Effect of Addition of Oregano Essential Oil and Calcium Chloride during Acid Adaptation on the Acid Tolerance Response and Concurrent Multiple Stress Resistance of Salmonella

To inhibit the development of adaptive acid tolerance response (ATR) and concurrent multiple stress resistance in Salmonella induced by the residual acid environment from carcass decontamination by organic acid spraying, the inhibitory effect of the addition of oregano essential oil and calcium chloride at different subinhibitory concentrations during acid adaptation on the ATR of Salmonella was investigated by determining the survival ability of Salmonella in acidic, thermal, hyperosmotic and oxidative environments before and after adaptation, and the possible mechanism was explored by determining the relative expression of stress resistance genes before and after treatments. The results indicated that the residual acid (pH 5.4) generated during carcass processing might induce ATR and heat-resistant cross-protection in Salmonella, suggesting a potential food safety risk. After 2 h treatment with hydrochloric acid at pH 3, the loss of the adapted strain in the 1/2 MIC oregano essential oil and 10 mmol/L CaCl₂ treatment group increased by 1.32 (lg(CFU/mL)) compared with the control group, and oregano essential oil combined with CaCl₂ had a synergistic inhibitory effect on ATR. The addition of oregano essential oil and calcium chloride during acid adaptation down-regulated the expression of the phoP and rpoS genes, which in turn intervened in the two-component system and global transcription factors, thereby suppressing ATR development. Sub-inhibitory concentrations of oregano essential oil inhibited the formation of the acid and heat resistance of acid-adapted Salmonella, but increased the resistance to hyperosmosis, whereas CaCl₂ at a concentration of 10 mmol/L combined with sub-inhibitory concentrations of oregano essential oil reduced the osmotic resistance. The results of the current study help to understand the effects of essential oil and CaCl₂ on the acid adaptation process of Salmonella and provide a theoretical basis for risk control for the use of organic acids in the beef industry.