Frozen vegetables have previously been associated with outbreaks of listeriosis in both the USA and Europe. An outbreak of Listeria monocytogenes serogroup 4 caused 53 cases in five European countries between 2015 and 2018. Whole genome sequencing (WGS) indicated that frozen sweet corn from a producer in Hungary was the source of illness. However, limited data is available on the prevalence of Listeria in frozen produce. A study of frozen fruit and vegetables from catering and retail premises in England was therefore carried out to assess their microbiological quality with respect to Listeria and Escherichia coli.

Between December 2018 and April 2019, 1050 frozen fruit and vegetable samples were collected. Of these, 99% were of a satisfactory or borderline microbiological quality. Eleven samples (1%) contained ≥100 cfu/g of Escherichia coli (considered unsatisfactory in products labelled as ready-to-eat). Listeria monocytogenes or other Listeria species were detected in six samples (2%) of fruit compared to 167 samples (24%) of vegetables and six samples (26%) of fruit and vegetable mixes, but none at a level of ≥100 cfu/g. Characterisation by WGS of 74 L. monocytogenes isolates identified ten genetic clusters indicating a common source. For 8 of the 10 clusters, the isolates came from homogenous food types: four were sweet corn, and there was one cluster each for beans, peas, peppers and broccoli. There were five genetic associations between isolates from frozen vegetables and from clinical cases of listeriosis, including two cultures from frozen beans that were indistinguishable from the 2015–2018 sweet corn outbreak strain.

This study indicates that L. monocytogenes was present in 10% of frozen vegetables and even though products are generally not ready-to-eat and are intended to be cooked prior to consumption, these have the potential to cause illness. Clear cooking and handling instructions are therefore required on these products to ensure that the health of consumers is not put at risk, and appropriate Good Manufacturing Practice measures should be followed by all fruit and vegetable freezing plants in order to reduce contamination with Listeria during processing.

以前，在美国和欧洲，冷冻蔬菜与李斯特菌病暴发有关。单核细胞增生李斯特菌血清群4的暴发在2015年至2018年期间在五个欧洲国家引起了53例病例。全基因组测序（WGS）表明，匈牙利生产商的冷冻甜玉米是病源。但是，有关冷冻产品中李斯特菌的流行情况的可用数据有限。因此，对英格兰餐饮和零售场所的冷冻水果和蔬菜进行了研究，以评估其相对于李斯特菌和大肠杆菌的微生物质量。

在2018年12月至2019年4月之间，收集了1050个冷冻的水果和蔬菜样品。其中99％的微生物质量令人满意或临界。11个样品（1％）中的大肠杆菌含量≥100 cfu / g （认为​​即食产品不令人满意）。在六个水果样本中（2％）检出了李斯特菌或其他李斯特菌菌种，而蔬菜中的167个样本（24％）和水果和蔬菜混合物中的六个样本（26％）检出了李斯特菌，但≥100 cfu / G。74 L的WGS表征 。单核细胞增生分离物鉴定出指示共同来源的十个遗传簇。在10个簇中的8个中，分离株来自同种食物类型：4个是甜玉米，每个豆，豌豆，辣椒和西兰花中都有一个簇。冷冻蔬菜分离物与李斯特菌病临床病例之间存在五个遗传关联，包括与2015–2018年甜玉米暴发菌株没有区别的两种冷冻豆培养物。

这项研究表明，大号。单核细胞增生病菌存在于10％的冷冻蔬菜中，即使产品通常不可以即食且打算在食用前进行烹饪，但它们仍有可能引起疾病。因此，需要对这些产品进行清晰的烹饪和处理说明，以确保不会危害消费者的健康，所有水果和蔬菜冷冻工厂均应遵循适当的良好生产规范措施，以减少加工过程中李斯特菌的污染。