食用农产品中单核细胞增生李斯特菌的污染状况与血清组分型

单核细胞增生李斯特菌是致死率较高的一类重要食源性致病菌。本研究共采集上海市部分超市、农贸市场及生产基地的4大类(生鲜肉、鸡蛋、牛奶和果蔬)共437份食用农产品样品,按照GB 4789.30-2010方法对其中的单核细胞增生李斯特菌进行分离、鉴定,利用多重PCR方法对收集的分离株进行血清组分型。结果显示,单核细胞增生李斯特菌的总检出率为14.9%(65/437),生鲜肉、鸡蛋、牛奶和果蔬中的污染率依次为27.2%(61/224),3.4%(2/58),2.0%(1/51)和1.0%(1/104);79株单核细胞增生李斯特菌分离株可被分为5个血清组:1/2a或3a(55.7%,n=44),1/2c或3c(21.5%,n=17),1/2b或3b或7(17.7%,n=14),4a或4c(3.8%,n=3)和4b或4d或4e(1.3%,n=1)。上述结果表明:上海地区农产品中存在不同程度的单核细胞增生李斯特菌污染,且部分分离株有较高的致病性,该结果为上海地区单核细胞增生李斯特菌流行病学的风险监测提供了数据支持。     

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%).     

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 and typing of Listeria monocytogenes in raw catfish fillets

Raw channel catfish fillets collected from three processing plants during four time periods were tested for the presence of Listeria species. Listeria monocytogenes was the predominant Listeria species found in these catfish fillets, with 25 to 47% prevalence. Other Listeria species, such as L. welshimeri, L. innocua, L. ivanovii, L. grayi, and L. seeligeri, were also found. L. monocytogenes isolates were further fingerprinted by a repetitive element PCR. Forty distinctive electrophoretic types (ETs) and three genetic clusters were determined by Dice coefficient analysis and UPGMA (unweighted pair group method using arithmetic averages). Twenty of 40 ETs were represented by a single isolate, and the other 20 ETs were represented by 2 to 11 isolates. Thirty-five ETs, represented by 76 isolates, were found in processing plant A, B, or C and designated plant-specific types. The remaining five ETs, represented by 21 isolates, were found in multiple plants and designated nonplant-specific types. In addition, 10 ETs from 52 isolates were found repeatedly during different seasons. Plant-specific and nonplant-specific L. monocytogenes coexisted in processed catfish fillets. Some isolates were persistently found in processed fillets, suggesting that either the current sanitation procedures used by these plants are inadequate or that these isolates originated from the natural habitats of the catfish. The results also suggest that the repetitive element PCR is a useful tool for differentiating L. monocytogenes subtypes and can be used for tracing the source of a contamination.     

Molecular characterization of Listeria monocytogenes from natural and urban environments

Characterization of 80 Listeria monocytogenes isolates from urban and natural environments differentiated 7 and 26 EcoRI ribotypes, respectively. Whereas the majority of isolates from the natural environment represented L. monocytogenes lineage II (12 of 13 isolates), urban isolates grouped evenly into lineages I and II (32 and 33 isolates, respectively) and included two lineage III isolates. Multilocus sequence typing of all natural isolates and a randomly selected subset of 30 urban isolates showed a higher overall diversity (Simpson index of discrimination [D] of 0.987 and 0.920, respectively) than did EcoRI ribotyping (D = 0.872 and 0.911, respectively). Combined analysis with ribotype and lineage data for 414 isolates from farm sources, 165 isolates from foods and food-processing environments, and 342 human clinical isolates revealed that lineage I was significantly more common among human (P < 0.0001) isolates, whereas lineage II was more common among isolates from the natural environment, farms, and foods (P < or = 0.05). Among a total of 92 ribotypes, 31 showed significant associations with specific isolate sources. One ribotype (DUP-1039C) was significantly associated with both natural environments and farms. A spatial analysis showed a marginal association between locations in the natural environment positive for L. monocytogenes and a proximity to farms. Our data indicate that (i) L. monocytogenes strains from different sources show a high level of diversity; (ii) L. monocytogenes subtypes differ significantly in their associations with different environments, even though populations overlap; and (iii) a higher proportion of isolates from environmental sources than from human clinical cases can be classified into L. monocytogenes lineage II, which supports the classification of this lineage as an environmentally adapted subgroup.     

