Low prevalence of Listeria monocytogenes in foods from Italy

Listeria monocytogenes is an important foodborne pathogen that causes gastrointestinal disorders, and, especially in immunocompromised people, serious extraintestinal diseases, such as septicemia and meningitis, as well as abortion in pregnant women. Many foods, from both plant and animal origin, have been involved in listeriosis outbreaks. This article reports the results of a 12-year survey (1993 through 2004) on the presence of L. monocytogenes in several kinds of food marketed in Italy. Of 5,788 analyzed samples, 121 (2.1%) were contaminated with L. monocytogenes. The highest prevalence was found in smoked salmon (10.6%) and in poultry meat samples (8.5%) and the lowest in red meat (0.3%). L. monocytogenes was not found in 154 samples of fresh seafood products. Fifty-two isolates were also serotyped by the agglutination method. The most common serotypes detected in the 52 strains tested were 1/2a (36.5%), followed by 1/2c (32.8%), 1/2b (13.5%), 4b (11.5%), 3a (3.8%), and 3b (1.9%). The results of the present study showed low levels of L. monocytogenes in the analyzed samples. A total of 61.5% of the 52 L. monocytogenes strains analyzed belonged to serotypes 1/2a, 1/2b, and 4b, namely the serovars that are most commonly involved in extraintestinal human listeriosis outbreaks. In the ready-to-eat samples, these three serotypes were 40.0% (1/2a), 17.1% (1/2b), and 14.3% (4b). This finding highlights the need to implement strict hygienic measures during the production, distribution, and sale of foods to reduce the risk of foodborne listeriosis in humans to an acceptable level.     

Low prevalence of Listeria monocytogenes in cull sows and pork

The goal of this study was to determine the prevalence of Listeria monocytogenes in sows slaughtered at a single Midwestern plant on two occasions (trial 1, n = 179 sows; trial 2, n = 160 sows). Fecal samples collected antemortem (trial 1) as well as animal tissues, and carcass swabs collected at the abattoir (trials 1 and 2) were analyzed. Eight isolates of L. monocytogenes were recovered from five samples that represented 0.18% of the total samples (n = 2,775). In trial 1, L. monocytogenes was detected in a tonsil sample (0.6%; 1 positive of 181 tonsils), in a carcass (0.6%; 1 positive of 179 carcasses), which was sampled prior to the organic rinse, and in two chopped meat block samples (1.2%; 2 positive of 165 samples). In trial 2, L. monocytogenes was only detected in a single chopped meat block sample (0.15%; 1 positive of 688 total samples). These data indicate the low prevalence of L. monocytogenes in the cull sow.     

Listeria prevalence and Listeria monocytogenes serovar diversity at cull cow and bull processing plants in the United States

Listeria monocytogenes, the causative agent of epidemic and sporadic listeriosis, is routinely isolated from many sources, including cattle, yet information on the prevalence of Listeria in beef processing plants in the United States is minimal. From July 2005 through April 2006, four commercial cow and bull processing plants were sampled in the United States to determine the prevalence of Listeria and the serovar diversity of L. monocytogenes. Samples were collected during the summer, fall, winter, and spring. Listeria prevalence on hides was consistently higher during cooler weather (28 to 92% of samples) than during warmer weather (6 and 77% of samples). The Listeria prevalence data collected from preevisceration carcass ranged from undetectable in some warm season samples to as high as 71% during cooler weather. Listeria on postintervention carcasses in the chill cooler was normally undetectable, with the exception of summer and spring samples from one plant where > 19% of the carcasses were positive for Listeria. On hides, L. monocytogenes serovar 1/2a was the predominant serovar observed, with serovars 1/2b and 4b present 2.5 times less often and serovar 1/2c not detected on any hides sampled. L. monocytogenes serovars 1/2a, 1/2c, and 4b were found on postintervention carcasses. This prevalence study demonstrates that Listeria species are more prevalent on hides during the winter and spring and that interventions being used in cow and bull processing plants appear to be effective in reducing or eliminating Listeria contamination on carcasses.     

