Purpose: The purpose of this study was to investigate the survival characteristics and changes in gene expression profiles associated with fatty acid biosynthesis of five serovars of Salmonella exposed to a low water activity food (sugar) over a 14-day period.
Methods: Five serovars of stationary phase Salmonella (S. Typhimurium ATCC 2486, S. Enteritidis H4267, S. Tennessee ARI-33, S. Tennessee S13952 and S. Tennessee K4643 [peanut butter outbreak strain]) were each inoculated into granular sugar (treatment; Aw=0.50) or sugar water (0.25% w/v; Aw= 0.99; control) and held aerobically over a 14-day period at 25°C. First, survival of the each strain was tested by serial diluting and spread plating on TSA and XLT-4 plates at selected sampling times (0, 1, 3, 5, 7, and 14 days). Then, gene expression was evaluated by extracting total RNA of each strain using RNeasy Mini Purification Kits, and Real-Time Reverse-Transcriptase Polymerase Chain Reaction (qRT-PCR) was performed to compare gene expression profiles. The Relative Expression Software Tool (REST; 2009) was used to analyze gene expression using recA and ffh as reference genes. All experiments were performed in triplicate.
Results: After 14 days incubation at 25°C, there was a 2.5 to 3.9 log reduction for all five strains. S. Tennessee ARI-33 and S. Tennessee K4643 displayed greater survival than S. Typhimurium, S. Enteritidis and S. Tennessee S13952, which showed no growth on XLT-4 after 14 days incubation. The fab A gene (unsaturated fatty acid biosynthesis) was observed to be up-regulated for all strains for at least one sampling time. S. Typhimurium and S. Enteritidis increased expression of the cfa gene (cyclopropane fatty acid biosynthesis) over 14 days and concurrently had a lower survival rate.
Significance: The results suggested that low water activity environments might trigger unsaturated fatty acid biosynthesis of Salmonella, and cyclopropane fatty acid synthesis is not favorable for survival of the five serovars tested over 14 days.