Occurrence of Campylobacter and Listeria monocytogenes in a poultry processing plant

The occurrence of Campylobacter and Listeria monocytogenes was studied in 645 samples from surfaces, water, and poultry products (chicken carcasses, chicken parts, viscera, and spoils) in a poultry processing plant in southern Brazil. The automated mini-VIDAS system was used to detect the presence of Campylobacter and L. monocytogenes on the samples. The positive samples were confirmed by conventional methods. Campylobacter and L. monocytogenes were found in 16.6 and 35.6% of the analyzed samples, respectively. The sampling points with the highest Campylobacter incidence were intestine (63.3%, 19 of 30 samples), gallbladder (46.7%, 14 of 30), carcasses before evisceration (33.33%, 10 of 30), and carcasses after plucking (30%, 9 of 30). For L. monocytogenes, the majority of positive samples were from frozen breast (100%, 15 of 15 samples), frozen wing (93.3%, 14 of 15), fresh breast (83.3%, 25 of 30), fresh wing (80%, 24 of 30), skin of breast and leg (76.7%, 23 of 30), frozen leg (60%, 9 of 15), and fresh leg (50%, 15 of 30).     

Molecular characterization of Listeria monocytogenes isolated from a poultry further processing facility and from fully cooked product

This study was undertaken to explore environmental sources of Listeria monocytogenes in a commercial chicken further processing facility and to compare the isolates obtained from this facility with others obtained from fully cooked product. In a survey conducted at the processing facility, 40 environmental sites (encompassing two production lines and representing areas in which raw and cooked products are processed) were cultured for L. monocytogenes. The resulting isolates were subjected to molecular subtyping by ribotyping, and these isolates were compared with 25 isolates collected by plant personnel from product contact surfaces and from fully cooked product. Eighty-nine environmental and product isolates were divided into 15 distinct ribogroups. Two ribogroups included isolates from fully cooked product; the members of these two ribogroups were subjected to further analysis by pulsed-field gel electrophoresis, resulting in four clusters. L. monocytogenes isolates from fully cooked product produced on line 1 were found to be indistinguishable from isolates collected from (i) drains on the raw-product side of line 1 and (ii) the floor surface in the cooked-product area of line 1. L. monocytogenes isolates from fully cooked product from line 2 were found to be indistinguishable from isolates collected from (i) the spiral freezer exit conveyor on line 2, (ii) raw product contact surfaces on line 1, and (iii) drains in the cooked-product area of line 1. These data suggest that L. monocytogenes can colonize a poultry further processing facility and eventually be transferred to fully cooked product.     

Mathematical model of Listeria monocytogenes cross-contamination in a fish processing plant

Listeriosis is a foodborne disease caused by the bacterium Listeria monocytogenes. The food industry and government agencies devote considerable resources to reducing contamination of ready-to-eat foods with L. monocytogenes. Because inactivation treatments can effectively eliminate L. monocytogenes present on raw materials, postprocessing cross-contamination from the processing plant environment appears to be responsible for most L. monocytogenes food contamination events. An improved understanding of cross-contamination pathways is critical to preventing L. monocytogenes contamination. Therefore, a plant-specific mathematical model of L. monocytogenes cross-contamination was developed, which described the transmission of L. monocytogenes contamination among food, food contact surfaces, employees' gloves, and the environment. A smoked fish processing plant was used as a model system. The model estimated that 10.7% (5th and 95th percentile, 0.05% and 22.3%, respectively) of food products in a lot are likely to be contaminated with L. monocytogenes. Sensitivity analysis identified the most significant input parameters as the frequency with which employees' gloves contact food and food contact surfaces, and the frequency of changing gloves. Scenario analysis indicated that the greatest reduction of the within-lot prevalence of contaminated food products can be achieved if the raw material entering the plant is free of contamination. Zero contamination of food products in a lot was possible but rare. This model could be used in a risk assessment to quantify the potential public health benefits of in-plant control strategies to reduce cross-contamination.     

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.     

