食源性单增李斯特菌与英诺克李斯特菌基因组特征比较

目的 通过全基因组测序对甘肃省市售食品中分离的单增李斯特菌和英诺克李斯特菌基因组特征进行比较分析。方法 收集2021—2022年甘肃省市售食品中分离的25株单增李斯特菌和7株英诺克李斯特菌作为研究对象,对菌株进行全基因组测序,分析其系统发育谱系、克隆复合群（CC）、序列型（ST）、毒力基因、抗性基因及泛基因组。结果 32株李斯特菌分属单增李斯特菌谱系Ⅰ和Ⅱ及英诺克李斯特菌3个群,单增李斯特菌分为10个亚群,英诺克李斯特菌分为5个亚群,与CC型保持一致,核心基因组多位点序列分型能将各谱系中不同CC型的菌株明显分开,谱系Ⅰ与英诺克李斯特菌的进化关系更近。25株单增李斯特菌均携带李斯特菌毒力岛LIPI-1和内化素基因,不携带LIPI-3,有2株ST87型菌株携带LIPI-4;7株英诺克李斯特菌均不携带LIPI-1和内化素基因,均携带LIPI-4,有5株菌携带LIPI-3。单增李斯特菌有16株携带SSI-1、3株携带SSI-2,7株英诺克李斯特菌均不携带SSI-1,有6株携带SSI-2。李斯特菌的泛基因组大小随着测序基因组数目的增加呈现线性增多,25株单增李斯特菌当菌株数量达到15后核心基因数目稳定在2 272个,占泛基因组基因数目的46.2%,25株单增李斯特菌和7株英诺克李斯特菌共同的核心基因1 487个,当菌株数量达到10后数目趋于稳定。结论 核心基因组多位点序列分型可将不同谱系不同克隆复合群的李斯特菌进行区分,英诺克李斯特菌与单增李斯特菌生化特性相似与其亲缘关系相近有关,致病性差异与英诺克李斯特菌缺失单增李斯特菌特有的毒力基因相关。     

Efficacy of pulsed light for shelf-life extension and inactivation of Listeria monocytogenes on ready-to-eat cooked meat products

Pulsed light (PL) was tested for its utility to improve the microbial quality and safety of ready-to-eat cooked meat products. Vacuum-packaged ham and bologna slices were superficially inoculated with Listeria monocytogenes and treated with 0.7, 2.1, 4.2 and 8.4 J/cm². PL treatment at 8.4 J/cm² reduced L. monocytogenes by 1.78 cfu/cm² in cooked ham and by 1.11 cfu/cm² in bologna. The effect of PL on lipid oxidation and sensory properties was also investigated. The 2-thiobarbituric acid values were very low and chromaticity parameters were within the normal values reported for cooked meat products. PL at 8.4 J/cm² did not affect the sensory quality of cooked ham, while treatments above 2.1 J/cm² negatively influenced the sensory properties of bologna. The combination of PL and vacuum packaging provided ham with an additional shelf-life extension of 30 days compared with only vacuum packaging. The shelf-life of bologna was not extended by PL.

Industrial relevance

The efficacy of pulsed light for the decontamination of surfaces offers excellent possibilities to ensure food safety and to extend shelf-life of ready-to-eat (RTE) products. The results of this study indicate that Listeria monocytogenes can be reduced by approximately 2 log cfu/cm² in RTE cooked ham and 1 log cfu/cm² in bologna using a fluence of 8.4 J/cm². This dose does not affect the sensory properties of ham and triples its shelf-life when compared with conventional RTE products. On the contrary, fluences above 2.1 J/cm² are not suitable for the treatment of bologna since sensory quality is modified.     

