Effect of lettuce cultivar and irrigation water source on the shift in phytobiota and survival of pathogenic E. coli and Salmonella spp. on lettuce during storage

Fresh produce, like lettuce, is a known vehicle for a number of recent foodborne outbreaks. Survival of bacterial pathogens might be influenced by the diverse phytobiota. However, the effects of lettuce cultivar and irrigation with different types of water sources, like reclaimed wastewater, on the dynamics of innate microbiota and survival of foodborne pathogens on lettuce leaves have not been well studied. Three cultivars of lettuce, including Annapolis, Celinet, and Coastline, were grown in a heated green-house and irrigated by primary-, and secondary-waste water started 6 weeks post-seeding, with ground well water as control. Nine types of the lettuce were sampled and inoculated with a cocktail inoculum of pathogenic E. coli and Salmonella (5 log CFU/mL), or sterile water as control. All of the inoculated and control lettuce samples were stored and tested one day and seven days after inoculation for total and inoculated bacteria by plate counting, PMA-qPCR, and 16S rDNA gene high through-put sequencing. Collected irrigation water samples were tested in day 0. Microbial compositions in three irrigation water sources were significantly different. Irrigation by different types of water sources did not alter the dominant bacteria on lettuce, but might affect the survival of foodborne pathogens during storage. Populations of total bacteria on lettuce remained at the same level or increased after 1 week storage. However, the populations of inoculated pathogenic E. coli and Salmonella enterica reduced significantly. Survival of inoculated foodborne pathogens varied on different lettuce cultivars. The compositions of most abundant bacterial communities on different lettuce cultivars grown in the same greenhouse were similar, which were not obviously affected by irrigation, inoculation of foodborne pathogens, and storage at refrigerated temperature. Data derived from this study can benefit current mitigation strategies of foodborne pathogens and spoilage bacteria during produce processing.