Summary of Awards to Date

Towards preventing internalization and persistence of human bacterial pathogens in fresh produce


Jan. 1, 2015 - Dec. 31, 2015

Funding Agency

University of California, Davis

Amount Awarded



Maeli Melotto, Ph.D.
University of California, Davis


Consumption of a healthy diet, including fresh fruits and vegetables, is critical for human health. Associated with this trend is the increased number of food poisoning outbreaks. Internal contamination of leaves is difficult, if not impossible, to be removed by standard sanitization procedures. Understanding human pathogen entry into leaves is therefore paramount in preventing food borne diseases. We have shown that stomata, the pores on leaf surfaces, can actively close and restrict the entry of human and plant pathogenic bacteria in Arabidopsis and lettuce leaves. In this study we will use a combination of approaches including bacterial infection and stomatal closure assays, thermoimaging, and microscopy with the objective to determine how efficiently various leafy vegetable can prevent penetration and persistence of Escherichia coli O157:H7 and Salmonella enterica Typhimurium in internal tissues. We will use commercially available leafy vegetables including parsley, cilantro, basil, arugula, spinach, and several varieties used in spring mix salads. Experiments will be conducted under several levels of air relative humidity relevant to produce production and packing facilities. This research has great potential to provide a mechanistic understanding of active plant defense against human pathogen internalization of fresh produce and to guide future development of appropriate prevention measures.

Technical Abstract

Plant genetic resistance to human pathogens has been overlooked mainly due to the fact that many of these pathogens do not cause visual symptoms in plants as plant pathogens do. It has been widely accepted that these symptomless associations between plant and human pathogens would be a passive process with no dramatic physiological changes occurring in either organism.  However, we and others have gathered mounting evidence suggesting that plants have the ability to actively respond to the presence of human pathogens and these responses involve components of the plant innate immunity system.

Innate immunity in plants and animals can be activated by highly conserved pathogen-associated molecular patterns (PAMPs). We have recently found that stomata, which are epidermal pores in plants responsible for water transpiration and CO2 uptake, play a critical role in innate immunity to restrict the entry of human and plant pathogenic bacteria (Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium SL1344, and Pseudomonas syringae pv. tomato DC3000) into plant leaves. These studies uncovered a novel functional output of innate immunity in plants and a new battleground in host-bacterium interactions that has important implications in plant association with human pathogenic bacteria. The long-term goals of my research are (i) to elucidate the signal transduction pathway leading to stomate-based defense against human and plant pathogenic bacteria, (ii) to characterize the in situ molecular responses of human pathogenic bacteria in the plant, and (iii) to provide fundamental knowledge on the genetic basis of plant defenses against human pathogens to guide the development of genetically resistant varieties of fresh produce for sustainable and environmentally friendly agricultural practices.

We have used model plants and produce (e.g., Arabidopsis and lettuce) to further fundamental knowledge in the field. With this proposed research, we aim to assess the genetic variation of stomate-based defense within the plant kingdom. Therefore, our specific objectives are guided by two questions: Do all plants have the genetic capability to mount stomatal immunity against to human pathogenic bacteria? If so, how robust is this response under variable air relative humidity conditions? We will systematically assess stomatal response to the bacterial strains SL1344 and O157:H7 in parsley, basil, spinach, cilantro, iceberg lettuce, and spring mix salad greens such as mustard greens, frisee, Swiss chard, arugula, endive, radicchio, and several lettuce varieties. We will conduct quantitative analyses (stomatal and bacterial infection assays, thermo-imaging, and microscopy) to effectively determine how efficiently innate immunity in leafy vegetables can be against human pathogens and whether certain plant species can clear these microbial populations from the apoplast.

This line of research in the field of food safety is still in its infancy, and basic experiments are still needed to address this issue. Thus, this is a proof-of-concept proposal and the primary outcome is related to the POC-Category 3 “Generate preliminary data for a novel fundamental research that fits a long-term CPS priority”. The successful completion of this project will allow for translational research from model systems to leafy vegetables to reduce pathogen load in the produce chain.