单核细胞增生李斯特菌毒力基因及其致病机制的研究进展

单核细胞增生李斯特菌是常见的食源性致病菌,广泛存在于环境中。单核细胞增生李斯特菌感染后主要表现为败血症、脑膜炎和单核细胞增多,也可导致孕妇流产、胎死宫内、新生儿死亡等。单核细胞增生李斯特菌致病性与其毒力基因及毒力岛密切相关,其机制是众多毒力因子在各调控因子复杂的网络调控下的结果。本综述旨在了解单核细胞增生李斯特菌毒力基因及其致病机制。     

食品中单核细胞增生李斯特菌微滴数字PCR定量检测方法建立

目的 建立食品中单核细胞增生李斯特氏菌的微滴式数字聚合酶链式反应（ddPCR）快速定量检测方法。方法 筛选单核细胞增生李斯特氏菌的特异性引物和探针,通过对目标菌纯菌液及人工污染样品的检测,比较ddPCR方法和平板计数法的定值效果,对ddPCR结果进行特异性、灵敏性和重复性分析。结果 本研究建立的单核细胞增生李斯特氏菌ddPCR检测方法具有良好的特异性、灵敏性和重复性。单核细胞增生李斯特氏菌纯菌液中定量限（LOQ）和检出限（LOD）均为136 CFU/mL,在鱿鱼圈和香肠样品中定量限分别为240 CFU/g和155 CFU/g。ddPCR在各梯度水平上变异系数均小于25%,ddPCR和平板计数定值对数值相对偏差均小于30%。结论 本研究建立的ddPCR方法能够快速、准确、灵敏、特异地定量检测食品中单核细胞增生李斯特氏菌。     

中国畜禽源单核细胞增生李斯特菌流行性的系统回顾及荟萃分析

目的 为明确我国畜禽源单核细胞增生李斯特菌（单增李斯特菌）的流行特性,本文通过荟萃（Meta）分析对我国畜禽源食品单增李斯特菌污染与流行情况进行了系统回顾,以期为我国畜禽肉生产过程中单增李斯特菌的防控提供参考依据。方法 首先在PubMed、Web of Science、万方数据库、中国知网等数据平台通过输入关键检索词检索2001—2022年发表的有关我国畜禽食品中单增李斯特菌检出率的文献,之后利用Stata软件对文献数据进行Meta分析。结果 共纳入中、英文研究文献277篇,总样本数据为110 066例,单增李斯特菌的合并检出率为7.2%（95%CI：6.4%~8.0%）。不同区域的亚组分析发现,吉林和黑龙江等地的检出率高于其他地区;不同时间的亚组分析发现单增李斯特菌的检出率随着时间的推移而降低。熟肉食品中单增李斯特菌的检出率（2.9%,95%CI：2.4%~3.5%）远低于生肉（10.7%,95%CI：9.5%~12.0%）;生肉中鸡肉的检出率为11.0%（95%CI：8.3%~14.0%）与猪肉的检出率10.9%（95%CI：7.9%~14.2%）略高于其他种类的生肉。结论 我国畜禽源食品单增李斯特菌检出率依然处于比较高的水平。     

一例孕产妇单核细胞增生李斯特菌感染的溯源调查及发病机制探讨

目的 针对一例孕产妇李斯特菌病开展病例调查和溯源分析,探讨感染来源和单增李斯特菌病的发病机制,为防控李斯特菌病提供依据。方法 开展现场流行病学调查,收集病例信息,采集病例血液标本、家庭冰箱内食品及厨房环境样本、家庭附近农贸市场的食品样本,针对不同来源样本中的单增李斯特菌进行检测。结果 病例经常食用从农贸市场购买的中式凉拌菜（5～7次/周）,在家自制或二次加工中式凉拌菜。其家庭冰箱冷藏室储存食物生熟不分,且生食水果在冰箱中出现腐烂现象。厨房的两块菜板生熟不分、清洗消毒不及时,厨房操作面存在交叉污染。检测结果显示,11份样本中共分离出3株单增李斯特菌,1株来自该病例的血液标本,2株来自厨房冰箱内食品涂抹和菜板涂抹。提示该病例由于食用污染食品导致感染并通过胎盘屏障感染胎儿。结论 本起事件是丰台区首次在食品和环境中尝试溯源单增李斯特菌病的感染来源。病例家庭冰箱内生肉和胡萝卜、厨房环境涂抹样本中均检出单增李斯特菌,明确了食品与环境交叉污染导致病例单增李斯特菌感染发病;医院的早期识别及处置是避免新生儿不良结局出现的重要保障。     

