Environmental adaptation and stress response of Salmonella enterica in peanut oil, peanuts and chia seeds

In North America, outbreaks of Salmonella in recent years have been linked to low-water activity (aw) foods, such as tree nuts, peanut butter and chia seed powder. The unusual emergence in microenvironments that should otherwise limit bacterial survival highlights the need for the elucidation of mechanisms that enhance Salmonella survival in low aw foods, which are currently poorly understood. The purpose of this study was to evaluate the response of Salmonella enterica to two stressors commonly encountered in low-aw food processing, desiccation and heat treatment. Five strains representing different serotypes of S. enterica were inoculated onto food matrices with varying aw: peanut oil (aw 0.521 ± 0.003), peanuts (aw 0.321 ± 0.20) and chia seeds (aw 0.585 ± 0.003) to identify survival characteristics in low-aw environments. To assess the effect of stress pre-adaptation on survival, peanut oil-desiccated cells and/or cells shocked at 45°C were subsequently subjected to 70°C. Lastly, the relative expression levels of five stress response or virulence genes (i.e. invA, fadA, otsB, rpoE and dnaK) were assessed following heat treatment or desiccation using quantitative polymerase chain reaction (qPCR). S. enterica exhibited long-term survival in the low-aw foods (up to 105 days) and showed a strain-specific response. S. Hartford and S. Thompson were identified as persistent in these low-aw foods, while Typhimurium was identified as the least persistent serotype. Furthermore, cells pre-exposed to six days of desiccation in peanut oil and/or 45°C heat for three minutes exhibited significantly (p