Stomatal response and human pathogen persistence in leafy greens under preharvest and postharvest environmental conditions

Human pathogens can internalize and persist inside crop plants leading to foodborne outbreaks and illnesses. Pathogenic bacteria might use natural openings on the plant surface, such as the stomatal pore, to penetrate the leaf interior and colonize the intercellular space. Once internalized, these pathogens often escape current sanitation procedures that are mostly efficient to clean the plant surface. Plants have evolved mechanism to quickly perceive the presence of bacteria and close the stomatal pore, potentially diminishing leaf contamination. In this study, we assessed the ability of several fresh leafy greens in mounting stomatal immunity against Escherichia toll O157:H7 and Salmonella enterica serovar Typhimurium (STm) SL1344 and determined the influence of environmental conditions (light, temperature, and humidity) on the effectiveness of the stomatal response and bacterial persistence in the leaf. We observed that, independent of the air relative humidity (RH), E. coli O157:H7 induces stomata closure in Butter Lettuce, Romaine, Basil, Spinach, and Cilantro when compared to the water control. However, STm SL1344 induces a variable stomatal response in different plant species and environmental conditions. Furthermore, endophytic STm 1344 population increases with high RH independently of other environmental conditions, whereas O157:H7 population size significantly increases in Romaine and Spinach only under post-harvest conditions. The phenotypic variability observed among the various STm-plant interactions provides opportunities to discovering the underlying genetic basis of both the plant and the pathogen, as well as to proposing additional plant-specific control measure to reduce pathogen load on/in leafy greens.