Changes in Barotolerance, Thermotolerance, and Cellular Morphology throughout the Life Cycle of Listeria monocytogenes

Changes in barotolerance, thermotolerance, and cellular morphology throughout the life cycle ofListeria monocytogenes were investigated. For part 1 of this analysis,L. monocytogenes ATCC 19115 was grown to log, stationary, death, and long-term-survival phases at 35°C in tryptic soy broth with yeast extract (TSBYE). Cells were diluted in whole milk that had been subjected to ultrahigh temperatures (UHT whole milk) and then high-pressure processed (HPP) at 400 MPa for 180 s or thermally processed at 62.8°C for 30 s. As cells transitioned from the log to the long-term-survival phase, the D400 MPa and D62.8°C values increased 10- and 19-fold, respectively. Cells decreased in size as they transitioned from the log to the long-term-survival phase. Rod-shaped cells transitioned to cocci as they entered the late-death and long-term-survival phases.L. monocytogenes strains F5069 and Scott A showed similar results. For part 2 of the analysis, cells in long-term-survival phase were centrifuged, suspended in fresh TSBYE, and incubated at 35°C. As cells transitioned from the long-term-survival phase to log and the stationary phase, they increased in size and log reductions increased following HPP or heat treatment. In part 3 of this analysis, cells in long-term-survival phase were centrifuged, suspended in UHT whole milk, and incubated at 4°C. After HPP or heat treatment, similar results were observed as for part 2. We hypothesize that cells ofL. monocytogenes enter a dormant, long-term-survival phase and become more barotolerant and thermotolerant due to cytoplasmic condensation when they transition from rods to cocci. Further research is needed to test this hypothesis and to determine the practical significance of these findings.