ProP Is Required for the Survival of Desiccated Salmonella enterica Serovar Typhimurium Cells on a Stainless Steel Surface

Abstract

dConsumers trust commercial food production to be safe, and it is important to strive to improve food safety at every level. Several outbreaks of food-borne disease have been caused by Salmonella strains associated with dried food. Currently we do not know the mechanisms used by Salmonella enterica serovar Typhimurium to survive in desiccated environments. The aim of this study was to discover the responses of S. Typhimurium ST4/74 at the transcriptional level to desiccation on a stainless steel surface and to subsequent rehydration. Bacterial cells were dried onto the same steel surfaces used during the production of dry foods, and RNA was recovered for transcriptomic analysis. Subsequently, dried cells were rehydrated and were again used for transcriptomic analysis. A total of 266 genes were differentially expressed under desiccation stress compared with a static broth culture. The osmoprotectant transporters proP, proU, and osmU (STM1491 to STM1494) were highly upregulated by drying. Deletion of any one of these transport systems resulted in a reduction in the long-term viability of S. Typhimurium on a stainless steel food contact surface. The proP gene was critical for survival; proP deletion mutants could not survive desiccation for long periods and were undetectable after 4 weeks. Following rehydration, 138 genes were differentially expressed, with upregulation observed for genes such as proP, proU, and the phosphate transport genes (pstACS). In time, this knowledge should prove valuable for understanding the underlying mechanisms involved in pathogen survival and should lead to improved methods for control to ensure the safety of intermediate-and low-moisture foods.

Thermal Resistance and Gene Expression of both Desiccation-Adapted and Rehydrated Salmonella enterica Serovar Typhimurium Cells in Aged Broiler Litter

“…Our finding is consistent with data from some previous studies (12,14,15). These observations may indicate that while the formation of filaments may be critical for long-term persistence in low-a w environments, this may not be the case when bacteria are desiccated under certain conditions, such as in aged broiler litter.…”

“…Conflicting results, however, were obtained in the present study; no induction of the fadA gene was detected in S. Typhimurium under desiccation conditions in aged broiler litter (P Ͼ 0.05). Similarly, some authors also observed no expression of the fadA gene in Salmonella in response to various desiccation conditions (11,12,14). This discrepancy could possibly be accounted for by the different water activities accompanying different methods used to develop a desiccation environment.…”

“…As these genes are not necessary for survival at high a w , only the expressions of a few essential genes in Salmonella remained active under these conditions. In the previous study by Finn et al (12), otsAB in S. Typhimurium, which had been upregulated under desiccation stress, returned to basal levels following a 30-min rehydration, although speculatively, it is possible that the regulations of the dnaK and grpE genes, involved in heat shock protein synthesis, are somehow linked, in a manner that is as yet undefined. While the dnaK gene was upregulated under desiccation stress, a situation similar to that of the grpE gene, their fold changes in expression simultaneously became much lower following rehydration (P Ͻ 0.05).…”

“…Two genes, otsA and otsB, are involved in trehalose biosynthesis (35). Finn et al (12) reported that otsAB was upregulated Ͼ11-fold in S. Typhimurium under desiccation stress. A recent study by Fong and Wang (13) also showed the upregulation of otsB in S. enterica cells after a 6-day desiccation in peanut oil.…”

“…To perform rehydration after 3-, 12-, and 24-h desiccation adaptations, the inoculated litter (10 g) was mixed with 30 ml of sterile water in a Whirl-Pak bag (Nasco, Fort Atkinson, WI) (12) and homogenized by using a Seward 400 Circulator Lab blender (Seward Ltd., Worthing, West Sussex, UK). The homogenate was kept at room temperature for 3 h.…”