Acid adaptation in Salmonella Enteritidis was characterized by phenotypic and gene‐expression analyses. S. Enteritidis cells at log‐phase and stationary‐phase were kept at pH 4.5 to 6.0 for 1 to 4 hours. All treatments induced various levels of acid tolerance response that were dependent on pH, exposure time and growth phase. This acid adaptation resulted in tolerance to 50 °C and 8% NaCl regardless of the growth phase. However, the tolerance of log‐phase and stationary‐phase cells to low temperatures (4 and –20 °C) was increased and decreased, respectively. RT‐qPCR analysis revealed that genes involved in tolerance to acid (SEN1564A and cfa), heat (rpoH, uspB, and htrA), salt (proP, proV, and osmW), and cold (cspA, cspC, and csdA) stress were generally upregulated after acid adaptation. These results provide an initial insight into mechanisms of acid adaptation and induced cross protection in S. Enteritidis. Practical Application Stress tolerance acquisition resulting from acid adaptation in foodborne pathogens poses a great threat to food safety. The current work showed that acid adaptation induced direct tolerance and cross‐tolerance to high temperature, low temperature, and salt in Salmonella Enteritidis, possibly due to the upregulation of stress tolerance‐related genes. These results provide key insights into acid adaptation mechanisms and efficient control of S. Enteritidis.

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