Previous findings showed that Listeria monocytogenes exhibits higher heat tolerance to thermal exposure at 60°C when pre-exposed to sublethal heat stress at 48°C for 30/60 min. The objective of this study was to investigate the morphological changes that occurred in L. monocytogenes serotype 1/2a cells as visualized by transmission electron microscopy (TEM) after exposure to sublethal heat stress at 48°C for 60 min and in combination with lethal concentration of carvacrol for 30 min. The TEM micrographs revealed thickening of cell wall and cell membrane, and clumping of cytoplasm when subjected to sublethal heat stress followed by carvacrol treatment. These studies indicate that L. monocytogenes cells when adapted to heat stress may alter its morphology to protect themselves against carvacrol. Further studies will investigate the effect of increased contact time with carvacrol and with other essential oils on the ultrastructural changes in L. monocytogenes cells adapted to heat stress.

The objective of this study was to investigate the differences in survival of L. monocytogenes EGD (Bug600) (serotype 1/2a) in various disinfectants and essential oils after sublethal heat stress at 48°C for 60 min. The survival of heat stressed L. monocytogenes cells was decreased in lethal acid (HCl or H3PO4 pH 2.5), lethal oxidative stress (H2O2 1000 ppm) and lethal quaternary ammonium compounds (QAC 2.5-3.5 ppm) compared to non-stressed control cells. By contrast, the survival of heat stressed L. monocytogenes cells was higher in lethal alkali (NaOH or KOH pH 12) than the control cells. Also, the survival of heat stressed L. monocytogenes cells was higher in lethal carvacrol (428 ppm) and bay oil (1100 ppm) but was decreased in lethal red thyme oil (300 pm) compared to control. These findings indicate that the heat stressed cells of L. monocytogenes are not easily killed by alkali-based disinfectants and essential oils containing carvacrol and bay oil. Therefore, disinfectants and essential treatments should be carefully considered when heat stressed cells of L. monocytogenes may be present.