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，特别是在即食的预包装沙拉中，在过去十年中急剧增加。主要的细菌污染物是沙门氏菌、大肠杆菌和李斯特菌。由单核细胞增生李斯特菌引起的李斯特菌病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，从而增加人类感染的风险。人们对李斯特菌最初是如何附着在沙拉叶子上的以及它是如何繁殖到危险水平的缺乏了解。沙拉的保质期也很短，因此，从收获到到达消费者手中的时间很短。目前的李斯特菌检测方法需要数天的培养，而且在确认任何污染之前，受污染的产品有可能到达消费者手中。因此，更好地了解李斯特菌在沙拉叶片上附着和繁殖的调控因素，以及更快地检测新鲜农产品李斯特菌污染的方法，将对食品安全做出重要贡献。食物香气是由一束挥发性有机化合物（VOCs）组成的，其成分受到食物储存条件和微生物在食物中的定植的影响。因此，食物的挥发性有机化合物气味包含有关储存条件和污染的信息，可以在几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（VOC）谱，可以在新鲜切面甜瓜上检测到李斯特菌的实际污染水平，初步结果表明，我们也可以在火箭叶片上检测到李斯特菌的污染。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，病原体在沙拉上的附着及其增殖受到病原体菌株、沙拉品种和沙拉叶年龄的影响。

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.