Differential Expression of Salmonella Tennessee Membrane-associated Genes in a Low Water Activity Food

Introduction:  Survival of Salmonella in low water activity foods has resulted in numerous product recalls and foodborne outbreaks.  It is pertinent to determine the adaptation mechanisms utilized by Salmonella to survive in this adverse environment for prolonged periods of time.  Once these adaptive mechanisms are defined, we can implement antimicrobials or processing conditions that are antagonistic to survival and reduce the likelihood of Salmonella contamination in these foods. 

Purpose: The purpose of this study was to investigate the changes in gene expression profiles associated with fatty acid biosynthesis of SalmonellaTennessee exposed to a low water activity food (sugar).

Methods:  Stationary phase Salmonella Tennessee (peanut butter outbreak strain K4643) was inoculated on sugar (treatment) or held in tryptic soy broth (TSB, control) and incubated for 1 h at 25°C. Total RNA was extracted using RNeasy Mini Purification Kits and the quantity, quality and purity of RNA was evaluated. This was followed by Real-Time Reverse-Transcriptase Polymerase Chain Reaction (qRT-PCR) to compare gene expression profiles. The Relative Expression Software Tool (REST) was used to analyze gene expression using recA and ffhas reference genes. The experiment was performed in triplicate.

Results:  Genes involved in unsaturated (fabA) and saturated (fabD) fatty acid biosynthesis were observed to be significantly down-regulated 3.8- and 10.9-fold, respectively, when Salmonella Tennessee was held for 1 h in sugar (P < 0.05).  The expression of cyclopropane fatty acid synthesis encoding gene cfa was found to be up-regulated 2.1-fold (P < 0.05). Expression of fabB, also involved in unsaturated fatty acid biosynthesis, was not significantly affected.

Significance:  These results indicate that cyclopropane fatty acid alteration of the lipid membrane may be one mechanism by which Salmonella adapts to low water activity environments.  Future research will determine if these/similar results are observed in other Salmonella serovars and if higher levels of cyclopropane fatty acids are found in phospholipids of Salmonella under similar situations.