Growth of Listeria monocytogenes on vacuum-packaged horsemeat for human consumption

In order to investigate the likelihood of Listeria monocytogenes (serotype 4b, ATCC 19115) growth on vacuum-packaged horsemeat at refrigeration temperature, fourteen horsemeat surface/volume homogeneous 150 g weight pieces were superficially inoculated with serotype 4b L. monocytogenes and vacuum packaged. The samples were stored at 4 ± 1 °C. Two pieces (one for pH determination and one for L. monocytogenes counts) were examined at days 0, 7, 14, 21, 28, 35 and 42. Surface pH did not show significant variations during the experiment. The average L. monocytogenes initial contamination level was 1.77log₁₀ CFU/g. A lag phase of 7 days was recorded. The exponential growth rate between day 7 to day 35 was 0.125log₁₀ CFU/day, corresponding to 3.51log₁₀ CFU/g in 28 days. At the end of the experiment the mean L. monocytogenes log₁₀ CFU/g was 5.78.     

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.     

Listeria monocytogenes in foods in Norway

Three-hundred-and-eighty-two samples of different retail food items in Norway (imported soft cheese, raw chicken, minced meat, fermented sausages, vacuum-packed processed meat products, smoked salmon, peeled shrimps, raw minced fish) and 78 carcass samples (sheep, pig, cattle), were screened for Listeria monocytogenes. Of the 460 samples investigated, 78 were found to contain L. monocytogenes. Five of these contained greater than 10(3) cfu/g, four greater than 10(2) cfu/g, while the remainder were shown to contain L. monocytogenes only after enrichment. L. monocytogenes was isolated most frequently from raw chicken, sporadically from soft cheese, shrimps, processed meat products and smoked salmon, and not at all from carcasses and fermented sausages.     

单核细胞增生李斯特菌的毒力因子

目的单核细胞增生李斯特菌(Listeria monocytogenes,Lm)属于革兰阳性无芽孢杆菌李斯特菌属,主要通过食物传播。Lm的致病性与毒力因子密切相关,研究其毒力因子对认识致病机理有着重要意义。方法对Lm重要的毒力因子(溶血素、磷脂酶、内化素、肌动蛋白、P60蛋白等)进行综述。结果溶血素是一个多功能的毒力因子,对于Lm逃离吞噬细胞囊是必需的。Lm能产生两种磷脂酶C:磷脂酰肌醇磷脂酶C和磷脂酰胆碱磷脂酶C,协助细菌的细胞内复制。肌动蛋白使得Lm在宿主细胞间能够扩散。内化素与Lm的侵袭力有关。P60蛋白是Lm的主要免疫原性抗原。结论Lm毒力因子研究的深入对李斯特菌病的防治将带来深远的影响。     

Acid responses of Listeria monocytogenes

The acid response and the correlated protein synthesis in Listeria monocytogenes were studied. The lowest pH value which L. monocytogenes could resist was dependent on the strain and the kind of acid used. Previous adaptation to an intermediary pH augmented bacterial resistance to a subsequent lethal acidic pH. The acid tolerance was also growth phase dependent. Organic volatile acids exerted a more deleterious effect on L. monocytogenes than inorganic acids, because weak acids infer a lower intracytoplasmic pH. The responses to acid adaptation (mildly acidic pH) and acid stress (lethal acidic pH) shared the majority of acid-induced proteins. The bacteria required more stress proteins to face severe acidic conditions. In order to obtain insights into the role these acid-induced proteins play in the mechanism of acid resistance, the proteins were analysed by two-dimensional electrophoresis and the most abundant were identified by peptide mass fingerprinting and MALDI-TOF mass spectrometry.     

