Effects on morphology and motility of Listeria monocytogenes cells subjected to sublethal stress of temperatures

Survival and growth of Listeria monocytogenes L56 (IMR isolate) was studied in trypticase soy broth (TSB) grown at 37oC before being subjected to three selected sublethal stress temperatures (55oC, 28oC and 4oC). The morphological changes and motility as affected by temperature stresses were determined using Scanning Electron Microscopy (SEM) and motility media, respectively. For this purpose, 10 ml broth containing L. monocytogenes previously grown in 300 ml of TSB at 37oC at log phase (12 hour) and stationary phase (19 hour) to 55oC, 28oC and 4oC were taken every 2 h intervals for measurement of morphological changes and motility. A total of 180 cells were measured lengthwise at 9,500x Scanning Electron Microscopy (SEM) magnification using SEMafore for each temperature treatments. The study demonstrated cells of both log and stationary phase of stressed temperature of L. monocytogenes showed a significant variation in morphology. Cells of log phase become elongated at 55oC but not at 28oC and 4oC, whereas cells of stationary phase were shorter and more coccoidal rather than elongated as in log phase cells at the three temperatures studied. Loss of motility was observed in cells stressed at stationary phase cells but not at log phase; suggesting that motility plays a role in survival of the organism under temperature stress.