UC Davis

Lettuce is often associated with contamination by human pathogens due to the multiple outbreaks and recalls that happen each year. This is because human pathogens, such as Escherichia coli O157:H7 (hereafter O157:H7), are capable of becoming internalized within the leaf tissue where they are resistant to standard industry cleaning procedures. Contamination events often occur on the farm while the plant is growing. This offers the plant and bacteria opportunities to interact which has shown to effect the extent of O157:H7 growth. The goal of these studies was to test the effect of environmental and genetic variables on O157:H7 persistence in the phyllosphere. The information discovered here will allow breeders the opportunity to intentionally produce genotypes that are non-conducive to human pathogen growth, adding to the mitigation strategies already in place. Here I discuss the topic of O157:H7 contamination of lettuce, including how and where it occurs as well as what interactions the bacteria and host are currently known to have. Then I provide details on screens conducted to test the effects of growth temperature on O157:H7 persistence in 25 lettuce genotypes, O157:H7 genetic variation on its persistence in Salinas lettuce, and lettuce genetic variation on O157:H7 persistence within 92 commercial genotypes and lettuce breeding lines. In these screens I used 3.5 week old plants that were vacuum infiltrated with O157:H7 to ensure consistent contamination level. Within the growth temperature screen, I also assessed the leaf area of the plants after infiltration. Through this I identified lettuce genotypes with a beneficial combination of traits to grow at elevated temperatures and be non-conducive to O157:H7 persistence. Furthermore, I found that the presence of O157:H7 did not alter the leaf area compared to plants that received a mock infiltration within this screen. Within the genetic screens I observed that within our O157:H7 isolates most showed a similar capacity to persist within Salinas lettuce. In contrast I observed a large range of O157:H7 persistence when tested in the 92 genotype lettuce panel. Here I also submitted tissue samples to undergo genotyping by sequencing (GBS) analysis allowing me to observe the population structure within the panel and confirm that it was consistent with previously published lettuce populations. Finally, combining the screen and GBS data I conducted the first genome-wide association (GWAS) study in lettuce regarding O157:H7 persistence, though I was unable to identify any significant single nucleotide polymorphisms (SNPs) associated with the trait. This body of work suggests that the lettuce genome harbors untapped potential to deal with increased growing temperatures and be non-conducive to O157:H7 growth within the phyllosphere.