Evaluation of the effect of defrosting practices of ground beef on the heat tolerance of Listeria monocytogenes and Salmonella Enteritidis

The effect of common defrosting practices of ground beef, including (i) defrosting in the refrigerator (5 °C for 15 h), (ii) defrosting at room temperature (25 °C for 12 h) and (iii) defrosting in the microwave, on the heat tolerance of artificially inoculated Listeria monocytogenes and Salmonella Enteritidis, was studied. The thermal inactivation of S. Enteritidis was not, overall, affected by defrosting practices. In contrast, defrosting at room temperature resulted, overall, in an increased heat tolerance of L. monocytogenes compared to the rest tested defrosting practices. Inactivation kinetics of the two pathogens for the different defrosting practices were determined by fitting the data to the Weibull model. The δ parameter of the Weibull model (heat challenge time (min) required for the first 1-log reduction) for S. Enteritidis and for defrosting at 25 °C, microwave defrosting, defrosting at 5 °C and for the control (fresh ground beef inoculated with the pathogens just before the heat challenge trials) was 1.13, 1.62, 1.60 and 0.96, respectively, while the corresponding values for L. monocytogenes were 20.13, 10.82, 9.95 and 9.47, respectively. The findings of this study should be useful in risk assessments and in developing food handling guidelines for the consumers.     

Comparing predicting models for heat inactivation of Listeria monocytogenes and Pseudomonas aeruginosa at different pH

Under the same experimental conditions it has been demonstrated that whereas survival curves of Listeria monocytogenes in the range of temperatures from 54 to 62 °C followed a first-order kinetic, those of Pseudomonas aeruginosa in the range of temperatures from 50 to 56 °C were not linear showing a shoulder followed by a linear region. The first order kinetic model did not describe survival curves of P. aeruginosa. A model based on the Weibull distribution (Log₁₀(N_t/N₀)=(1/−2.303)*(t/b)ⁿ)) accurately described the inactivation kinetics of both microorganisms at the three pHs of 4, 5.5, 7.4 investigated. For both microorganisms, the b value depended on the treatment temperature and the pH of the treatment medium. Whereas for L. monocytogenes the n value was independent of the treatment conditions, for P. aeruginosa the n value depended on the pH of the treatment medium.The model based on the Weibull distribution was capable of accurately predicting the treatment time to inactivate five Log₁₀ cycles of both microorganisms at the three pHs investigated.     

Modelling the growth of Listeria monocytogenes in fresh green coconut (Cocos nucifera L.) water

The behaviour of Listeria monocytogenes in the fresh coconut water stored at 4 °C, 10 °C and 35 °C was studied. The coconut water was aseptically extracted from green coconuts (Cocos nucifera L.) and samples were inoculated in triplicate with a mixture of 5 strains of L. monocytogenes with a mean population of approximately 3 log₁₀ CFU/mL. The kinetic parameters of the bacteria were estimated from the Baranyi model, and compared with predictions of the Pathogen Modelling Program so as to predict its behaviour in the beverage. The results demonstrated that fresh green coconut water was a beverage propitious for the survival and growth of L. monocytogenes and that refrigeration at 10 °C or 4 °C retarded, but did not inhibit, growth of this bacterium. Temperature abuse at 35 °C considerably reduced the lagtimes. The study shows that L. monocytogenes growth in fresh green coconut water is controlled for several days by storage at low temperature, mainly at 4 °C. Thus, for risk population this product should only be drunk directly from the coconut or despite the sensorial alterations should be consumed pasteurized.     

