Purpose: The objective of this study was to investigate the molecular and physiological mechanisms of cells exposed to low water activity.
Methods: Total RNA-Seq was used to determine transcriptional changes of Salmonella enterica ser. Typhimurium inoculated on filters and equilibrated at aw 0.11 and control aw (aw 1.0). KEGG Orthology classification was performed and knock-out mutants of genes of interest were created to assess their role on Salmonella’s ability to survive desiccation. Wild-type (WT) and mutant strains were inoculated on micro glass-beads, dried, equilibrated to aw 0.11 and 1.0 and enumerated, while phenotypical changes were compared by scanning electronic microscopy.
Results: The transcriptional analysis revealed 290 genes upregulated in cells at aw 0.11 compared to 1.0, including two virulence genes: sopD and sseD. The viability of sopD and sseD mutants dried on beads was significantly lower than WT cells (≥1 log CFU, P≤0.02). After exposure to aw 0.11 the differences in viability were greater than 2 log CFU between WT and mutants (P<0.01). Mutant cells had very distinct morphology from the WT with shorter and rounder cells. Micrographs of dried cells and exposed to aw 0.11 depicted an evident indentation in the middle of the cell and the lack of solid exopolymeric substances, with only a fibrous and filamentous net present.
Significance: Understanding which cellular components and molecular regulators are involved in Salmonella’s ability to survive thermal processing on low aw matrices is fundamental to combat persistence of this foodborne pathogen on low aw matrices and its acquired thermal tolerance. This work discovered the connection of two genes with Salmonella’s survival to desiccation.