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


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<0.05) higher resistance to 70°C heat treatment. qPCR revealed various degrees of up- and down-regulation amongst the characterized genes and across different strains under the desiccation and heat treatments. Serotypes Hartford and Thompson displayed the highest up-regulation in otsB and fadA, genes vital in desiccation response, consistent with their persistence in the survival assays. Moreover, differential expression of dnaK, a gene important for heat-tolerance was also observed across all Salmonella strains. The current research emphasizes the adaptable nature of S. enterica to stresses encountered in low-aw food processing. Additionally, unique stress response characteristics among Salmonella strains highlight the need for tailored mitigation strategies regarding high-risk Salmonella strains in the food industry.

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