Publication Date

Fall 2015

Advisor(s) - Committee Chair

Ritchie Taylor (Director), Vijay Golla, and Colin Farrell

Degree Program

Department of Public Health

Degree Type

Master of Public Health in Environmental Health


Foodborne illness continues to be a substantial public health issue in the United States, with fresh produce being one of the leading causes of outbreaks. Understanding routes of contamination of fresh produce and how pathogens survive on plant surfaces is paramount in improving food safety and reducing risk to public health. The objectives of this study were to select environmental E.coli isolates as pathogen surrogates of Salmonella typhimurium and E.coli O157:H7, assess lettuce plant contamination by spray irrigation water, and evaluate a common industry quality control (QC) E.coli strain (ATCC 25922). Selections of E.coli surrogates were made utilizing biofilm and leaf attachment data from lab scale assays. Five surrogates were found to be similar in biofilm formation and leaf attachment capabilities of the pathogens, while the common QC strain was significantly different than Salmonella in both biofilm formation and leaf attachment (p < 0.05). Persistence of surrogates, pathogens and the QC strain on lettuce plants was assessed in greenhouse scale experiments, where it was found that all isolates were above detection levels for 22 days. Die-off rates were calculated for all isolates, with the QC strain having the greatest rate of die-off in the first experiment (k = -4.52) and the second greatest in the second experiment (-2.82) while the pathogens and selected surrogates had statistically similar and lower rates of die-off. Based on this information, current policies concerning the sampling and management of irrigation waters and crops for microbial safety may be insufficient. It is recommended that sampling methods and frequencies be adjusted for irrigation waters and fresh produce, and the use of projected die-off rates not be used for the determination of time intervals needed before a crop is safe to harvest.


Environmental Public Health | Public Health