Behaviour of Escherichia coli O157:H7 and Salmonella enterica during greenhouse butterhead lettuce production

Fruits and vegetables, together called fresh produce, represent an important part of the human diet. However, since the mid 1990’s foodborne illness outbreaks linked to consumption of fresh fruits and vegetables were documented worldwide. Especially leafy vegetables, such as lettuce, are frequently involved. Most identified outbreaks were caused by Salmonella and Escherichia coli O157:H7, enteric bacterial pathogens which are traditionally associated with food products of animal origin like meat, dairy and eggs. For this thesis, butterhead lettuce was chosen as model plant because of its economic value for Belgium and the lack of knowledge about this typical northern European lettuce type regarding contamination with bacterial enteric pathogens. The research focused on factors influencing the contamination of the crop before harvest, also called preharvest contamination, by using artificial contamination of seeds, irrigation water and growing lettuce plants. Furthermore, experiments were conducted to gain insight into the underlying genetic mechanisms that these pathogens use to survive on fresh produce.