Mechanism of S. aureus ATCC 25923 in response to heat stress under different water activity and heating rates

Water activity (aw) and heating rates are two major factors altering heat resistance of food-borne pathogens. The specific mechanism of heat resistance triggered by these two factors is still unclear. In this study, S. aureus ATCC 25923 was inoculated in walnut shells with two aw values (0.965, HC samples; 0.586, LC samples), and heated to 60 °C with two heating rates of 5 °C/min (HT and LT samples, respectively) and 0.5 °C/min (LST samples) by a heating block system (HBS). The transcriptional response of S. aureus ATCC 25923 to aw and heating rate was characterized by using RNA-Seq. The results showed that the cell integrity of S. aureus ATCC 25923 was damaged under all heat treatment conditions. Genes involved in the pathway of ribosome were all up-regulated in HT samples while down-regulated in LST samples. Some genes (lysC, vraS, vraR, sgt and etc) involved in cell-wall synthesis were down-regulated more than 2-fold in HT samples. Genes involved in the pathways of energy metabolism were also down-regulated in LST samples. The cells in LT samples prevented external damage by stimulating the synthesis of cell wall components and inhibiting membrane function related metabolisms as compared to those in HT samples. The suppression of energy metabolism in LST samples indicated that cells under this condition needed to reduce metabolic activity to survive. These results provide valuable insights into survival strategies of S. aureus ATCC 25923 under different thermal treatment conditions and clarify the mechanism of heat resistance altered by aw and heating rate for S. aureus ATCC 25923.