Investigation of Listeria monocytogenes contamination pattern in local Chinese food market and the tracing of two clinical isolates by RAPD analysis

This study was undertaken to investigate the contamination pattern of Listeria monocytogenes in local Chinese food markets and to trace two clinical isolates. Random amplification polymorphic DNA (RAPD) was developed to track the source of L. monocytogenes in ready-to-eat (RTE) food products and from clinical origin. Three random primers, PB1, PB4 and HLWL74, were used to subtype all the L. monocytogenes strains isolated from RTE food products, fresh food products, environmental sewages in the markets and two clinical meningitis patients. It was shown that all the 49 isolates could be classified into 5, 4 and 4 types using these three random primers, PB1, PB4, and HLWL74, respectively. Twenty-seven composite profiles were identified by a combination of the three primers. The same composite profiles of L. monocytogenes could be found both in the fresh food products, environmental sewages and RTE food products, suggesting that the L. monocytogenes in the RTE food products may come from those in the fresh food products or sewage in the same market. The composite profiles of the two clinical isolates were the same as those of strains isolated from RTE food products, indicating that the disease might have resulted from the consumption of the RTE food products contaminated with L. monocytogenes. The results show that RAPD could be a powerful tool for the investigation of contamination pattern of L. monocytogenes in Chinese food markets and also for tracking the source of L. monocytogenes in clinical patients.     

可生食蔬菜中单核细胞增生李斯特菌生存特征研究

目的 探究可生食蔬菜品种、温度、接种部位对单核细胞增生李斯特菌（以下简称单增李斯特菌）存活的影响,为可生食蔬菜中单增李斯特菌的风险评估和关键控制措施提供理论依据。方法 以冻干定量单增李斯特菌为菌株来源,以彩椒、洋葱、黄瓜、圣女果和生菜5种可生食蔬菜的表面和切面为单增李斯特菌的接种点,在4 ℃、25 °C条件下培养7 d,定期监测每份样本中的单增李斯特菌的菌量,对其生长情况进行分析。结果 单增李斯特菌冻干菌种不同瓶间菌量均匀（F=1.923,P<0.05）,-20 ℃储存28 d后的复苏率为93.3%±4.2%。在4 ℃条件下,除了彩椒表面、黄瓜切面、生菜表面和生菜切面外,单增李斯特菌在其他蔬菜上放置7 d后均未见显著生长（δ<0.5 log₁₀ CFU/mL）。在25 ℃条件下,单增李斯特菌在彩椒、洋葱、圣女果、生菜以及黄瓜切面上均呈现为支持生长［δ为（1.16±0.35）~（2.68±0.18）log₁₀ CFU/mL］。单增李斯特菌在黄瓜切面、生菜表面和切面放置7 d后,菌量仍持续增长,在生菜的表面和切面生长趋势和浓度基本一致。结论 单增李斯特菌在可生食蔬菜上的存活能力与蔬菜种类、表面与切面、储存温度等条件密切相关,温度的控制对降低其在可生食蔬菜中的风险至关重要。生菜和切后的黄瓜作为单增李斯特菌高风险食品,应引起风险评估的重视。     

Characterization of Listeria monocytogenes strains isolated from Gorgonzola cheese rinds

Listeria monocytogenes is a foodborne pathogen frequently present in ripened soft cheeses. Forty-three strains of L. monocytogenes isolated from the rind of ripened Gorgonzola cheeses produced in 24 different dairy plants were characterized by biotyping, serotyping, and molecular typing. Biotyping was performed by studying two phenotypes closely associated with virulence, such as hemolytic and phospholipase C activities. Traditional typing techniques did not allow a discrimination among the 43 strains studied. All strains showed a good hemolytic activity on blood agar, and only slight differences were observed when titration of hemolytic activity of culture supernatants was performed. Also phospholipase activities were quite similar for all the strains. Concerning serotyping, all strains belonged to serotype 1/2a. The molecular characterization was performed by RAPD-PCR. Combined cluster analysis following PCR amplification experiments allowed to group L. monocytogenes strains into few distinguishable profiles. At a level of similarity of 80%, the 43 strains were grouped into only 5 composite profile groups. Although isolated in 24 different plants, the presence of a few closely related strains demonstrated a possible relationship between these cheese isolates; a special ability of these strains to adapt to Gorgonzola cheese processing environment could be suggested.     

