Purpose: This study aimed to investigate whether acid or alkaline adaptation alters Salmonella acid/heat resistance and virulence gene expression, as well as, to find out whether the alternation in Salmonella resistance was due to the changes in membrane lipid composition/expression of stress-related genes or both of them.
Methods: S. Enteritidis was cultivated at different pHs (5.3-9.0) adjusted by LA or TSP. Adapted cells were subject to simulated gastric fluid (pH=2.0) and thermal treatment (54, 56, 58, and 60 °C) to determine its acid/heat resistance. D-values were calculated based on survivor curves. Membrane lipid composition was determined by gas chromatography. The transcription levels of stress-related genes (rpoS and rpoH) and virulence-related genes (spvR, hilA, sefA, and avrA) were evaluated by real-time PCR. Mean values were compared using ANOVA.
Results: Results showed that cells adapted at pH 5.3 had significantly (P < 0.05) higher acid (D-value = 7.9 min) and heat resistance (D60oC-values = 0.87 min), followed by the control and alkali-adapted cells. The increased resistance was correlated with the decreased membrane fluidity; whereas no upregulation of rpoS and rpoH was found in acid-adapted cells. About 6.0, 2.1, and 2.5-fold upregulation of spvR, avrA, and hilA were observed in cells adapted to pH 9.0, whereas sefA had its highest expression level in the control cells.
Significance: This study indicates that the regulation of the cytoplasmic membrane rather than the stress-related genes may play a more crucial role in conferring acid and heat resistance on acid adapted S. Enteritidis.