A Quantitative Microbial Risk Assessment (QMRA) was performed to estimate the expected number of listeriosis cases due to the consumption, on the last day of shelf life, of 20 000 servings of multi-ingredient sandwiches produced by a medium scale food producer in Italy, by different population strata, defined by infection susceptibility (healthy, susceptible, transplant recipients and total population).

First, all the sandwich ingredients were analysed for pH, Aw, salt and organic acids content and submitted to challenge tests at three different temperatures (4, 6 and 10 °C) to evaluate their suitability for L. monocytogenes growth. Next, a stochastic model was constructed simulating the contamination of the ingredients that were the best (bean cream) and worst (cheese cream) growth substrates. For each substrate, an exposure assessment was performed, estimating the number of L. monocytogenes within each serving. Then, two dose-response models were alternatively applied: the first used a fixed r value for each of the three population groups, while the second considered a variable r value (lognormal distribution), taking into account the variability in strain virulence and different host subpopulations susceptibility.

The stochastic model predicted zero cases for total population for both the substrates by using the fixed r approach, while 3 cases were expected when a higher variability (in virulence and susceptibility) was considered in the model; the number of cases increased to 45–52 in the worst scenario (bean cream contamination) assuming all servings would be consumed by transplant recipients.

An uncertainty analysis was performed by considering alternative scenarios: a higher mean bacterial concentration (+ 0.5 Log CFU/g) or higher standard deviation (+ 0.5) determined evident increases in the expected number of cases, almost doubling the risk. A similar effect was also exerted by an extended storage time (from 72 to 96 h), in particular in the worst case scenario. Finally, different protective interventions were evaluated (70/30 N₂/CO₂ packaging, home cooking or their combination). Both the interventions resulted in a strong decrease of the risk; MAP packaging, should be regarded as the most promising one, as it can be performed by the producer, who can assure a strict control of the treatment performances.

进行了定量微生物风险评估（QMRA），以估计由于在货架期的最后一天食用了由意大利一家中型食品生产商生产的2万份多成分三明治而导致的李斯特菌病病例数，根据不同的感染人群（健康，易感人群，移植受者和总人群）定义。

首先，分析所有三明治成分的pH，Aw，盐和有机酸含量，并在三种不同温度（4、6和10°C）下进行挑战性测试，以评估其对单核细胞增生李斯特氏菌生长的适用性。接下来，建立一个随机模型来模拟最佳（豆奶油）和最差（奶酪奶油）生长基质成分的污染。对于每种底物，进行暴露评估，以估计单核细胞增生李斯特菌的数量每份内。然后，交替使用两个剂量反应模型：第一个模型针对三个人群中的每个群体使用固定的r值，而第二个模型考虑了菌株毒力和不同宿主的变异性，将其视为变量r值（对数正态分布）亚群易感性。

随机模型使用固定r方法预测两种底物的总种群为零，而模型中考虑到较高的变异性（毒力和敏感性）时，则为3例。假设所有份量都将由移植接受者食用，那么在最坏的情况下（豆奶油污染），病例数增加到45-52。

通过考虑替代方案进行了不确定性分析：较高的平均细菌浓度（+ 0.5 Log CFU / g）或较高的标准偏差（+ 0.5）确定了预期病例数的明显增加，风险几乎翻了一番。延长的存储时间（从72小时到96小时）也产生了类似的效果，尤其是在最坏的情况下。最后，评估了不同的防护措施（70/30 N ₂ / CO ₂包装，家庭烹饪或它们的组合）。两种干预措施均大大降低了风险；MAP包装应被视为最有前途的包装，因为它可以由生产商执行，生产商可以确保严格控制处理性能。