Date of Award


Document Type


Degree Name

Master of Science (MS)


Nutrition and Food Sciences

First Advisor

Andrea Etter


Salmonella enterica is a widespread species of bacteria that consists of over 2,600 different serovars, with fewer than 100 serovars accounting for most human salmonellosis cases globally. Serovars are classified by their surface antigens and are commonly grouped into two groups – serovars that cause typhoid fever are known as typhoidal Salmonella enterica, and serovars that cause salmonellosis are non-typhoidal S. enterica. The CDC estimates that non-typhoidal S. enterica (NTS) causes 1.35 million illnesses annually in the United States, with 26,500 hospitalizations and 420 deaths. Of these illnesses, most are outbreak-associated and can be traced back to a common source.The objective of this work is to determine if outbreak-associated strains of S. enterica display enhanced stress tolerance compared to S. enterica strains that have not been involved in an outbreak. S. enterica serovars Enteritidis, Heidelberg, Typhimurium, Newport, and I 4,5,[12]:i:- (monophasic Typhimurium) outbreak-associated (OA) and non-outbreak-associated (NOA) strains were evaluated for enhanced stress tolerance. To determine the attachment capacity of these strains, 21 OA and 4 NOA isolates were grown statically in 96-well polystyrene plates for 24, 72, and 120 hours in 1X (nutrient- rich) and 1/20X (nutrient-poor) trypticase soy broth (TSB) conditions, at 22oC and 4oC. Sanitizer minimum inhibitory concentration (MIC) was determined for 23 OA and 6 NOA isolates through static incubation of cultures in 1X and 1/20X trypticase soy broth (TSB), with serial dilutions of Chlorox (bleach) and Inspexx-250 (PAA) at 22oC for 24 hours using OD600. Heat tolerance of 9 OA isolates was evaluated; aliquots were taken at the start of stationary phase and serially diluted into phosphate-buffered saline, and then incubated at 56°C and pour-plated at 0-, 3-, 6-, 9-, 15-, 30-, 45-, and 60-minutes post- heat-shock. Plates were incubated at 37oC for 36 hours, after which colony counts were recorded. Statistically significant differences between isolates for all assays were determined via analysis of variance (ANOVA) with Tukey’s HSD test, with significance defined at padj < 0.05. In 1/20X TSB at 4oC, three Salmonella Heidelberg OA, two monophasic OA, and one NOA monophasic isolate attached significantly better than ≥3 others. At 22oC, five Heidelberg OA, two monophasic OA, and one NOA monophasic isolate attached significantly better than ≥3 others. In 1X TSB at 4oC and 22oC, only one OA Heidelberg attached significantly better than ≥3 others. MICs for bleach, 1X TSB were >200ppm and >100ppm in 1/20X TSB (padj < 0.01). MICs for PAA, 1X TSB were >175ppm vs 25.9ppm in 1/20X TSB (padj < 0.01). After the initial period of 6 minutes, all isolates showed a significant decrease in growth (padj < 0.05). At 45 minutes post-scald, 3 isolates plated were below detectable limits (less than 1 CFU/mL), followed by 5 isolates at 60 minutes post-scald. One isolate was still detectable at 60 minutes with a count of 3.43*103 CFU/mL. Findings suggest that there is no significant difference in attachment capacity and MICs between the outbreak-associated and non-outbreak-associated isolates. The sanitizer tolerance and heat tolerance observed in several of the strains are high and could pose a threat to the food processing industry.



Number of Pages

97 p.

Available for download on Friday, August 16, 2024