Filament Formation in Listeria monocytogenes

Listeria monocytogenes strains SLCC 5764 cells were challenged with various NaCl concentrations and levels of pH. Cells were propagated in tryptic soy broth containing 250 to 1,500 mM NaCl or TSB at pH values ranging from 3.5 to 9.5.Microscopic analyses of the listerial cells following incubation revealed morphological changes under several adverse conditions. Filament formation occurred at NaCl concentrations above 1,000 mM with an increase in filament length as NaCl concentration increased. The same phenomenon was observed at pH values of 5.0 to 6.0 (adjusted with citric acid) and at pH > 9.0 (NaOH). The length of the filaments increased as the growth environment became more challenging. Cellular elongation of L. monocytogenes cells grown under these conditions suggests that an adaptation mechanism for survival may be involved. Since long filament structures may form under certain stressful conditions, these observations may be useful in preventing misidentification of L. monocytogenes .