Inactivation and occurrence of sublethal injury of Salmonella Typhimurium under mild heat stress in broth

Thermal processing of food products continues to be the standard commercial method used for enhancing microbiological safety. The influence of cell characteristics of Salmonella Typhimurium strains, such as growth phase, cell density, and the availability of nutrients and metabolites was evaluated regarding a 3?Log₁₀ reduction of surviving cells when exposed to a mild heat treatment of 55?°C. Using nonselective and selective agars it was revealed that under thermal stress Salmonella Typhimurium suffered from injuries on both outer and cytoplasmic membrane, which consequently led to cell death. Compared to stationary cells, those in exponential growth phase were more prone to thermal injury, especially on the cytoplasmic membrane. Occurrence of thermal inactivation and injury decreased with the increase of cell density, and with the increased availability of metabolites produced during cell growth, but showed no apparent correlation with nutrients in the culture broth. The results enlighten the importance of cell growth phase, cell density and metabolite availability in Salmonella Typhimurium thermal resistance. Consequently these factors should be well considered when models are constructed and used to predict thermal effects on the survival of the pathogen. Moreover, our data show that attention should be paid when selective media are used to detect sublethally injured cells.