Heat resistance of Listeria monocytogenes

The heat resistance data on Listeria monocytogenes in culture media and foods are summarized. Most heat resistance data for foods have been obtained in dairy, meat, poultry, and egg products. Limited data have been published on seafood, fruits, and vegetables. The methodologies employed have evolved over time; hence data from earlier experiments are not directly comparable to more recent studies. Many factors influence the heat resistance of L. monocytogenes. Variation exists among different strains in their ability to withstand heat treatment. In addition, heat resistance is influenced by age of the culture, growth conditions, recovery media, and characteristics of foods such as salt content, a(w), acidity, and the presence of other inhibitors. Listeriae are more heat resistant than most other nonspore-forming foodborne pathogens, and thus, processing recommendations based on data from experiments with Salmonella spp. or pathogenic Escherichia coli may not be sufficient to eliminate similar numbers of L. monocytogenes. The data provided in this review may prove useful for food processors in determining appropriate times and temperatures for producing foods free of vegetative pathogens.     

PCR-焦磷酸测序检测单增李斯特氏菌研究

摘要 目的：建立结合PCR-焦磷酸测序检测单核细胞增生李斯特氏菌( Listeria monocytogenes, LMO)的方法。方法：根据LMO的hly基因设计扩增引物和测序引物,特异地扩增目的片段,再制备单链模版在测序引物引导下进行焦磷酸测序,测序结果与GenBank中的hly基因序列比对判断LMO。结果：扩增引物和测序引物表现出良好的特异性,16株LMO均扩增出大小249bp的DNA片段,焦磷酸测序结果与hly基因序列100%匹配,而非LMO对照菌株未扩增出DNA条带,焦磷酸测序结果阴性。结论：建立的方法特异性高,是快速从DNA序列水平上检测LMO的有效手段。     

食品中单核细胞增生李斯特氏菌调查及毒力研究

为了解和掌握各类食品中单核细胞增生李斯特氏菌（Lm)污染状况及菌株的毒力情况,采集5类265件食品,菌株鉴定应用API细胞鉴定系统;毒力测定采用溶血实验、多聚酶链反应（PCR）以及小鼠致病力实验的方法进行,结果显示Lm检出率为4.90%;其中从生肉、熟肉、灌汤和速冻食品中的检出率分别为15.00%、2.48%、9.52%;从熟肉和速冻食品中还检出其它李斯特氏菌,检出率分别为4.96%、5.92%;乳制品和水产品中未检出Lm。13株Lm毒力测定显示,溶血实验与小鼠的致病力和PCR测定的溶血素基因无必然联系,而溶血素基因与内化素基因阳性结果相同,Lm对多种抗生素敏感,其中对氨氨苄青霉素、头孢唑啉和环丙沙星敏感率为100%。     

Aerosol studies with Listeria innocua and Listeria monocytogenes

Aerosol studies of Listeria monocytogenes in food processing plants have been limited by lack of a suitable surrogate microorganism. The objective of this study was to investigate the potential of using green fluorescent protein-labeled strains of Listeria innocua as a surrogate for L. monocytogenes for aerosol studies. These studies were conducted in a laboratory bioaerosol chamber and a pilot food-processing facility. Four strains of L. innocua and five strains of L. monocytogenes were used. In the laboratory chamber study, Listeria cells were released into the environment at two different cell numbers and under two airflow conditions. Trypticase soy agar (TSA) plates and oven-roasted breasts of chicken and turkey were placed in the chamber to monitor Listeria cell numbers deposited from aerosols. A similar experimental design was used in the pilot plant study; however, only L. innocua was used. Results showed that L. monocytogenes and L. innocua survived equally well on chicken and turkey breast meats and TSA plates. No-fan and continuous fan applications, which affected airflow, had no significant effect on settling rates of aerosolized L. monocytogenes and L. innocua in the bioaerosol chamber or L. innocua in the pilot plant study. Listeriae cell numbers in the air decreased rapidly during the first 1.5 h following release, with few to no listeriae detected in the air at 3 h. Aerosol particles with diameters of 1 and 2 microM correlated directly with the number of Listeria cells in the aerosol but not with particles that were 0.3, 0.5, and 5 microM in diameter. Results indicate that L. innocua can be used as a surrogate for L. monocytogenes in an aerosol study.     

