Effect of NaCl on Thermal Resistance, Antibiotic Resistance, and Human Epithelial Cell Invasion of Listeria monocytogenes

This study evaluated the effects of NaCl on heat resistance and Caco-2 cell invasion of Listeria monocytogenes in broth media and sausage. A 10-strain mixture of L. monocytogenes was inoculated in tryptic soy broth containing 0.6% yeast extract (TSBYE), and sausage formulated with 0, 2, 4, and 6% NaCl. The medium was stored at 7, 15, 20, and $25^{\circ}C$ for 3-16 d, and medium samples were withdrawn at the appropriate time and challenged to 55, 60, and $63^{\circ}C$ to evaluate the thermal resistance of the pathogen. Sausage samples were stored at 7 and $25^{\circ}C$, and they were exposed to $63^{\circ}C$ to evaluate thermal resistance. NaCl-habituated L. monocytogenes strains NCCP10811 and NCCP10943 were examined for 12 antibiotics and Caco-2 cell invasion assay (only L. monocytogenes NCCP10943). Bacterial populations of L. monocytogenes generally increased (p<0.05) during the heat challenge as NaCl concentrations increased in both TSBYE and sausage samples. The antibiotic resistance of L. monocytogenes was not observed ($p{\geq}0.05$) when it was exposed to a single concentration of NaCl in TSBYE, but the pathogen obtained resistance to some antibiotics when exposed to a sequential increase of NaCl concentration. Invasion efficiency of L. monocytogenes NCCP10943 was not increased ($p{\geq}0.05$) with NaCl concentration increase. These results indicate that NaCl may increase the resistance of L. monocytogenes to heat and to some antibiotics, but may not increase Caco-2 cell invasion of L. monocytogenes.