Modeling the Physiological Response of Salmonella to Heat Shock during Slow Cooking Processes

Introduction: Sublethal heating, as can occur during slow cooking of meat products, is known to induce increased thermal resistance in Salmonella. However, very few studies have addressed the kinetics of this response.  Several recent studies have reported improved thermal inactivation models that include the effect of prior sublethal history on subsequent thermal resistance; however, none of these models were based on cellular-level responses to the sublethal conditions.

Purpose: To quantify heat stress regulation gene transcript levels in Salmonella enterica subsp. entericaserovar Montevideo strain S5-403 subjected to various time-temperature profiles, in order to better model the physiological response to sublethal heat stress that can occur during slow cooking operations.

Methods: Salmonella culture (100 μl) was incorporated into 0.2 ml PCR tubes containing 100 μl of TSB .  Samples were then subjected to various heating profiles (including a sublethal holding period at 40 or 45°C for 5, 10, 15, 30, 60, or 180 min) using a PCR thermocycler.  RNA was then extracted from Salmonella in the heat-treated samples and its quality assessed on an Agilent 2100 Bioanalyzer.  cDNA synthesized from the extracted RNA was used to assay for and quantify heat stress regulation gene transcript levels for stress genes rpoH, dnaK, clpB, ibpA, htpG, and degP using the ABI Prism 7000 Detection System (Applied Biosystems). 

Results: The transcript levels were evaluated in terms of the influence of sublethal holding temperature, holding time, and their interaction.  For example, in comparing the initial, maximum, and final log copy numbers for each gene, ibpA displayed significant changes in transcript level over time, with a significant increase during the first 15 min at 40°C (P < 0.05), followed by a decrease to a final level (at 180 min) that was still greater than the initial level (P < 0.05).

Significance: The quantitative description of the cellular-level responses to sublethal thermal injury will enable improved population-level models formulated on these underlying kinetics, rather than purely mathematical constructs.