Salmonella survival and differential expression of fatty acid biosynthesis-associated genes in a low-water-activity food.

UNLABELLED: The purpose of this study was to investigate the difference in expression of fatty acid biosynthesis genes and survival of different serotypes of Salmonella when incubated in a low-water-activity (aw ) food over a 14-day period. Stationary cells of five strains of Salmonella enterica belonging to 3 different serovars (Typhimurium ATCC 2486, Enteritidis H4267, Tennessee ARI-33, Tennessee S13952 and Tennessee K4643) were inoculated into granular sugar (aW   = 0·50) and held aerobically over a 14-day period at 25°C. Survival was determined by enumerating colonies on TSA and XLT-4 plates at 0, 1, 3, 5, 7 and 14 days. Correspondingly, gene expression was evaluated for three selected genes involved in fatty acid biosynthesis and modification (fabA, fabD and cfa). After 14 days of incubation, the population was reduced from 2·29 to 3·36 log for all five strains. Salmonella Tennessee ARI-33 and Salm. Tennessee K4643 displayed greater survival than Salm. Typhimurium and Salm. Enteritidis. The increased expression of the cfa gene (involved in cyclopropane fatty acid biosynthesis) over 14 days was found associated with strains with a lower survival rate. The fabA gene (involved in unsaturated fatty acid biosynthesis) was observed up-regulated for all strains for at least one sampling time and for Salm. Tennessee ARI-33 for all time points tested, suggesting its potential role in enhancing Salmonella survival in low aw foods. SIGNIFICANCE AND IMPACT OF THE STUDY: Numerous outbreaks of salmonellosis associated with low-water-activity foods have been reported. Therefore, the adaptive mechanisms utilized by Salmonella to survive in low-water-activity foods for prolonged periods of time need to be better understood. The results in this study showed that low-water-activity environments increase expression of gene fabA, which is involved in unsaturated fatty acid biosynthesis of Salmonella, while the increased expression of cfa, associated with cyclopropane fatty acid synthesis, was associated with decreased survival over 14 days.