Heat resistance of Salmonella enterica is increased by pre-adaptation to peanut oil or sub-lethal heat exposure

• Exposure to a six-day incubation in peanut oil increases Salmonella heat resistance at 70 °C.
• Exposure to a 3-min heat treatment at 45 °C increases Salmonella heat resistance at 70 °C.
• Quantitative PCR revealed differential transcriptional profiles among Salmonella strains.
• The number and lethality of hurdles should be considered to avoid cross-protection.
• High-risk Salmonella strains need to be included in risk profiling.

Cross-protection represents a considerable challenge in the food industry where hurdled interventions are often employed to reduce Salmonella contamination. The heat resistance of Salmonella strains from five serotypes (i.e., Typhimurium, Enteritidis, Tennessee, Thompson and Hartford) at 70 °C was determined by measurement of viable cell populations before and after adaptation to two common stresses employed in low-water activity food processing, desiccation and sub-lethal heat treatment. Survival of Salmonella at 70 °C significantly increased (p < 0.05) following the six-day incubation in peanut oil (aw 0.52 ± 0.00) and/or the exposure to a sub-lethal heat treatment at 45 °C for 3 min. Quantitative PCR revealed upregulation of two desiccation stress-related genes, fadA and otsB, following the peanut oil incubation, whereas heat treatment induced upregulation of a heat-resistance gene, dnaK. Invasion gene invA and alternative sigma factor rpoE were downregulated following either of the treatments. Interestingly, different Salmonella strains yielded different transcriptional profiles. The strain-specific resistance phenotypes and transcriptional profiles provided further insights into the mechanisms employed to tolerate desiccation and heat stresses in the food industry.