Characterization of Listeria monocytogenes and Listeria innocua from a vegetable processing plant by RAPD and REA

The incidence of Listeria monocytogenes in a vegetable processing plant was investigated over a 23-month period. Frozen ready-to-eat vegetable samples, well as the plant environment, were sampled. The molecular subtyping techniques, Random Amplified Polymorphic DNA (RAPD) and Restriction Endonuclease Analyses (REA), were performed to help investigate the origin and routes of Listeria dissemination.

The low and sporadic incidence of L. monocytogenes made it impossible to establish an epidemiological sequence in the processing plant, though a case of cross-contamination between tomato and ratatouille was detected. Listeria innocua subtyping, however, allowed us to determine the prevalence of several strains in vegetables, and their presence on machinery samples suggested the possibility of cross-contamination during processing.

The low incidence of L. monocytogenes indicated that the risk of listeriosis transmission by vegetable consumption is low. On the other hand, the isolation of the same strain of L. innocua in several surveys pointed out the risk of colonisation on surfaces and machinery. The persistence of Listeria spp. is a cause for concern as can lead to future contamination of vegetables processed in the plant and to a possible increased risk for health. Therefore, periodic controls for the presence of Listeria spp. and a further review of the cleaning and disinfection procedures used in frozen vegetable plants are recommended.     

Prevalence of Listeria monocytogenes in poultry production in France

This study aimed to update and create a data set from laying hens and broilers regarding contamination by Listeria monocytogenes. Two hundred laying-hen flocks were sampled, with 88 flocks reared in cages and 112 reared on the floor. One hundred forty-five broiler flocks were sampled, with 85 conventional and 60 free-range flocks. A total of 774 and 725 samples were analyzed from laying hens and broilers, respectively. L. monocytogenes was detected in 31 of 200 flocks, yielding an estimated prevalence of 15.5% in laying-hen flocks. Among positive flocks, there appeared a significant (P = 0.004) difference between caged and floor-reared hens, with a higher detection in dust samples from floor-reared hens. In positive caged hen flocks, significant (P = 0.028) differences between dust and fecal samples appeared, with a higher detection in feces than in dust samples. In broiler flocks, L. monocytogenes was isolated in 46 of 145 flocks, yielding an estimated prevalence of 32% (28% in conventional flocks versus 37% in the free-range flocks). L. monocytogenes was isolated in samples taken from conventional flocks with a lower frequency than in free-range flocks (13 versus 18%, respectively). The serotyping of L. monocytogenes strains showed that the majority belonged to type 1/2a in laying-hen flocks (74.3%) and in broiler flocks (40.5%). A significant difference (P = 0.007) between laying hens and broilers was shown for serogroup 4 and for serovar 1/2b (P = 0.007); these serogroups were more prevalent in broilers (40%) than in laying hens (5.7%).     

Control of Listeria monocytogenes growth in a ready-to-eat poultry product using a bacteriophage

A bacteriophage (phage) that infected strains of the species Listeria monocytogenes as well as Listeria ivanovii and Listeria welshimeri, but not Listeria grayi or Listeria innocua, was isolated from sheep faeces. The phage had a contractile tail and an icosohedral head indicating that it was a myovirus, and was morphologically similar to phage A511. At 30 °C, phages added at 5.2 × 10⁷ PFU ml⁻¹ prevented the growth in broth of L. monocytogenes present at approximately twice this concentration for 7 h, but re-growth occurred such that the concentration after 24 h incubation was similar in both control and phage-treated cultures. At the same temperature, but on the surface of vacuum-packed ready-to-eat chicken breast roll, there was an immediate 2.5 log₁₀ CFU cm⁻² reduction in pathogen concentration following addition of phages and then re-growth. However, at a temperature reflecting that at which a chilled food might be held (5°C), this re-growth was prevented over 21 days incubation. The data suggest a dose-dependent rapid reduction in pathogen concentration followed by no continued phage-mediated effect. These results, alongside other published data, indicate that a high concentration of phages per unit area is required to ensure significant inactivation of target pathogens on food surfaces.     