The occurrence of Listeria monocytogenes in cheese from a manufacturer associated with a case of listeriosis

A case of listeriosis was associated with the consumption of a soft cheese produced in England. Goats cheese and other products from the same food manufacturer were examined for the presence of Listeria over the following 11 months. Listeria monocytogenes was isolated from 16 of 25 cheese samples on retail sale, 12 of 24 cheese samples obtained directly from the factory, and from shelving within the plant. Phage-typing of 68 isolates of L. monocytogenes from cheese samples and the factory showed that 66 (97%) were indistinguishable from the strain isolated from the patient's cerebrospinal fluid and stool. L. monocytogenes was not isolated from seven goats milk or two yoghurt samples. Listeria innocua was isolated from 10 cheese samples, two of which contained no other species of Listeria. Levels of L. monocytogenes shortly after production were low (<10/g), but were higher (10⁵–10⁷ cfu/g) in six of the 16 cheese samples obtained from retail outlets. Multiplication of L. monocytogenes was demonstrated in cheeses contaminated at the factory and held at 4°C in the laboratory.     

Effects of curing sodium nitrite additive and natural meat fat on growth control of Listeria monocytogenes by the bacteriocin-producing Lactobacillus curvatus strain CWBI-B28

In realistic model meat systems, the separate and combined effects of fat content and sodium nitrite on the antilisterial activity of the bacteriocin of Lactobacillus curvatus CWBI-B28 were studied. In laboratory fermentations where Listeria monocytogenes was co-cultured at 4 °C with bacteriocin-producing CWBI-B28 in lean pork meat (fat content: 13%) without added nitrite, a strong antilisterial effect was observed after one week. The effect was maintained for an additional week, after which a slight and very gradual rebound was observed. Both added nitrite (20 ppm) and a high-fat content (43%) were found to antagonise this antilisterial effect, the Listeria cfu count reached after six weeks being 200 times as high in high-fat meat with added nitrite than in lean meat without nitrite. This antagonism could not be attributed to slower growth of the bacteriocin-producing strain, since CWBI-B28 grew optimally in fat-rich meat with 20 ppm sodium nitrite. Bacteriocin activity was also measured in the samples. The observed activity levels are discussed in relation to the degree of antilisterial protection conferred.     

成都市冷藏即食预包装熟肉制品中单核细胞增生李斯特菌定量检测及污染风险研究

目的 为探明了解冷藏即食预包装熟肉制品中单核细胞增生李斯特菌（单增李斯特菌）污染情况及潜在风险,本研究针对短保质期的冷藏即食预包装熟肉制品开展单增李斯特菌定量污染风险调查。方法 2022年5~8月,从成都市采集样品64份,依据采用传统培养分离鉴定和RT-PCR分子检测方法进行单增李斯特菌的定性和定量检测,分离菌株提取DNA后进行二代全基因组测序并开展相关流行病学特征分析。结果 检测结果发现64份即食熟肉样品中,6份样品检出单增李斯特菌,检出份数为6/64。6份阳性样品均由某同一工厂生产,其中5份为卤肉三拼样品。定量检测结果表明,卤肉三拼的单增李斯特菌污染水平为30~200 CFU/g;单增李斯特菌阳性的猪肉样品污染水平为<10 CFU/g。阳性样品中的6株分离菌株经全基因组测序分析鉴定为单增李斯特菌,均含有LIPI-1毒力岛相关基因,多序列位点分型发现主要的序列型（ST）为ST3（n=2）和ST121（n=2）,其次是ST8和ST9。单核苷酸多态性（SNP）结果分析发现同一工厂分离到的单增李斯特菌菌株LM22071911和LM22080802之间检测到23个SNP差异,单增李斯特菌株LM22062706和LM22080806之间检测到13个SNP差异,说明在该工厂加工环节存在两种克隆群的单增李斯特菌直接传播的风险。结论 成都市部分市售冷藏即食预包装熟肉制品污染多种李斯特菌,需加强源头控制。     

