Stress Conditioning Modulates Phenolic Antimicrobial Responses in Salmonella Typhimurium and Listeria innocua

Environmental stress adaptation can alter bacterial responses to antimicrobial interventions used in food processing and preservation. This study evaluated how prior acid, osmotic, and cold conditioning affected the susceptibility and growth behavior of Salmonella enterica serovar Typhimurium and Listeria innocua during exposure to carvacrol and thymol. Cells were conditioned in acidic tryptic soy broth (TSB, pH 5.5), osmotic (TSB supplemented with 2.5% NaCl), or low temperature (4°C) before antimicrobial treatment. Susceptibility was assessed using modified broth microdilution and minimum bactericidal concentration assays, while recovery was evaluated using 24 h optical‐density growth curves and primary growth modeling. Stress conditioning produced modest, organism‐ and compound‐specific changes in susceptibility. In S. Typhimurium , osmotic and cold conditioning increased thymol MIC from 2.5 to 5.0 mM. In L. innocua , osmotic and cold conditioning increased the carvacrol MIC from 2.5 to 5.0 mM and cold conditioning increased the thymol MIC from 2.5 to 5.0 mM. Under selected treatment conditions, adapted cells also showed shorter apparent lag phases than unstressed cells. In L. innocua time–kill assays, 0.5× MIC permitted population growth, whereas 1× MIC produced condition‐dependent reductions followed by partial regrowth in several treatments; a consistent sustained bactericidal effect was not observed across all conditions. These findings indicate that prior physiological history can influence phenolic antimicrobial susceptibility and recovery behavior and should be considered when validating natural antimicrobial interventions.