the same plants (n = 111) were also analyzed by the FDA method, which was modified to include L. monocytogenes plating medium (LMPM) and the L. monocytogenes confirmatory plating medium (LMCM) used in the LMDS method. Listeria monocytogenes was detected in 6.3% of cheese and 11.0% of environmental samples. Crates, drains, and floor samples showed the highest contamination rates, with 55.6, 30.0, and 20.6% L. monocytogenes positive samples, respectively. Finished products and food contact surfaces were positive in only one plant. The FDA method showed a higher sensitivity than the LMDS method for detection of L. monocytogenes from environmental samples. The addition of LMPM and LMCM media did not further enhance the performance of the FDA method for L. monocytogenes detection from finished products. Molecular subtyping (PCR-based allelic analysis of the virulence genes actA and hly and automated ribotyping) was used to track contamination patterns. Ribotype DUP-1044A, which had previously been linked to a 1998 multistate human listeriosis outbreak in the United States, was the most commonly identified subtype (20/36 isolates) and was isolated from 2 plants. This ribotype was persistent and widespread in one factory, where it was also responsible for the contamination of finished products. We hypothesize that this ribotype may represent a clonal group with a specific ability to persist in food processing environments. While previous listeriosis outbreaks were linked to Latin-style fresh cheeses made from unpasteurized milk, the presence of this organism in pasteurized cheese products illustrates that persistent environmental contamination also represents an important source of finished product contamination.

Longitudinal studies on Listeria in smoked fish plants: impact of intervention strategies on contamination patterns

Four ready-to-eat smoked fish plants were monitored for 2 years to study Listeria contamination patterns and the impact of plant-specific Listeria control strategies, including employee training and targeted sanitation procedures, on Listeria contamination patterns. Samples from the processing plant environment and from raw and finished product were collected monthly and tested for Listeria spp. and Listeria monocytogenes. Before implementation of intervention strategies, 19.2% of raw product samples (n = 276), 8.7% of finished product samples (n = 275), and 26.1% of environmental samples (n = 617) tested positive for Listeria spp. During and after implementation of Listeria control strategies, 19.0% of raw product samples (n = 242), 7.0% of finished product samples (n = 244), and 19.5% of environmental samples (n = 527) were positive for Listeria spp. In one of the four fish plants (plant 4), no environmental samples were positive for L. monocytogenes, and this plant was thus excluded from statistical analyses. Based on data pooled from plants 1, 2, and 3, environmental Listeria spp. prevalence was significantly lower (P < 0.05) for nonfood contact surfaces and the finished product area and for the overall core environmental samples after implementation of control strategies. Listeria prevalence for floor drains was similar before and after implementation of controls (49.6 and 54.2%, respectively). Regression analysis revealed a significant positive relationship (P < 0.05) between L. monocytogenes prevalence in the environment and in finished products before implementation of control strategies; however, this relationship was absolved by implementation of Listeria control strategies. Molecular subtyping (EcoRI ribotyping) revealed that specific L. monocytogenes ribotypes persisted in three processing plants over time. These persistent ribotypes were responsible for all six finished product contamination events detected in plant 1. Ribotype data also indicated that incoming raw material is only rarely a direct source of finished product contamination. While these data indicate that plant-specific Listeria control strategies can reduce cross-contamination and prevalence of Listeria spp. and L. monocytogenes in the plant environment, elimination of persistent L. monocytogenes strains remains a considerable challenge.

The microbial ecology of high-risk, chilled food factories; evidence for persistent Listeria spp. and Escherichia coli strains

AIMS

The intention of this study was to provide evidence of any Listeria spp. or Escherichia coli strain persistence, and to identify whether strains of these organisms adapt to specific environmental or product niches in food factories.

METHODS AND RESULTS

A 3-year assessment of the microbial ecology of four, ready-to-eat food-processing factories was undertaken in which approx. 196 000 and 75 000 product and environmental samples were examined for Escherichia coli and Listeria spp. respectively. A total of 152 E. coli isolates (44 environmental and 108 product in 62 ribogroups) and 260 Listeria spp. isolates (174 environmental and 86 product in 30 ribogroups) were identified and ribotyped. The overall prevalence of E. coli (0.08%), all Listeria spp. (0.35%) and L. monocytogenes (0.23%) was very low. Some 10 E. coli ribogroups and 14 Listeria spp. ribogroups showed evidence for persistence, defined as the isolation of the same strain, from the same site, over a prolonged time period. The majority of E. coli strains were product niche oriented whilst the majority of Listeria spp. strains were environmental niche oriented.

CONCLUSION

Current UK high-risk food factory designs, personnel hygiene and cleaning and disinfection regimes are sufficient to control Listeria spp. and E. coli to very low levels.

SIGNIFICANCE AND IMPACT OF THE STUDY

Persistent strains of these organisms, however, can remain within factory high-risk production areas over considerable time periods, warranting an examination of the strain persistence mechanisms and alternative hygiene controls.

Impact of intervention strategies on Listeria contamination patterns in crawfish processing plants: a longitudinal study

Two ready-to-eat crawfish processing plants were monitored for 2 years to study the impact of Listeria control strategies, including employee training and targeted sanitation procedures, on Listeria contamination. Environmental, raw material, and finished product samples were collected weekly during the main processing months (April to June) and tested for Listeria spp. and Listeria monocytogenes. Before implementation of control strategies (year 1), the two processing plants showed Listeria spp. prevalences of 29.5% (n = 78) in raw, whole crawfish, 5.2% (n = 155) in the processing plant environment, and 0% (n = 78) in finished products. In year 2, after plant-specific Listeria control strategies were implemented, Listeria spp. prevalence increased in raw crawfish (57.5%, n = 101), in the processing plant environment (10.8%, n = 204), and in the finished product (1.0%, n = 102). Statistical analysis showed a significant increase in Listeria spp. prevalence (P < 0.0001) and a borderline nonsignificant increase in L. monocytogenes prevalence (P = 0.097) on raw material in year 2. Borderline nonsignificant increases were also observed for Listeria spp. prevalence in environmental samples (P = 0.082). Our data showed that Listeria spp. prevalence in raw crawfish can vary significantly among seasons. However, the increased contamination prevalence for raw materials only resulted in a limited Listeria prevalence increase for the processing plant environment with extremely low levels of finished product contamination. Heat treatment of raw materials combined with Listeria control strategies to prevent cross-contamination thus appears to be effective in achieving low levels of finished product contamination, even with Listeria spp. prevalences for raw crawfish of more than 50%.

