Salad contamination with Listeria - improving food safety through understanding plant-bacterial interactions and development of novel detection method

食用沙拉等新鲜农产品是人类饮食的重要组成部分，可以预防一系列癌症和心血管疾病。不幸的是，与受污染蔬菜有关的疾病爆发，特别是在即食预包装沙拉中，在过去十年中急剧增加。主要细菌污染物为沙门氏菌、大肠杆菌。大肠杆菌和李斯特菌。李斯特菌病，由细菌病原体单核细胞增生李斯特菌引起，对孕妇和老年人特别危险。与其他食源性病原体不同，李斯特菌可以在低温（4 ℃）下繁殖，增加人类感染的风险。人们对李斯特菌如何附着在沙拉叶上以及它如何繁殖到危险水平缺乏了解。沙拉的保质期也很短，因此，收获和到达消费者之间的时间非常短。目前李斯特菌的检测方法需要几天的培养，并且在确认任何污染之前，受污染的产品可能会到达消费者手中。因此，更好地了解李斯特菌如何在沙拉叶上附着和繁殖的调控因素，以及更快地检测李斯特菌对新鲜农产品污染的方法，将对食品安全做出重要贡献。食品香气由一束挥发性有机化合物（VOCs）组成，其组成受食品储存条件和微生物在食品中的定植影响。因此，食物中的挥发性有机化合物包含有关储存条件和污染的信息，可以在几个小时内使用气相色谱质谱法（GC-MS）进行测量。我们最近表明，通过分析挥发性有机化合物概况，可以在鲜切甜瓜上检测到李斯特菌污染的现实水平，初步结果表明，我们也可以检测到火箭叶上李斯特菌的污染。我们小组的进一步数据表明，在沙门氏菌和大肠杆菌污染的情况下。病原菌在色拉上的附着和增殖受病原菌菌株、色拉种类和色拉叶龄的影响。

Consumption of fresh produce such as salads is an important component of the human diet, protecting against a range of cancers and cardiovascular disease. Unfortunately, disease outbreaks linked to contaminated vegetables, especially in ready-to-eat pre-packed salads, have increased sharply over the last decade. Major bacterial contaminants are Salmonella, E. coli and Listeria. Listeriosis, caused by the bacterial pathogen Listeria monocytogenes is specifically dangerous for pregnant women and elderly people. Unlike other foodborne pathogens, Listeria can multiply at cold temperatures (4 oC) increasing the risk of human infection. There is a lack of understanding of how Listeria attaches to salad leaves in the first place and how it manages to multiply to dangerous levels. Salads have also a short shelf-life and, therefore, the time between harvest and reaching the consumer is very short. Current detection methods for Listeria require several days of culturing and risk contaminated products reaching the consumers before any contamination can be confirmed. Consequently, a better understanding of the factors regulating how Listeria attaches and multiplies on salad leaves, and faster method of detection of contamination of fresh produce by Listeria, would make an important contribution to food safety.Food aroma is composed of a bouquet of volatile organic compounds (VOCs) whose composition is affected by food storage conditions and colonisation of food by microorganisms. The VOCs bouquet of a food, therefore, contains information about storage conditions and contamination and can be measured using gas chromatography mass spectrometry (GC-MS) within a matter of hours. We recently showed that realistic levels of contamination with Listeria could be detected on fresh cut melon through the analysis of VOC profiles, and preliminary results indicate that we can also detect contamination with Listeria on rocket leaves. Further data from our group show that, in the case of contamination with Salmonella and E. coli, attachment of the pathogen to the salad and its multiplication is affected by the strain of the pathogen, and the salad species and age of the salad leaf.