Bacterial and plant factors that influence adhesion of enterohaemorrhagic E. coli and Salmonella enterica to salad leaves

Until now, research into transmission of food-borne pathogens has largely focused on farm animal reservoirs. Although transmission of enteric pathogens via salad leaves is on the rise, only a few studies have addressed the molecular mechanisms used by human enteric bacterial pathogens to attach to plants. Likewise there have been few studies investigating variation between plant genotypes on bacterial adhesion and/or survival and those that have not addressed the genetic basis of any differences. In this project we aim to investigate how human bacterial pathogens attach to salad leaves (in particular lettuce). We will determine the effect of bacterial growth condition on the ability of the organism to attach to leaves. Similarly, we will determine the effect of plant growth conditions on susceptibility to bacterial attachment. Using genetic approaches, we will determine which bacterial genes are involved in leaf attachment and we will utilise a lettuce mapping population of recombinant inbred lines to initiate studies aimed at gaining an understanding of the genetic basis of a plant's susceptibility to bacterial adhesion. We will also determine survival and replication rates of Salmonella and E. coli on the different lettuce lines. The ultimate goal of our studies is to provide the underpinning knowledge of the interaction between the plant and enteric bacteria in the phyllosphere to allow the development of strategies to reduce the risk of crop contamination and to increase food safety.

Traditionally, research into food borne pathogens in the UK has focused on zoonosis. However, recent surveys have shown that while there is a reduction in transmission of non-typhoidal serovars of Salmonella enterica via contaminated poultry products, there is a steep rise in transmission via salad leaves, particularly, but not exclusively, lettuce. Similarly, although Infections with enterohaemorrhagic E. coli (EHEC) are usually linked to consumption of contaminated beef products, sever outbreaks in Sakai city in 1996 and the US in 2006 were traced to consumption of contaminated radish sprouts and pre-packed lettuce, respectively. Only a few studies have addressed the molecular mechanisms used by human enteric bacterial pathogens to attach to plants. Likewise there have been few studies investigating variation between plant genotypes on bacterial adhesion and survival and those that have been carried out do not address the genetic basis of any differences. In this project we will determine the mechanisms used by S. enterica & EHEC (O157 and non-O157) to attach to salad leaves. We will determine the effect of bacterial growth conditions on leaf attachment potential. We will use immunofluoresence microscopy, scanning electron microscopy and viable counts to determine the level and pattern of adhesion and to visualize potential adhesion factors. We will determine survival and replication rates in planta. Combining with site directed mutagenesis, this will reveal common and species-specific leaf adhesion and survival strategies. We will also investigate host factors that contribute to leaf susceptibility to colonisation. We will determine the effect of plant growth conditions and age on colonisation by S. enterica & EHEC. Using genetic and genomic approaches we will initiate studies to determine the genetic basis of lettuce to susceptibility to colonisation. These studies will underpin development of new risk assessments and improvements in food safety.