Comparative analysis of acid resistance in Listeria monocytogenes and Salmonella enterica strains before and after exposure to poultry decontaminants. Role of the glutamate decarboxylase (GAD) system

Data on the ability of chemical poultry decontaminants to induce an acid stress response in pathogenic bacteria are lacking. This study was undertaken in order to compare the survival rates in acid broths of Listeria monocytogenes and Salmonella enterica strains, both exposed to and not exposed to decontaminants. The contribution of the glutamate decarboxylase (GAD) acid resistance system to the survival of bacteria in acid media was also examined. Four strains (L. monocytogenes serovar 1/2, L. monocytogenes serovar 4b, S. enterica serotype Typhymurium and S. enterica serotype Enteritidis) were tested before (control) and after exposure to trisodium phosphate, acidified sodium chlorite, citric acid, chlorine dioxide and peroxyacids (strains were repeatedly passed through media containing increasing concentrations of a compound). Stationary-phase cells (10⁸ cfu/ml) were inoculated into tryptic soy broth (TSB) acidified with citric acid (pH 2.7 and 5.0) with or without glutamate (10 mM) added, and incubated at 37 °C for 15 min. Survival percentages (calculated from viable colonies) varied from 2.47 ± 0.67% to 91.93 ± 5.83%. L. monocytogenes cells previously exposed to acid decontaminants (citric acid and peroxyacids) showed, when placed in acid TSB, a higher (P < 0.05) percentage of survival (average 38.80 ± 30.52%) than control and pre-exposed to non-acidic decontaminants strains (22.82 ± 23.80%). Similar (P > 0.05) survival percentages were observed in previously exposed to different decontaminants and control Salmonella strains. The GAD acid resistance system did not apparently play any role in the survival of L. monocytogenes or S. enterica at a low pH. This study demonstrates for the first time that prior exposure to acidic poultry decontaminants increases the percentage of survival of L. monocytogenes exposed to severe acid stress. These results have important implications for the meat industry when considering which decontaminant treatment to adopt.     

咸阳市市售食品中单核细胞增生李斯特菌基因组特征及耐药和致病基因分析

目的 采用全基因组测序技术对从咸阳市市售食品中分离的64株单核细胞增生李斯特菌（单增李斯特菌）的基因组特征,耐药和致病性基因进行分析。方法 收集咸阳市市售食品中分离的64株单增李斯特菌,利用微量肉汤法进行药敏测定,同时进行全基因组测序,原始序列经拼接后利用生物信息学软件进行基因组注释、系统发育树构建及基因组特征和遗传原件分析。结果 64株分离株对于氨苄西林、青霉素、美罗培南、复方新诺明、万古霉素5种抗生素结果均为敏感。其中2株分离株对2种抗生素产生抗性,分别为四环素和红霉素。全基因组测序分析表明,64株分离株分属3个谱系,分为15个克隆群（CC）,以谱系Ⅰ和谱系Ⅱ为主;2株耐药株基因型与表型一致,耐药基因上下游遗传环境分析表明,这些基因的可能来源为猪丹毒杆菌、肠球菌和外源质粒。所有分离株均携带致病基因岛LIPI-1和LIPI-2,部分谱系Ⅰ菌株携带LIPI-3或LIPI-4,具有潜在致病风险。携带质粒和抗性相关基因的主要为谱系Ⅱ菌株。inlA基因提前终止突变均发生在谱系Ⅱ,可能降低菌株毒力。结论 咸阳市市售食品中单增李斯特菌基因组结构相对稳定,菌株存在获得性耐药,且因携带更多毒力基因而产生潜在的高致病性菌株。谱系Ⅰ和谱系Ⅱ菌株在毒力基因、抗性相关基因和质粒携带方面均具有差异,显示不同CC型菌株的毒力和环境适应性存在差异,为咸阳市单增李斯特菌的监测和防控提供了参考数据。     

