Investigating Stress Tolerance of Multiple Salmonella enterica Strains Associated With Foodborne Outbreaks

Salmonella enterica is a foodborne pathogen commonly found in food processing environments. Although control methods such as heat treatment and sanitizers are often used, S. enterica has evolved strategies for survival and persistence to overcome pathogen control. This study assessed 43 outbreak-associated (OA) and nonoutbreak associated (NOA) S. enterica isolates from serovars Enteritidis, Heidelberg, Newport, Typhimurium, and monophasic Typhimurium (I 4,[5],12:i:-) for enhanced stress tolerance. Heat shock at 56°C, minimum inhibitory concentrations (MICs) for sanitizers sodium hypochlorite (NaOCl) and peroxyacetic acid (PAA), and crystal violet microtiter assays were used to evaluate heat tolerance, sanitizer tolerance, and attachment capacity, respectively. Most isolates (n = 34/43) carried at least one antimicrobial resistance gene, and nearly half (n = 21/43) displayed genotypic and/or phenotypic resistance to ampicillin, ciprofloxacin, or ceftriaxone. Most isolates carried genes conferring resistance to gold (n = 43/43) and arsenic (n = 41/43), and tolerance to mercury, copper, and silver was common among monophasic Typhimurium and Heidelberg isolates. Efflux pump qacEdelta1 was detected among eight Heidelberg isolates. We found enhanced stress tolerance (i.e., an unusually high ability to survive and adapt to various environmental stresses) to sanitizers and enhanced attachment capacity, indicating biofilm formation. Isolates evaluated for heat tolerance survived at least 15 min at 56°C and three survived > 60 min. Overall, we found evidence of enhanced tolerance to individual stresses across both OA and NOA S. enterica. There were no strong patterns based upon serovar or OA/NOA status; however, we did find that specific enhanced stress tolerance profiles may have contributed to outbreak characteristics.