Relationship between superoxide dismutase and pathogenic mechanisms of Listeria monocytogenes

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Abstract
L. monocytogenes was examined for superoxide dismutase (SOD) activity. Two catalase-negative strains possessed at least 2-fold greater SOD activities than the catalase-positive L. monocytogenes strains examined. Growth conditions such as aeration and Fe concentration influenced the specific activity of SOD obtained from cells cultured in defined media. L. monocytogenes SOD from crude extracts and after partial purification was analyzed by polyacrylamide gel electrophoresis. Fe was associated with the single band of SOD activity detected in the gels. SOD activity appeared to be primarily extracytoplasmic. Survival of organisms in a superoxide-generating medium was studied, with photoactivation of riboflavin used as the source of free radical formation. Virulent, catalase-positive L. monocytogenes strains were relatively resistant to killing in a pH 7 superoxide-containing medium. An intact-cell assay for SOD was developed, which used the superoxide-generating system and employed the superoxide-dependent oxidation of sulfite, added to the medium, and inhibition of this oxidation by SOD. Maximal SOD activities of intact cells were observed when 100-400 .mu.g (dry weight) of viable Listeria cells/ml was added to the medium. A possible role for SOD in the pathogenesis of listeric infection is discussed.