Assessments of plant and microbial interactions of Escherichia coli O157:H7 and Salmonella on romaine lettuce and development of molecular tools for detection and source matching of pathogenic and commensal Escherichia coli applied to regional investigati

Growth kinetics and adherence dynamics of Escherichia coli O157:H7 isolates and Salmonella serotypes were compared on Romaine lettuce leaves which were inoculated with S. Michigan, S. Agona, S. Montevideo, or one of three E. coli O157:H7 isolates and were recovered over 72 hours. The results show a significant increase (p<0.05) in recovered colony forming units (CFU)/leaf over 72 hours from the primary and secondary washes for all bacteria inoculated on greenhouse- and field-grown Romaine lettuce.;A multiplex real-time PCR assay was developed to identify pathogenic E. coli; it generated a unique melt-curve permitting easy interpretation of the presences or absence of pathogenic E. coli. Forty-one environmental bacterial species, 29 E. coli samples, and 16 human pathogens were tested and yielded no false-positives.;Evaluating the power of discrimination of microarray and PFGE at source matching 16 environmental E. coli isolates found microarray to have more discriminating power. This level of discrimination surpasses the PFGE analysis' sensitivity, therefore proper and more precise interpretation of subspecies relatedness is likely. The microarray's analysis, a more comprehensive genomic assessment than PFGE, yielded better agreement of origin for the E. coli isolates.;The influence of biosurfactant-producing Pseudomonas fluorescens on growth kinetics and adherence dynamics of E. coli O157:H7 on greenhouse-grown Romaine lettuce was investigated. The effect of biosurfactant secreted by P. fluorescens 123 (biosurfactant +) or the presence of P. fluorescens EG3 (biosurfactant -) on the adherence of E. coli O157:H7 on non-wounded greenhouse-grown Romaine lettuce over 72 hours was examined. Data showed, despite increased removal of CFU/leaf, an infectious dose remained. Recoveries of E. coli O157:H7 with P. fluorescens EG3 and E. coli O157:H7 with P. fluorescens 123 were compared, 88.8% (R2=0.888) of the variance can be explained by the presence of biosurfactant and the effect of time ( p<0.05). Approximately 60% and 86% of the variance in the recovery between E. coli O157:H7 alone, with P. fluorescens EG3, or with P. fluorescens 123, respectively, can be explained by treatment and time (R2=0.597, R 2=0.861) (p<0.05).