A Simple Assay to Assess Salmonella enterica Persistence in Lettuce Leaves After Low Inoculation Dose

Lettuce Genotype-Dependent Effects of Temperature on Escherichia coli O157:H7 Persistence and Plant Head Growth

Lettuce has been commonly associated with the contamination of human pathogens, such as Escherichia coli O157:H7 (hereafter O157:H7), which has resulted in serious foodborne illnesses. Contamination events may happen throughout the farm-to-fork chain, when O157:H7 colonizes edible tissues and closely interacts with the plant. Environmental conditions have a significant impact on many plant-microbe interactions; however, it is currently unknown whether temperature affects O157:H7 colonization of the lettuce phyllosphere. In this study, we investigated the relationship between elevated growth temperatures, O157:H7 persistence, and lettuce head growth using 25 lettuce genotypes. Plants were grown under optimal or elevated temperatures for 3.5 weeks before being inoculated with O157:H7. The bacterial population size in the phyllosphere and lettuce head area was estimated at 0- and 10-days postinoculation (DPI) to assess bacterial persistence and head growth during contamination. We found that growing temperature can have a positive, negative, or no effect on O157:H7 persistence depending on the lettuce genotype. Furthermore, temperature had a greater effect on head area size than the presence of O157:H7. The results suggested that the combination of plant genotype and temperature level is an important factor for O157:H7 colonization of lettuce and the possibility to combine desirable food safety traits with heat tolerance into the lettuce germplasm.

It takes two to attach - endo-1,3-β-d-glucanase as a potential receptor of mannose-independent, FimH-dependent Salmonella Typhimurium binding to spinach leaves

Currently, fresh, unprocessed food has become a relevant element of the chain of transmission of enteropathogenic infections. To survive on a plant surface and further spread the infections, pathogens like Salmonella have to attach stably to the leaf surface. Adhesion, driven by various virulence factors, including the most abundant fim operon encoding type 1 fimbriae, is usually an initial step of infection, preventing physical removal of the pathogen. Adhesion properties of Salmonella's type 1 fimbriae and its FimH adhesin were investigated intensively in the past. However, there is a lack of knowledge regarding its role in interaction with plant cells. Understanding the mechanisms and structures involved in such interaction may facilitate efforts to decrease the risk of contamination and increase fresh food safety. Here, we applied Salmonella genome site-directed mutagenesis, adhesion assays, protein-protein interactions, and biophysics methods based on surface plasmon resonance to unravel the role of FimH adhesin in interaction with spinach leaves. We show that FimH is at least partially responsible for Salmonella binding to spinach leaves, and this interaction occurs in a mannose-independent manner. Importantly, we identified a potential FimH receptor as endo-1,3-β-d-Glucanase and found that this interaction is strong and specific, with a dissociation constant in the nanomolar range. This research advances our comprehension of Salmonella's interactions with plant surfaces, offering insights that can aid in minimizing contamination risks and improving the safety of fresh, unprocessed foods.

Intraspecies competition among Salmonella enterica isolates in the lettuce leaf apoplast

Multiple Salmonella enterica serovars and strains have been reported to be able to persist inside the foliar tissue of lettuce (Lactuca sativa L.), potentially resisting washing steps and reaching the consumer. Intraspecies variation of the bacterial pathogen and of the plant host can both significantly affect the outcome of foliar colonization. However, current understanding of the mechanisms underlying this phenomenon is still very limited. In this study, we evaluated the foliar fitness of 14 genetically barcoded S. enterica isolates from 10 different serovars, collected from plant and animal sources. The S. enterica isolates were vacuum-infiltrated individually or in pools into the leaves of three- to four-week-old lettuce plants. To estimate the survival capacity of individual isolates, we enumerated the bacterial populations at 0- and 10- days post-inoculation (DPI) and calculated their net growth. The competition of isolates in the lettuce apoplast was assessed through the determination of the relative abundance change of barcode counts of each isolate within pools during the 10 DPI experimental period. Isolates exhibiting varying apoplast fitness phenotypes were used to evaluate their capacity to grow in metabolites extracted from the lettuce apoplast and to elicit the reactive oxygen species burst immune response. Our study revealed that strains of S. enterica can substantially differ in their ability to survive and compete in a co-inhabited lettuce leaf apoplast. The differential foliar fitness observed among these S. enterica isolates might be explained, in part, by their ability to utilize nutrients available in the apoplast and to evade plant immune responses in this niche.