Prevalence and characterization of Listeria monocytogenes isolated from soft cheese

A survey was made in 1995–1996 for Listeria spp. in 63 soft cheeses, made from raw ewe's milk using traditional methods, in the Province of Beira Baixa (Portugal). Listeria spp. were isolated from 47 (75%) of the cheeses, L.monocytogenes was isolated from 29 (46%), and L.innocua but not L.monocytogenes from 18 (29%). Of 24 isolates of L.monocytogenes that were serotyped, 20 were serotype 4b, three were serotype 1/2b and one was serotype 1/2a. Phage typing of isolates of L.monocytogenes and L.innocua showed that in some cases a particular phage type was associated with cheese from a particular source. Twenty four strains of L.monocytogenes tested were able to grow at 30°C in culture medium adjusted with HCl to a pH in the range from 4.4 to 6.0 within 3 days; in the pH range 4.4–6.8 a representative strain grew most rapidly at pH 6.8. The pH range in the cheeses during maturation was between about 5.2–6.4. Whether L.monocytogenes could multiply in the cheeses would depend on factors such as concentration of organic acids and of salt, and storage temperature.     

Reduction and survival of Listeria monocytogenes in ready-to-eat meats after irradiation

A five-strain Listeria monocytogenes culture was inoculated onto six different types of ready-to-eat (RTE) meats (frankfurters, ham, roast beef, bologna, smoked turkey with lactate, and smoked turkey without lactate). The meats were vacuum packed and stored at 4 degrees C for 24 h prior to irradiation. Populations of L. monocytogenes were recovered by surface plating on nonselective and selective media. The margins of safety studied include 3-log (3D) and 5-log (5D) reduction of pathogenic bacteria to achieve an optimal level of reduction while retaining organoleptic qualities of the meats. A 3-log reduction of L. monocytogenes was obtained at 1.5 kGy when nonselective plating medium was used. The dosages for 3-log reduction were 1.5 kGy for bologna, roast beef, and both types of turkey and 2.0 kGy for frankfurters and ham on the basis of use of selective medium. The D10-values ranged from 0.42 to 0.44 kGy. A 5-log reduction of L. monocytogenes was obtained at 2.5 kGy with nonselective medium. With selective medium, the dosages were 2.5 kGy for bologna, roast beef, and both types of turkey and 3.0 kGy for frankfurters and ham. Survival of L. monocytogenes in the same RTE meat types after irradiation was also studied. Meats were inoculated with 5 log L. monocytogenes per g and irradiated at doses of 2.0 and 4.0 kGy. Recovery of the surviving organisms was observed during storage at temperatures of 4 and 10 degrees C for 12 weeks. Preliminary results showed no growth in meats irradiated at 4.0 kGy. Survivors were observed for irradiated meats at 2.0 kGy stored at 10 degrees C after the second week. No growth was observed in samples irradiated at 2.0 kGy stored at 4 degrees C until the fifth week.     

Prevalence and contamination levels of Listeria monocytogenes in retail foods in Japan

Retail foods in Japan were surveyed for the presence and contamination levels of L. monocytogenes. It was isolated from 12.2, 20.6, 37.0 and 25.0% of 41 minced beef, 34 minced pork, 46 minced chicken and 16 minced pork-beef mixture samples, respectively. MPN values were higher than 100/g in five (10.9%) minced chicken samples, but lower than 100/g in all minced beef, pork and pork-beef mixture samples. The organism was also isolated from 5.4% of the 92 smoked salmon samples at MPN values lower than 10/g, and from 3.3% of 213 ready-to-eat raw seafood samples at MPN values from lower than 0.3 to higher than 100/g. None of the 285 vegetable samples were contaminated with L. monocytogenes. These findings indicate that ready-to-eat raw seafoods are relatively high risk among the foods surveyed in this study.     

