Listeria monocytogenes was examined for superoxide dismutase (SOD) activity. Two catalase-negative strains possessed at least twofold greater SOD activities than the catalase-positive L. monocytogenes strains examined. Growth conditions such as aeration and iron 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. Iron 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 superoxidecontaining 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 to 400 ,g (dry weight) of viable Listeria cells per ml was added to the medium. A possible role for SOD in the pathogenesis of listeric infection is discussed. Facultative intracellular pathogens such as Listeria monocytogenes must possess means of overcoming the nonspecific immune responses mediated by phagocytic cells and various humoral factors. The sequence in phagocytic killing of bacteria includes the formation of a toxic superoxide (*02) free radical (5) which is later eliminated by the enzyme superoxide dismutase (SOD) (22). Indirect evidence also exists for a role of * 02 in leukocyte-mediated antimicrobial activity. Baehner et al. (6) showed that the nitroblue tetrazolium reduction by leukocytes is dependent on *°02 generated by phagocytic cell metabolism, and Curnutte et al. (11) observed that neutrophils obtained from patients with chronic granulomatous disease produced low or undetectable quantities *°2 SODs from Escherichia coli have been found to contain manganese (16) or iron (31). Yost and Fridovich (32) demonstrated that E. coli cells grown in iron-rich media are more resistant to killing by phagocytes than E. coli grown in irondeficient media. Increased resistance appeared to correlate with increased levels of iron-SOD rather than with catalase or other effects of iron.

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Spleen cells from Trypanosoma musculiinfected mice were unable to respond to sheep erythrocyte antigen in vitro; moreover, they suppressed the responses of normal spleen cell cultures in dose-dependent fashion.

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