Impacts of Xanthomonas Foliar Disease on the Fate and Biology of Salmonella enterica

The phyllosphere—above-ground foliage of plants—houses a consortium of multi-trophic interactions, including those between host plant, enteric human pathogens, and pathogenic phytobacteria. The interactions between these players have consequences for human health. Following arrival via irrigation water, Salmonella enterica resides as a commensal on plant surfaces for long periods, though its populations slowly decline. And yet, salmonellosis outbreaks associated with consumption of raw produce cause illnesses, hospitalizations, and tons of food waste every year. Xanthomonas pathogens, which cause bacterial leaf spot diseases, dramatically alter the leaf apoplast into an aqueous habitat and likely serve as biomultipliers for S. enterica.

My thesis builds on existing work examining S. enterica’s ability to benefit from Xanthomonas disease through determining how Xanthomonas infection alters S. enterica movement, replication, and virulence. Chapter 1 introduces the relationship between phytobacterial disease and food safety by highlighting drivers of S. enterica persistence in the phyllosphere. Subsequent chapters expand on my research findings, starting with Chapter 2, in which we demonstrated Xanthomonas-induced watersoaking transforms an inhospitable host into an environment that permits S. enterica internalization into the apoplast and passive movement to distal tomato leaf tissue. In Chapter 3, we demonstrated that Xanthomonas vitians infection progress and environmental humidity additively enhance S. enterica’s ability to rapidly multiply within lettuce leaves. We then tested the ability of these replicating S. enterica cells to cause infection in an animal host in Chapter 4 and demonstrated that X. vitians-infected lettuce affects S. enterica physiology in a way that alters murine infection. Chapter 5 provides a unique review of S. enterica’s interaction with biomultipliers in the phyllosphere using kid-friendly language. Lastly, the broader impact of this research and future directions are described in Chapter 6.

My work examining S. enterica’s fate in multiple hosts, including tomato, lettuce, and mouse holds multidisciplinary significance. Our hope is that these findings will lead to wider recognition of diseased plants as a potential reservoir for S. enterica, leading to better risk assessment procedures for predicting S. enterica burden in the field that account for both climate factors and the presence of foliar disease.