Prevalence and genetic diversity of Listeria monocytogenes in the tonsils of pigs

This study was set up to establish the prevalence of Listeria monocytogenes in the tonsils of sows and fattening pigs from five Finnish slaughterhouses and to evaluate the genetic similarity of L. monocytogenes strains isolated from the tonsils. A total of 271 pig tonsils (132 tonsils from fattening pigs and 139 from sows) from five different slaughterhouses in various parts of Finland were studied from June 1999 to March 2000. Overall, 14 and 4% of pig tonsils harbored L. monocytogenes and Listeria innocua, respectively. The prevalence of L. monocytogenes in tonsils of fattening pigs (22%) was significantly higher than in sows (6%). The isolates (n = 38) recovered from tonsils showed a wide genetic diversity by means of 24 different pulsed-field gel electrophoresis (PFGE) types presented by the strains. Moreover, in numerical analyses of restriction patterns, no association was found between the clustering of strains and the slaughterhouses, and strains showing a similar PFGE type were recovered from pigs of different slaughterhouses. The high prevalence of L. monocytogenes showing various PFGE types in the tonsils of pigs could indicate a potential source of contamination of pluck sets, carcasses, and the slaughterhouse environment and of subsequent processing steps.     

Characterisation of Danish isolates of Listeria monocytogenes by multilocus enzyme electrophoresis.

A total of 84 strains of Listeria monocytogenes were analysed by multilocus enzyme electrophoresis at twelve enzyme loci. Eight enzyme loci were polymorphic with between 2 and 4 alleles per locus. Fourteen electrophoretic types (ETs) were identified. Among 62 human clinical isolates from Denmark, 8 different ETs were defined. Two ETs, designated ET 1 and ET 6, accounted for 77% of the human clinical isolates investigated. These ETs are identical with those responsible for several epidemics in Switzerland and in the United States. Comparison of 58 isolates of L. monocytogenes, typed by MEE, in relation to phage typing showed that phage typing was more discriminatory than MEE. The ability of MEE to distinguish between phage types of Epi-type and other phage types, however, was almost optimal. MEE typed 23 of 24 strains of Epi-type as belonging to ET 1. In contrast ET 1 was not found in 26 strains with phage types other than the Epi-type.     

Prevalence of Listeria monocytogenes during production and postharvest processing of cabbage

From November 1999 to May 2000, analyses of 425 cabbage, 205 water, and 225 environmental sponge samples from four cabbage farms with packing sheds and from two packing sheds in the Rio Grande Valley and Uvalde, Tex., were conducted to determine whether Listeria monocytogenes was present. Samples were tested by the Food and Drug Administration method for the isolation of Listeria spp., and confirmed isolates were DNA fingerprinted by repetitive-element sequence-based polymerase chain reaction (rep-PCR). L monocytogenes was isolated from 3% (26 of 855) of the samples. Twenty of these isolates were obtained from cabbage (7 isolates from farms and 13 from packing sheds). Three isolates were from water samples(two from farms and one from a packing shed), and three were from environmental sponge samples of packing shed surfaces. Rep-PCR-generated fingerprints of 21 of the isolates revealed 18 distinctive banding patterns. Four isolates from environmental sponge samples of conveyor belts and from cabbage samples shared identical banding patterns, suggesting common sources of contamination. These identical environmental isolates suggest that contact with packing shed surfaces may be a source of contamination of cabbage. However, the cabbage samples could have arrived contaminated, since they were not washed.     

Minimal water activity levels for growth and survival of Listeria monocytogenes and Listeria innocua.

