Differential phenolic metabolite and ROS responses in lettuce following infiltration with Salmonella and Escherichia coli O157:H7 accompanied bacterial log reductions.

While lettuce immune responses to enteropathogens have been studied at the molecular and physiological levels, plant secondary metabolite responses have received little attention. We evaluated romaine lettuce phenolic metabolite responses to Escherichia coli O157:H7 and Salmonella enterica Enteritidis infiltrated into the leaf apoplast. Evaluating spectrophotometric profiles of leaf extracts, we detected shifts in overall phenolics and developed a semi-quantitative method to measure representative phenolics absorbing maximally at 255, 273, 280 and 329 nm, based on known standards for quercetin, gallic acid, catechin and chlorogenic acid, respectively. We also measured reactive oxygen species (ROS) accumulation and bacterial counts following enteropathogen infiltration in pre- and post-harvest leaves. Live and heat-killed Salmonella elicited phenolics 4 h post-inoculation (hpi) (p < 0.05) which declined or returned to control levels 24 hpi, while exhibiting a lag time in ROS production. Phenolics in leaves inoculated with live or heat-killed E. coli O157:H7 remained mostly unchanged relative to controls, while ROS was detected at both timepoints. Salmonella populations declined by 2 log at 4 hpi and E. coli O157:H7 by 1.2 log. Post-harvest leaves had a lower phenolic content than pre-harvest leaves and exhibited lower log reductions (p < 0.05). The spectrophotometric method proved to be a useful and efficient tool to elucidate lettuce phenolic shifts in response to enteropathogens, indicating pathogen-specific responses influenced by lettuce harvest state. This research demonstrates that the complex plant responses to enteropathogens involve plant secondary metabolite shifts that intersect with basal plant immunity, suggesting direct and/or indirect links to enterobacterial population decline in the apoplast.