A cross-sectional study on the prevalence of Listeria monocytogenes and Salmonella in New York dairy herds

As part of our long-term objective of assessing risk for Listeria monocytogenes and Salmonella spp. in dairy herds, we carried out a cross-sectional study to determine the prevalence of the two organisms. The study population consisted of a sample of dairy herds enrolled in the Quality Milk Promotion Services at Cornell during the period of April 1998 to March 1999. The sample was stratified by geographical region to assure representation. Four hundred and four dairy farms were enrolled in the study. In-line milk filters were collected from each farm for bacteriological examination of L. monocytogenes and Salmonella spp. Four hypothesized risk factors were evaluated for their association with the likelihood of the presence of each of the two organisms using logistic regression analysis. Listeria monocytogenes was isolated from 51 (12.6%) of the milk filters. We found region-specific differences in the rate of farms with positive milk filters for this pathogen. Salmonella spp. were isolated from 6 (1.5%) milk filters. One isolate was confirmed as Salmonella enterica Serotype Typhimurium DT 104. There was no significant association between any of the hypothetical risk factors and the likelihood of Salmonella spp. isolation. Our study demonstrated that both L. monocytogenes and Salmonella spp. were prevalent in milk filters in New York dairy herds and that Salmonella was isolated at a significantly lower rate then L. monocytogenes.     

Reduced detectability of Listeria monocytogenes in the presence of Listeria innocua

Recent foodborne crises have demonstrated the importance of monitoring food safety. In terms of microbiological criteria, food safety requires the reliable detection of pathogens such as Listeria monocytogenes along the food chain by appropriate analytical methods. However, indications exist that accompanying Listeria innocua strains suppress the growth of L. monocytogenes during selective enrichment, which may cause reduced or even inhibited detection. To study these effects, the limit of detection of L. monocytogenes was investigated in the presence of L. innocua using the International Organization for Standardization standard method ISO 11290-1 and the VIDAS LDUO system, an automated method based on enzyme-linked fluorescence technology. The challenge was to provide low initial Listeria concentrations at sufficient precision to quantify the influence on the probability of detection of L. monocytogenes. The application of reference materials appropriate for quantitative test methods and a standardized dilution procedure were necessary to ensure accurate CFU levels of defined proportions of mixtures of both Listeria species. During selective enrichment, overgrowth of L. monocytogenes by L. innocua could be confirmed, leading to high rates of false-negative results. Moreover, with both methods, a significant decrease in the detectability of L. monocytogenes could be quantified at ratios of 2:1 at very low concentrations representative of natural contamination levels often found in foods and environments. It is concluded that there is a need to improve existing procedures with respect to selective enrichment, as well as the detection techniques.     

食品中单核细胞增生李斯特氏菌的风险评估

微生物风险评估主要评估食品中的微生物性病原可能对人群引起的潜在危害,以指导风险管理者制定相应的管理措施。单核细胞增生李斯特氏菌是一种重要的食源性致病菌,由于该菌引发的疾病致死率较高,且其暴发常出现于工业加工食品中而引起了世界范围内的广泛关注。JEMRA及美国FDA/FSIS分别对即食食品中单核细胞增生李斯特氏菌进行了定量评估并发布了完整的评估报告。各国也有对本国特定食品中单核细胞增生李斯特氏菌进行评估的文献报道。但在数据不足的情况下,也可通过定性/半定量的风险分级工具对不同来源的风险进行分级,并确定优先性。由于各国人群消费模式、消费量的不同,以及食品制作和处理方法上的差异会对暴露评估的结果产生影响,从而影响每份食品风险的大小,因此,各国有必要根据本国的情况对特定食品中的单核细胞增生李斯特氏菌进行相应的评估。     

