Purpose: Due to scope of the Foster Farms salmonellosis outbreak, we hypothesized that food isolates associated with the outbreak have enhanced resistance to processing stresses such as heat and antimicrobials. The purpose of this study was to compare the heat resistance of nine isolates associated with the Foster Farms outbreak with other common serovars.
Methods: Salmonella Typhimurium 14028, Salmonella Heidelberg SL476 and nine food isolates representing six strains associated with the outbreak were heat shocked at 56°C to mimic hard scald during poultry processing. Aliquots were serially diluted and pour plated or spread plated at 0, 2.5, 5, and 7.5 min (log phase) and 0, 15, 30, 45, and 60 min (stationary phase); time-to-equilibrium was approximately 2.2 min. Statistically significant differences between isolates and reference strains were determined by repeated measures analysis of variance (ANOVA) and Dunnett’s test, with significance defined at P<0.05.
Results: There were no significant heat resistance differences among isolates in log phase cells; all but one isolate reached the detection limit by 7.5 min. During stationary phase, 6 of 9 and 2 of 9 outbreak-associated food isolates showed significantly increased heat resistance compared to Salmonella Typhimurium 14028 and Salmonella Heidelberg SL476, respectively (P<0.05). Final average survival ranged 0 to 3.3 log CFU/ml.
Significance: Increased heat resistance of many of the outbreak-associated food isolates may contribute to survival through processing (e.g., scald) or final cook steps. Further research is crucial for understanding the variation in heat resistance and other stress among and within common S. enterica serovars in order to improve food safety practices and regulations.