Purpose: The objective of this study was to identify the genes activated by NaCl, which prevents L. monocytogenes from sensitizing against heat stress and increases the pathogenicity.
Methods: Nine strains of L. monocytogenes were inoculated individually in 10 ml tryptic soy broth plus 0.6% yeast extract, which was supplemented with 0, 1, 2, and 4% NaCl to be exposed to NaCl, and the cells were then subjected to at 60°C for 60 min to select L. monocytogenes strains, which are not heat-sensitized by NaCl. After heat challenge, L. monocytogenes strains were categorized into heat-sensitized strains (HS) and non-sensitized strains (NS). mRNA was then extracted from HS and NS, and cDNA was prepared to analyze transcriptomes (inlA, inlB, opuC, ctc, betL, osmC, and gbuB) by qRT-PCR.
Results: After heat challenge L. monocytogenes strains were categorized in to HS (L. monocytogenes strains NCCP 10805, NCCP 10806, NCCP 10807, NCCP 10810, NCCP 10811, and NCCP 10920) and NS (L. monocytogenes strains NCCP 10808, NCCP 10809, and NCCP 10943). Of seven tested genes, gene expression levels of pathogenicity related genes (inlA and inlB) and stress response genes (opuC, betL, and ctc) increased (P < 0.05) for NS (L. monocytogenes strains NCCP 10808 and NCCP 10809) after being exposed to NaCl as NaCl concentration increased. However, only betL gene expression level for HS (L. monocytogenes NCCP 10811) was increased (P < 0.05) as NaCl concentration increased.
Significance: The effect of NaCl on heat-sensitizing L. monocytogenes is strain-dependent, and opuC and ctc genes may prevent NS from heat-sensitizating by NaCl. Moreover, NaCl also increases invasion-related genes (inlA and inlB).