Perez-Lewis, Keila L. (2015-12). Novel Interventions for Reducing Pathogen Attachment and Growth on Fresh Produce. Doctoral Dissertation.
The objectives of this research were to 1) identify the native microbiota on surfaces of fresh fruit and leafy greens; 2) identify microorganisms antagonistic towards Salmonella enterica Typhimurium LT2 and Escherichia coli O157:H7 ATCC 700728 both in vitro and on produce surfaces; and 3) evaluate the ability of antimicrobial-bearing nano-encapsulates to prevent pathogen attachment and growth on produce surfaces. Produce (cantaloupe, tomato, endive, and spinach) was sampled from two farms for each produce type (n=30). Aerobic bacteria, lactic acid bacteria (LAB), yeasts/molds, enterococci, and coliforms were enumerated using appropriate media. For each sample, 4-12 isolated colonies from each medium were submitted to biochemical identification. Antagonism of recovered isolates against pathogens was determined using the Agar Spot method. Produce was spot-inoculated with a suspension of bacteria showing in-vitro antagonistic activity against S. enterica Typhimurium LT2 and E. coli O157:H7 then stored at 25?C for 24 h. Each sample was spot-inoculated with a suspension including both pathogens and stored at 25?C. At 0, 6, 12, and 24 h of storage, loose and strong attachment of pathogens on the surface was determined. Geraniol-loaded NPs were prepared by flash nanoprecipitation. Inhibition of Escherichia coli O157:H7 and S. Typhimurium LT2 was tested in vitro and on produce at 5?C, 15?C, and 25?C for up to 10 days. The organisms isolated from the surface of the various produce commodities were diverse; 1,389 isolates were isolated from the surfaces of cantaloupes, tomatoes, spinach, and endive. Of these, 109 (7.8%) showed antagonism activity in vitro against S. Typhimurium LT2 and 91 (6.6%) against E. coli O157:H7. Staphylococcus antagonistic isolates showed larger zones of inhibition against both pathogens than the other antagonistic isolates recovered from produce. On produce surfaces, the endive-recovered isolate Escherichia coli and the cantaloupe-recovered isolated Escherichia hermannii depressed the growth of both pathogens. Geraniol loaded NPs inhibited S. Typhimurium and E. coli O157:H7 growth at 0.4 and 0.2 wt.%, respectively. Pathogen reductions on treated produce ranged from 1.2 to 6.0 log10 CFU/cm^2. In summary, antimicrobial NPs and microorganisms naturally present on produce may be useful for the post-harvest decontamination of fresh produce, from cross-contaminating microbial pathogens.