Killing of Listeria monocytogens by conventional and germfree rat sera

Serum from both germfree and conventional rats, but not plasma or plasma serum, killed Listeria monocytogenes in vitro by a calcium-dependent mechanism that was independent of either complement or lysozyme and was not inhibited by the addition of iron. The listericidin was purified by passing either rat serum or platelet lysate through a nitrocellulose filter (0.2 micrometer) and eluting the activity from the filter with 0.02 N HCl. The partially purified listericidin was heat stable (56 degrees C for 30 min), removed by absorption with zymosan or bentonite, sensitive to treatment with trypsin or pronase, and inhibited by the addition of citrate (0.045 M), suggesting that the serum listericidin is a cationic protein. The development of serum listericidal activity, which could be important in the innate resistance of rats to L. monocytogenes, was dependent on both age and microbial status. Although some discrepancies exist between the serum listericidin and previous descriptions of serum beta-lysin, we believe that the rat serum listericidin is a similar cationic protein.