Following initial range-finding experiments, total count determinations were used to determine minimal water activity (aW) levels for the growth and survival of Listeria monocytogenes (L.m.) and L. innocua (L.i.). Media containing three different humectants; NaCl, sucrose or glycerol were used to determine minimal aW levels for growth in the above media which were 0.92, 0.92 and 0.90 respectively. The growth minima for L.i. were similar, or slightly higher than for L.m. in these media. Survival rates were generally lower in NaCl-adjusted media than in systems adjusted with sucrose or glycerol. Survival of L.m. and L.i. in these experiments was similar.     

Effect of temperature and pH on survival of Listeria monocytogenes in coleslaw

The survival and growth of Listeria monocytogenes in fresh coleslaw, pH 3.9, and in coleslaw adjusted to pH 4.0, 5.0, 6.0 or 7.0 before inoculation was studied at three temperatures (4, 15 and 25 degrees C). L. monocytogenes was not detectable after 5 days incubation in fresh coleslaw nor in coleslaw adjusted to pH 4.0. Coleslaw at pH 5.0 was also inhibitory to L. monocytogenes at all three temperatures studied. A decline in viable numbers of L. monocytogenes in coleslaw at pH 6.0 occurred at 4 degrees C and at 15 degrees C, whereas at 25 degrees C the viable count of L. monocytogenes increased initially and remained high after incubation for 25 days. L. monocytogenes grew rapidly in coleslaw at pH 7.0 at all three temperatures studied, followed by an equally rapid decline in viable count.     

Listeria monocytogenes, Yersinia enterocolitica, and Salmonella in dairy plant environments.

In order to determine the presence of the three environmental pathogens in dairy plants, six milk and four ice cream plants in a three state area were sampled. A total of 353 environmental samples were taken over three replications. Bacterial counts were performed on the environmental samples for chi-square analysis. Salmonella spp. were not isolated from any of the environmental samples. Yersinia enterocolitica was isolated from 6.8% of the environmental samples. Listeria monocytogenes was isolated from 6.5% of the environmental samples. Listeria spp. other than L. monocytogenes were isolated from 9.3% of the environmental samples. The presence of Y. enterocolitica was significantly related to high bacterial counts for six microbiological tests. The presence of L. monocytogenes was not related to high bacterial counts.     

Prevalence and genetic characterization of Listeria monocytogenes in retail broiler meat in Estonia

The prevalence and genetic diversity of Listeria monocytogenes in raw broiler legs at the retail level in Estonia were studied. A total of 240 raw broiler legs (120 from Estonia and 120 of foreign origin, which had been imported to Estonia from Denmark, Finland, Hungary, Sweden, and the United States) from 12 retail stores in the two largest cities in Estonia (Tallinn and Tartu) were investigated from January to December 2002. Of these, 70% were positive for L. monocytogenes. The prevalence of L. monocytogenes in broiler legs of Estonian origin (88%) was significantly higher than in broiler legs of foreign origin (53%) (P < 0.001). Altogether, 169 (106 Estonian and 63 imported) L. monocytogenes isolates were characterized by pulsed-field gel electrophoresis (PFGE) typing after treatment with the restriction enzyme AscI. The isolates showed a wide genetic diversity, with 35 different PFGE types obtained. Of these, 11 PFGE types came only from isolates of broiler legs of Estonian origin, 4 of Danish origin, 2 of Finnish origin, and 4 of Hungarian origin. Fourteen PFGE types came from isolates of broiler legs that originated from various countries. The strains that shared the same PFGE types from isolates of Estonian origin were recovered from broiler legs that came from different stores over the course of several months. Seventy-one L. monocytogenes isolates, including all PFGE types, were serotyped, and three serotypes (1/2a, 1/2b, and 4b) were obtained. Serotype 1/2a accounted for 96% of the isolates.     

