Influence of Plasmids on Catalase and Superoxide Dismutase Activities in Listeria monocytogenes.

Two of nine strains of Listeria monocytogenes examined were found to contain plasmid DNA. Strain 19112 contained a 31.1 kb plasmid and strain 7644 contained a 49.4 kb plasmid. Each of the strains was cured of its plasmid by heat treatment at 46°C. Both the wild-type and plasmid-negative forms of each strain were screened for cadmium resistance. The 31.1 kb plasmid of L. monocytogenes 19112 was shown to confer resistance to cadmium. Listeria monocytogenes 7644 did not show resistance to cadmium. The plasmids for both strains were isolated and purified by CsCl density-gradient centrifugation. Each plasmid was electroporated back into the respective plasmid-negative strain. The catalase (CA) and superoxide dismutase (SOD) activities were determined for the wild type, plasmid-negative and electroporated strains. There was a significant decrease in CA and SOD activities upon loss of the plasmid from each strain of L. monocytogenes . Strain 19112 showed a 36% decrease in CA activity and an 81% decrease in SOD activity as a result of plasmid removal. Strain 7644 showed a 22% decrease in both CA and SOD activities following plasmid loss. Catalase and SOD activity levels increased for both strains following reinsertion of the plasmid through electroporation. Catalase and SOD activity levels of L. monocytogenes 7644 were higher for the transformed strain than those of the wild type. Catalase and SOD activity levels of transformed L. monocytogenes 19112 were less than in the corresponding wild type. It appears that plasmids in L. monocytogenes strains 19112 and 7644 may be involved in influencing the regulation of the production of CA and SOD. Plasmid copy number may influence the level of activity of these enzymes.

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 .