Date of Award

12-2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Food Science

Committee Chair

Haley F. Oliver

Committee Member 1

Arun K. Bhunia

Committee Member 2

Amanda J. Deering

Committee Member 3

Manpreet Singh

Committee Member 4

Leslie N. Csonka

Abstract

The U.S. food industry employs multiple strategies to prevent foodborne illness including the use of antimicrobials, sanitizers, and heat to reduce or eliminate pathogens in food products. However, Listeria monocytogenes and Salmonella enterica isolates have widely varying tolerance to common processing stresses. Additionally, both pathogens display considerable variation in virulence among serovars and strains. The goal of these studies was to characterize the range of virulence and persistence capabilities for L. monocytogenes and S. enterica isolates. To this end, we determined the prevalence of inlA mutations in L. monocytogenes collected from retail deli surfaces, and used PFGE to study the persistence of L. monocytogenes in the deli environment. We tested nine Salmonella Heidelberg outbreak-associated isolates and two reference isolates for tolerance to heat and PAA-based sanitizers, and determined their ability to attach to abiotic surfaces. Additionally, we characterized the heat tolerance phenomenon in three Salmonella Heidelberg isolates using RNA-sequencing. We found that some L. monocytogenes strains persisted within delis for more than two years, despite the use of enhanced daily SSOPs and deep cleaning procedures. We also found these isolates rarely (< 2% prevalence) contained virulence-reducing mutations in inlA, in contrast to the high prevalence of those mutations (>35%) in food isolates. This indicates that cross-contamination of L. monocytogenes to food in delis is a significant public health risk. Six of nine Salmonella Heidelberg isolates had enhanced heat tolerance; two had higher heat tolerance than Salmonella Heidelberg reference strain SL476 (p < 0.05). The heat-tolerant outbreak-associated strains also attached better to an abiotic surface under stress conditions than heat-sensitive isolates or SL476. RNA-sequencing revealed that heat-tolerant isolates had higher expression of genes encoding heat shock proteins, stress tolerance systems, and virulence factors at 37°C. This indicates that some outbreak-associated isolates may be more likely to survive processing stresses and cause illness in consumers.

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