Incidence and principal sources of Listeria spp. and Listeria monocytogenes contamination in processed meats and a meat processing plant

The occurrence and distribution of listeriae in a meat processing plant was studied to determine the major sources and routes of contamination. Listeria monocytogenes and other Listeria spp. were isolated from 51% and 49% of samples of frozen raw meat taken from several incoming lots. Turkey necks and breasts, pork trimmings and lard were the principal sources of initial contamination. As a consequence, listeriae colonized certain processing sites where raw materials were handled and hygienic conditions were not strict. Mainly tumbled meats were contaminated heavily during tumbling as the need to operate tumblers continuously did not enable their proper cleaning and disinfection on a daily basis. Also the use of mechanically deboned turkey-neck meat in cooked sausages raised contamination at a pre-cooking stage. Listeriae survived in tumbled meats cooked in boilers at core temperatures below 70°C, and in country-style sausages heated to 65–68°C. In contrast, listeriae were killed in oven-cooked tumbled meats and emulsion-type sausages heated to 72–75°C, and in fully ripened salamis. Heat survivors appeared to be the main cause of post-process contamination as spreading of listeriae in the cutting room was restricted to processing lines where precontaminated meat products were handled. The possible reasons leading to heat survival of listeriae and the measures taken to control the problem were discussed.     

Distribution of Listeria monocytogenes molecular subtypes among human and food isolates from New York State shows persistence of human disease--associated Listeria monocytogenes strains in retail environments

While there is considerable information available regarding Listeria monocytogenes contamination patterns in food processing plants, our understanding of L. monocytogenes contamination and transmission in retail operations is limited. We characterized 125 food, 40 environmental, and 342 human clinical L. monocytogenes isolates collected in New York State from 1997 to 2002 using automated ribotyping and hly allelic variation. All environmental isolates were obtained from retail establishments and the majority of food isolates (98 isolates) were obtained from foods that were prepared or handled at retail. Overall, food and/or environmental isolates from 50 different retail establishments were characterized. The 125 food and 40 environmental isolates were differentiated into 29 and 10 ribotypes, respectively. For 16 retail establishments, we found evidence for persistence of one or more specific L. monocytogenes strains as indicated by isolation of the same EcoRI ribotype from food or environmental samples collected in a given establishment on different days. The human isolates were differentiated into 48 ribotypes. Statistical analyses showed that two ribotypes were significantly (P < 0.0001) more common among food isolates as compared with human isolates. However, a total of 17 ribotypes found among the human clinical isolates were also found among the food and environmental isolates. We conclude that L. monocytogenes, including subtypes that have been linked to human disease, can persist in retail environments. Implementation of Listeria control procedures in retail operations, which process and handle products that permit the growth of L. monocytogenes, are thus a critical component of a farm-to-table L. monocytogenes control program.     

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.     

A processing plant persistent strain of Listeria monocytogenes crosses the fetoplacental barrier in a pregnant guinea pig model

The foodborne pathogen Listeria monocytogenes can cause infection in immunocompromised humans and in the fetuses of pregnant women. We have demonstrated that one group of genetically similar L. monocytogenes strains (random amplified polymorphic DNA [RAPD] type 9) dominate and persist in several independent fish processing plants. The purpose of the present study was to determine the virulence potential of one RAPD type 9 strain (La111), one human clinical strain (Scott A), and one monkey clinical strain (12443) in a pregnant guinea pig model. Animals were orally exposed to 10(8) CFU of L. monocytogenes in whipping cream on gestation day (GD) 36 and euthanized on GD 42, 45, or 56. Strains 12443 and Scott A were shed from treated animals for 20 days, whereas La111 was shed only in the first 10 days. Strains 12443 and Scott A were recovered from maternal liver, spleen, and gallbladder on all 3 days of euthanization, whereas La111 was recovered only at GD 45 and 56. Scott A was not isolated from any placentas or fetuses. For dams treated with 12443, 22% of the fetuses were positive for L. monocytogenes, and surprisingly, treatment of dams with La111 resulted in 56% infected fetuses. L. monocytogenes was isolated from 16 and 20% of placentas for 12443 and La111, respectively. The study demonstrates that a food processing plant persistent strain of L. monocytogenes is able to cross the fetoplacental barrier in pregnant guinea pigs. Furthermore, we demonstrate that although information can be gained from model virulence assays, assessment of the virulence potential of a strain may require more complex hosts.     