Effects of high-pressure processing on Listeria monocytogenes, spoilage microflora and multiple compound quality indices in chilled cold-smoked salmon

AIMS

To evaluate the effect of high-pressure processing (HPP) on Listeria monocytogenes, microbial and chemical changes and shelf-life in chilled cold-smoked salmon (CSS).

METHODS AND RESULTS

First, challenge tests with L. monocytogenes were carried out using HPP of the product at 0.1 (control), 150, 200 and 250 MPa. Secondly, storage trials with the naturally contaminated product and HPP at 0.1 (control) and 200 MPa were realized. Shelf-life, microbial changes and chemical changes were determined and existing predictive models and multiple compound quality indices evaluated. HPP with 250 MPa did not inactivate L. monocytogenes but significant lag phases of 17 and 10 days were observed at ca 5 and 10 degrees C, respectively. HPP with 200 MPa had a marked effect on both colour and texture of CSS.

CONCLUSIONS

High-pressure processing was unable to prevent growth of L. monocytogenes or spoilage of chilled CSS. Existing mathematical models allowed growth rates of L. monocytogenes and shelf-life of samples without high-pressure treatments to be predicted.

SIGNIFICANCE AND IMPACT OF THE STUDY

High-pressure processing seems more appropriate for new types of salmon products than for a classical product like CSS where consumers expect specific quality attributes.

Tracking of Listeria monocytogenes in smoked fish processing plants

Four smoked fish processing plants were used as a model system to characterize Listeria monocytogenes contamination patterns in ready-to-eat food production environments. Each of the four plants was sampled monthly for approximately 1 year. At each sampling, four to six raw fish and four to six finished product samples were collected from corresponding lots. In addition, 12 to 14 environmental sponge samples were collected several hours after the start of production at sites selected as being likely contamination sources. A total of 234 raw fish, 233 finished products, and 553 environmental samples were tested. Presumptive Listeria spp. were isolated from 16.7% of the raw fish samples, 9.0% of the finished product samples, and 27.3% of the environmental samples. L. monocytogenes was isolated from 3.8% of the raw fish samples (0 to 10%, depending on the plant), 1.3% of the finished product samples (0 to 3.3%), and 12.8% of the environmental samples (0 to 29.8%). Among the environmental samples, L. monocytogenes was found in 23.7% of the samples taken from drains, 4.8% of the samples taken from food contact surfaces, 10.4% of the samples taken from employee contact surfaces (aprons, hands, and door handles), and 12.3% of the samples taken from other nonfood contact surfaces. Listeria spp. were isolated from environmental samples in each of the four plants, whereas L. monocytogenes was not found in any of the environmental samples from one plant. Overall, the L. monocytogenes prevalence in the plant environment showed a statistically significant (P < 0.0001) positive relationship with the prevalence of this organism in finished product samples. Automated EcoRI ribotyping differentiated 15 ribotypes among the 83 L. monocytogenes isolates. For each of the three plants with L. monocytogenes-positive environmental samples, one or two ribotypes seemed to persist in the plant environment during the study period. In one plant, a specific L. monocytogenes ribotype represented 44% of the L. monocytogenes-positive environmental samples and was also responsible for one of the two finished product positives found in this plant. In another plant, a specific L. monocytogenes ribotype persisted in the raw fish handling area. However, this ribotype was never isolated from the finished product area in this plant, indicating that this operation has achieved effective separation of raw and finished product areas. Molecular subtyping methods can help identify plant-specific L. monocytogenes contamination routes and thus provide the knowledge needed to implement improved L. monocytogenes control strategies.

Persistent and nonpersistent Listeria monocytogenes contamination in meat and poultry processing plants

Contamination analysis of persistent and nonpersistent Listeria monocytogenes strains in three meat processing plants and one poultry processing plant were performed in order to identify factors predisposing to or sustaining persistent plant contamination. A total of 596 L. monocytogenes isolates were divided into 47 pulsed-field gel electrophoresis (PFGE) types by combining the restriction enzyme patterns of AscI (42 patterns) and ApaI (38 patterns). Persistent and nonpersistent strains were found in all plants. Nonpersistent PFGE types were found mostly at one sampling site, with the processing environment being the most common location, whereas the persistent strains were found at several sampling sites in most cases. The processing machines were frequently contaminated with persistent L. monocytogenes PFGE types, and it was of concern that surfaces having direct contact with the products were contaminated. The role of the processing machines in sustaining contamination and in contaminating the products appeared to be important because the final product of several processing lines was contaminated with the same L. monocytogenes PFGE type as that found in the processing machine. The proportion of persistent PFGE types in heat-treated products was eight times higher than in the raw products, showing the importance of the persistent PFGE types as contaminants of the final heat-treated products. The contamination status of the processing lines and machines appeared to be influenced by the compartmentalization of the processing line, with poor compartmentalization increasing L. monocytogenes contamination. The separation of raw and post-heat treatment areas seemed especially important in the contamination status of post-heat treatment lines.