Nisin treatments to control Listeria monocytogenes post-processing contamination on Anthotyros, a traditional Greek whey cheese, stored at 4°C in vacuum packages

Post-processing contamination and growth of Listeria monocytogenes in whey cheeses stored under refrigeration is an important safety concern. This study evaluated commercially available nisin (Nisaplin^®) as a biopreservative to control L. monocytogenes introduced post-processing on Anthotyros, a traditional Greek whey cheese, stored at 4°C in vacuum packages for up to 45 days. The whey used (pH 6.5–6.7) was from Feta cheese manufacture, and it was subjected either to natural acidification (pH 5.3, readjusted to 6.2 with 10% NaOH) prior to heating, or to direct acidification (pH 6.0–6.2) at 80°C with 10% citric acid. Nisin was added either to the whey (100 or 500 IU g⁻¹) prior to heating, or to the cheese (500 IU g⁻¹) prior to packaging, also inoculated with ca. 10⁴ cfu g⁻¹ of L. monocytogenes strain Scott A. In cheese samples without nisin, L. monocytogenes (PALCAM agar) exceeded 7 log cfu g⁻¹ after the first 10 days of storage, irrespective of the whey acidification method. All nisin treatments had an immediate lethal effect (0.7–2.2 log reduction) on L. monocytogenes populations at inoculation (day 0), which was more pronounced with 500 IU g⁻¹ added to the whey. This treatment also suppressed L. monocytogenes growth below the inoculation level for 30 and 45 days in naturally and directly acidified samples, respectively. All other treatments had weak antilisterial effects. Nisin reversed the natural spoilage flora of Anthotyros cheese from Gram-positive to Gram-negative, and this ecological alteration was far more pronounced in the most effective antilisterial treatments.     

Persistence of Listeria monocytogenes strains isolated from products in a Polish fish-processing plant over a 1-year period

Seventy-one presumptive Listeria monocytogenes strains were isolated over a year from 152 samples comprising raw fish (salmon, seatrout) and their products (mainly, vacuum-packed cold-smoked sliced salmon) in a selected Polish fish-processing plant. Contamination of raw materials was at the level of 4.3–15.4%, whereas final products revealed significantly higher contamination (up to 77.8%) than regarded by other studies as typical (up to 40%). Strains were identified using conventional microbiological methods (including API®LISTERIA tests) and the PCR technique (aimed at iap gene fragment detection). A random amplification polymorphic DNA (RAPD) technique was applied to analyse their intraspecies diversity. RAPD typing revealed an incidence of eight RAPD types. Three of them were isolated over 8–10 months during the plant monitoring. It suggested that they were a persistent element of ‘in-house’ microflora and the applied typing technique produced evidence that fish products could be probably contaminated at the last stages of fish processing (e.g. smoking, slicing, and/or packaging). Their occurrence was probably supported by clone selection caused by ineffective application of cleaning and sanitizing procedures. The possibility of colonization of the production environment by fish-originated L. monocytogenes was also proven. Strains that belonged to a dominant RAPD type were additionally subjected to restriction fragment length polymorphism-pulsed field gel electrophoresis (RFLP-PFGE). RFLP-PFGE confirmed intraspecies similarity of strains belonging to a dominant RAPD type. A subset of strains from salmon samples was also characterized by serotyping. Contrary to earlier reports, they belonged mainly (91.7%) to the serotype 4.