Survival of Listeria monocytogenes in experimental chorizos

Chorizos-Mexican-style raw-meat sausages-are a concern in California because their production in small ethnic food markets is unregulated. Their formulation may cause them to appear cooked to the consumer, who may eat the raw sausage without prior proper cooking. Bacterial pathogens in such products may cause illness or even death. Survivability of Listeria monocytogenes in chorizos was evaluated under different storage conditions selected on the basis of an initial survey of uninspected chorizos in California. Sausages were formulated to five different initial water activity (aw) levels (0.85, 0.90, 0.93, 0.95, 0.97), stored under four conditions (refrigeration, "Ref," 6 to 8 degrees C under convective air circulation; room temperature, "RT," 24 to 26 degrees C under convective air circulation; hood, "Hd," 24 to 26 degrees C under forced air circulation; and incubation, "Inc," 30 to 31 degrees C under convective air circulation), and sampled after 1, 2, 4, and 7 days. The initial pH was 4.8 and remained near 5.0 from day 1 of the sampling period. An inoculated-pack study using a five-strain cocktail of L. monocytogenes was performed twice for each initial aw. Results indicated that the three lowest initial aw levels (0.85, 0.90, 0.93) and the Hd and Inc storage conditions were more effective (P < or = 0.05) at reducing L. monocytogenes levels in chorizos than the two highest initial aw levels (0.95 and 0.97) and the Ref storage condition, irrespective of storage time. These results can provide a scientific basis for guidelines given to uninspected chorizo producers in California and reduce the risk of foodborne illness.     

Survival of Listeria monocytogenes in ground beef

Listeria monocytogenes, due to its association with animals and animal products and its proven pathogenicity, is an organism of potential importance to the meat industry. The survival of L. monocytogenes in ground beef held at 4 degrees C for 2 weeks was investigated. The ground beef was inoculated with L. monocytogenes type 1 or type 4 at a level of 5 x 10(5) to 7 x 10(6) CFU/g and then packaged in either oxygen-permeable or oxygen-impermeable bags. Packages were sampled at random at 0, 2, 3, 5, 7, 11, and 14 days post-inoculation, and assayed for L. monocytogenes counts and pH. The number of L. monocytogenes in ground beef remained constant throughout the sampling period, and survival was not affected by package permeability to oxygen. Listeriae were not isolated from the control ground beef. The pH of the meat increased slightly during storage, but was always in the range of pH 5.6 to 5.9. It appears that L. monocytogenes in ground beef can survive without any substantial increase or decrease in viable cell population during refrigerated storage for 14 days.     

Thermal resistance of Listeria monocytogenes in foods

Recent studies on the heat resistance of Listeria monocytogenes in dairy products, the discrepancies in results and some of the difficulties involved in comparing interlaboratory experiments are reviewed. In addition, some current work on the thermal resistance of the organism in meat products is discussed.     

Identification of Listeria innocua surrogates for Listeria monocytogenes in hamburger patties

ABSTRACT:  Listeria innocua M1 has been used by many researchers as a nonpathogenic thermal processing surrogate for Listeria monocytogenes . However, L. innocua M1 has been criticized because its thermal survivability characteristics are not as closely parallel to L. monocytogenes as some would like in a variety of foods and processing conditions. The present study was conducted to compare multiple L. innocua and L. monocytogenes strains to validate L. innocua M1 as the ideal surrogate under high-temperature thermal processing conditions for L. monocytogenes . The D - and z -values of L. innocua M1, L. innocua strain SLCC 5639 serotype (6a), SLCC 5640 (6b), SLCC 2745 (4ab), and L. monocytogenes F4243 (4b) were calculated for raw hamburger patties. Hamburger patties were inoculated with 10^7–8 CFU/g of L. monocytogenes or L. innocua . Samples were heat treated at 4 temperatures (62.5 to 70 °C). At each temperature, the decimal reduction time ( D -value) was obtained by linear regression of survival curves. The D - and z -values were determined for each bacterium. The D -values of L. innocua and L. monocytogenes serotypes ranged from 3.17 to 0.13 min at 62.5 to 70 °C, and the z -values of L. innocua and L. monocytogenes were 7.44 to 7.73 °C. Two of the 4 L. innocua serotypes used in this experiment have the potential for use as surrogates in hamburger meat with varying margins of safety. L. innocua M1 should serve as the primary nonpathogenic surrogate with the greatest margin of safety in verifying a new thermal process to destroy L. monocytogenes .