Listeria monocytogenes cells injured by high hydrostatic pressure and their recovery in nutrient-rich or -free medium during cold storage

Cells of Listeria monocytogenes suspended in phosphate-buffered saline (PBS) were treated by high hydrostatic pressure (HHP; 500MPa, 25 degrees C, 10min), diluted by ten folds using trypticase soy broth (TSB) or PBS, and stored at cold temperatures of 0-15 degrees C. Viable cell count in TSB increased logarithmically close to the initial count at each storage temperature, while that in PBS increased temporarily and subsequently decreased to almost nondetectable level except the case at 15 degrees C, where it showed logarithmic increase thereafter. Based on proliferation experiments where their healthy cells were inoculated to TSB or to PBS containing their heat-killed dead cells, it was suggested that increase in the viable count of HHP-treated cells in TSB and PBS could be ascribed to the recovery of colony forming ability and/or proliferation depending on the cold storage temperature.