Prevalence of Listeria monocytogenes and Salmonella in ready-to-eat food in Catalonia, Spain

Listeria monocytogenes and Salmonella are pathogenic bacteria that can contaminate food products during or after processing. Ready-to-eat (RTE) food does not undergo any treatment to ensure its safety before consumption, and therefore risk of foodborne disease must be considered if these pathogens are present in the food. To evaluate the prevalence of these pathogens in RTE food, 140 RTE fish product samples, 501 RTE meat product samples, 462 RTE dairy samples, and 123 RTE dishes and desserts, providing a total of 1,226 samples, were collected from retail stores and food industry and analyzed for the presence of L. monocytogenes. A total of 1,379 samples consisting of 187 RTE fish products and 569 RTE meat products, 484 RTE dairy products, and 139 RTE dishes and desserts were collected and analyzed for the presence of Salmonella. L. monocytogenes was isolated from 20% of frozen Atlantic bonito small pies, 7.9% of smoked salmon samples, 11.1% of the pork luncheon meat samples, 6.2% of frozen chicken croquettes, 16.9% of cured dried sausage samples, 12.5% of cooked ham samples, and 20% of cooked turkey breast samples. L. monocytogenes was also found to be present in 1.3% of fresh salty cheese samples and 15.1% of frozen cannelloni samples. Salmonella was isolated from 1.2% of smoked salmon samples, 1.5% of frozen chicken croquettes, 2% of cooked ham samples, and 11.1% of cured dried sausage samples. Overall, occurrence of these pathogens in RTE foods was similar to that previously reported in the literature.     

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.     

Prevalence of Listeria monocytogenes in 13 dried sausage processing plants and their products

The aims of the present study were: (i) to investigate the occurrence of Listeria monocytogenes in dried sausage processing plants on surfaces before and during processing, (ii) to study the contamination in meat and sausages at different stages of maturation, (iii) to assess the distribution of L. monocytogenes in the different plants and products studied. Thirteen dried sausage processing plants were sampled at two different times of the working day. The studies were repeated twice to evaluate the persistence of the pathogen. A total of 1029 samples were collected. Among swabbed samples, 15% were positive before the beginning of the working day and 47.3% during working day. Results showed that effectiveness of cleaning and disinfecting operations could be linked with the complexity of processing lines and machines used. The presence of L. monocytogenes in mixed meat amounted to 71.6% of the collected samples. A decrease of the contamination rate in dry sausage was noted, particularly during the drying stage. Nevertheless 3 sausages studied presented a low contamination rate (<3 cfu/g) when ready for consumption. A total of 996 strains of L. monocytogenes were characterised by biochemical tests and serotyping. A majority of isolates were 1/2a (49.5%), 1/2c (19.5%) and 1/2b (13%) strains. A high heterogeneity of serotypes was observed in all plants, raw meat and in sausages during maturation.     

Prevalence and level of Listeria monocytogenes and other Listeria species in retail pre-packaged mixed vegetable salads in the UK

As part of the European Commission (EC) co-ordinated programme for 2005, a study of pre-packaged ready-to-eat (RTE) mixed salads containing meat or seafood ingredients from retail premises was undertaken in the UK to determine the frequency and level of Listeria monocytogenes in these products. Almost all (99.8%; 2682/2686) samples were of satisfactory/acceptable microbiological quality. Two (0.1%) samples exceeded EC legal food safety criteria due to the presence of L. monocytogenes in excess of 100 cfu g(-1) (1.7 x 10(2), 9.9 x 10(2)cfu g(-1)) while another two (0.1%) were unsatisfactory due to L. welshimeri levels over 100 cfu g(-1) (1.2 x 10(3), 6.0 x 10(3) cfu g(-1)). Overall contamination of Listeria spp. and L. monocytogenes found in samples of mixed salads in the UK was 10.8% and 4.8%, respectively. Almost twice as many salad samples with meat ingredients were contaminated with Listeria spp. and L. monocytogenes (14.7% and 6.0%, respectively) compared to samples with seafood ingredients (7.4% and 3.8%, respectively). Pre-packaged mixed salads were contaminated with Listeria spp. and L. monocytogenes more frequently when: collected from sandwich shops; not packaged on the premises; stored or displayed above 8 degrees C. This study demonstrates that the control of L. monocytogenes in food manufacturing and at retail sale is essential in order to minimize the potential for this bacterium to be present in mixed salads at the point of consumption at levels hazardous to health.