Plasmid profiles of Listeria species associated with poultry processing

Since Listeria is frequently associated with poultry products, a study was undertaken to determine sources and prevalence of Listeria species and plasmid profile types in a South African poultry processing plant. Listeria were isolated by a standard enrichment procedure from chicken carcass neck skins sampled at six points during processing and from swabs of two equipment surfaces. The survey was repeated five times at monthly intervals (July-November). Isolates from Listeria positive samples were identified to species level and plasmid profiles of each isolate were determined. Listeria innocua was detected on the rubber fingers of the plucking machines and all neck skin samples after evisceration. Listeria monocytogenes strains were detected on the rubber fingers, packaging funnel and all neck skins after spin chilling. Of the Listeria species isolated, 77·2% corresponded to 1 of 2 different plasmid profile types which were distributed throughout all the Listeria positive sampling points. The frequency of occurrence of both species varied from month to month, but both predominant plasmid profile types occurred whenever Listeria positive samples were found. It was concluded that Listeria contamination of carcasses in the plant was due to faecal or gut contamination from the chickens or the presence of endemic Listeria strains.     

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.     

加工环境中单核细胞增生性李斯特菌耐药特征分析

目的分析在流通过程中加工环境来源的食源性李斯特菌耐药性及耐药基因携带情况,探讨加工环境对该菌的影响。方法收集2016~2017年分离自加工环境中的71株单核细胞增生性李斯特菌,利用微量肉汤稀释法对8种抗菌药物进行药物敏感性分析,并采用PCR鉴定11种耐药基因(tet A、tet M、tet S、erm A、erm B、erm C、mec A、aac(6’)-Ib、van A、van B、cfr)。结果 71株Lm对庆大霉素(11.27%)、氨苄西林(4.22%)、头孢他啶(98.59%)、环丙沙星(97.18%)、四环素(21.12%)、红霉素(15.49%)、林可霉素(92.96%)均具有不同程度的耐药性,对万古霉素敏感。其中以耐受3~6种抗菌药为主,耐药情况较为严重。耐药基因检测表明四环素类tet A,tet M为主要耐药基因,检出率高达50%以上,其次为红霉素类erm A、erm B、erm C,检出率约11%,氨基糖苷类aac(6’)-Ib的检出率为5.63%。结论加工环境单核细胞增生性李斯特菌的耐药性呈上升趋势,其耐药表型与耐药基因型并不完全一致,从而表明加工环境中存在影响单核细胞增生性李斯特菌耐药性因素,同时为食源性李斯特菌病的防治及预警提供参考依据。     

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.     

Prevalence of Listeria monocytogenes subtypes in bulk milk of the Pacific Northwest

The prevalence of Listeria monocytogenes in bulk milk from three Pacific Northwest states was assessed for 474 herds at three time points. For samples collected in November 2000 and June 2001, the L. monocytogenes prevalence levels were 4.9 and 7.0%, respectively. All isolates were subtyped by serotyping and by pulsed-field gel electrophoresis (PFGE). Forty-nine of the 55 isolates belonged to serogroup 1/2a, while 6 belonged to serogroup 4. Subtyping by PFGE revealed that isolates from 31 herds shared 10 patterns; there was a weak but significant association between PFGE subtype and geographical distance. Six herds were positive for L. monocytogenes at both time points. Of these six herds, four had indistinguishable PFGE patterns at both time points. Twenty-five of the 33 herds that were positive in June 2001 were sampled again in June 2002. L. monocytogenes was recovered from 17 of these 25 herds (68%), with the ApaI restriction enzyme digestion profiles (REDP) for 8 herds being identical to those of isolates recovered from these herds the previous year. The ApaI REDP for the bulk milk isolates were compared with those for isolates recovered from environmental and human samples that were collected by the Washington Department of Health (n = 23). Analysis of ApaI digestion profiles revealed that only two of the Washington Department of Health isolates had digestion profiles similar to those for isolates from bulk milk; however, further analysis with the use of a second enzyme (AscI) was capable of discriminating between isolates from the two sources. Thus, we found no direct REDP matches between bulk milk and clinical isolates.     

