1a. Objectives (from AD-416)
The overall objective of this study is to assess the persistence and transfer of norovirus, Salmonella, Escherichia coli O157:H7 and avian pathogenic E. coli (APEC) on leafy greens and basil to gain information to better enhance the safety of produce and to reduce transmission of these pathogens in the field onto leafy greens. Technion-Israel will use basil and spinach as the food vectors of study for persistence and mechanisms of attachment of Salmonella, while the University of Delaware and USDA-EMFSL in Beltsville, will evaluate lettuce, spinach and basil in their studies of norovirus and bacteria. The University of Delaware will investigate persistence of noroviruses, APEC and nonpathogenic E. coli on plant surfaces in a comparison study, while USDA-EMFSL will evaluate E. coli O157:H7, APEC, nonpathogenic E. coli, and Salmonella simulating field conditions using lower, more realistic population levels and different irrigation regimes.
1b. Approach (from AD-416)
Objective 1. University of Delaware: To investigate the persistence of norovirus on lettuce, spinach and basil with viral detection and counting by plaque assay and RT-PCR, and to determine the sites of adherence on produce using confocal microscropy. Objective 2. Technion–Israel: To determine the effect of the irrigation regime on transfer and survival of Salmonella in plants; irrigation methods that will be investigated: Dripping vs. spraying, day vs. night, summer vs. wintercrops, daily vs. multiple short irrigation. Objective 3. USDA EMFSL: To determine the fate of enterohemorrhagic, avian pathogenic, non-pathogenic E. Coli, and Salmonella introduced to leafy green foliar surfaces in irrigation water at levels stated in the California Leafy Green Marketing Agreement.
3. Progress Report
Leafy greens are the produce commodity most likely to be contaminated by a foodborne pathogen. Irrigation water is an established source for this contamination. LGMA states irrigation water used for foliar applications may contain 126 MPN/100 mL of generic E. coli; no single sample can be >235 MPN/100 mL. Total organic carbon (TOC) content is one water quality indicator. Baby spinach plants were spray-irrigated with water containing E. coli O157:H7, and different TOC levels were used to determine epiphytic survival. E. coli O157:H7 in irrigation water that complied with LGMA standards did not survive for >24 h when applied onto surfaces of spinach plants, supporting the validity of these guidelines with regard to TOC and E. coli O157:H7. Repeated irrigation of spinach with irrigation water containing low levels of E. coli O157:H7 did not increase the persistence of E. coli O157:H7 on spinach leaves. Higher levels of total organic carbon also did not enhance E. coli O157:H7 survival on spinach leaves.