Incidence of Listeria monocytogenes in two milk processing environments, and assessment of Listeria monocytogenes blood agar for isolation

A year-long survey of two Northern Ireland milk processing plants for Listeria monocytogenes was carried out. Sample sites included the milk processing environment (walls, floors, drains, and steps), processing equipment, raw and pasteurised milk. The FDA listeria-selective enrichment procedure was used to process samples and an additional agar medium, L. monocytogenes Blood Agar (LMBA), was utilized as part of the isolation procedure in order to compare its performance to that of the recommended Oxford and Palcam agars. LMBA proved to be a very useful tool and was able to detect L. monocytogenes from 94.1% of sites compared to the 76.5% and 79.4% detection rate displayed by Oxford and Palcam agars, respectively. The overall incidence of listeria on equipment was 18.8% (6.3% L. monocytogenes), in the environment was 54.7% (40.6% L. monocytogenes) and in raw milk 44.4% (22.2% L. monocytogenes). On one occasion, L. welshimeri was isolated from pasteurised milk, probably demonstrating post-pasteurisation contamination of product. The main environmental sources of L. monocytogenes were considered to be a floor drain and stainless steel steps.     

Prevalence of Listeria monocytogenes in broilers at the abattoir, processing plant, and retail level.

The environment and products from two broiler abattoirs and processing plants and raw broiler pieces at the retail level were sampled for Listeria monocytogenes in order to evaluate the contamination level of the broiler carcasses and products. Sampling started in the slaughtering process and finished with raw broiler meat or ready-to-eat cooked product. Sampling sites positive for L. monocytogenes at the broiler abattoir were the air chiller, the skin-removing machine, and the conveyor belt leading to the packaging area. The L monocytogenes contamination rate varied from 1 to 19% between the two plants studied. Furthermore, 62% (38 of 61) of the raw broiler pieces, bought from retail stores, were positive for L. monocytogenes. Altogether, 136 L. monocytogenes isolates were obtained for serotyping and pulsed-field gel electrophoresis (PFGE) characterization performed with two rare-cutting enzymes (ApaI and AscI). Altogether three serotypes (1/2a, 1/2c, and 4b) and 14 different PFGE types were obtained using information provided from both ApaI and AscI patterns for discrimination basis. The two broiler abattoirs studied did not share the same PFGE types. However, the same PFGE types found in the raw broiler pieces at the retail level were also found in the broiler abattoirs where the broilers had been slaughtered.     

Listeria monocytogenes in Gorgonzola: subtypes, diversity and persistence over time

L. monocytogenes represents a primary concern in the production of Gorgonzola, a Protected Designation of Origin (PDO) Italian blue-veined cheese produced only in the Piedmont and Lombardy regions. L. monocytogenes isolates (N=95) obtained from Gorgonzola rinds, paste, and production/ripening environments were serotyped and then genotyped using Pulsed Field Gel Electrophoresis (PFGE). The goal of this study was to investigate the variability of L. monocytogenes PFGE-types across different PDO Gorgonzola manufacturers (N=22). The majority of the strains (88%) were serotyped as 1/2a. PFGE identified 2 major pulse-types grouping 62 strains, detected from different plants and years, suggesting the presence of persistent and niche-adapted L. monocytogenes. In 9 plants, environmental strains shared the same pulse-types with strains from rinds or paste, suggesting a possible transmission pathway. Encouragingly, L. monocytogenes was retrieved from only 1 paste, indicating that production processes were under control in 21 plants. In the remaining plant, un-effective pasteurization or cross-contamination during production processes could be the cause of the contamination. Consequently, it is imperative that producers operate under the total respect of the Good Manufacturing Practices and following the principles of the Hazard Analysis Critical Control Point plans, in order to contain contamination throughout the whole processing.