Abstract
Our studies reveal that normal rabbit blood contains a potent bactericidin active against Listeria monocytogenes. The factor is present in greatest amounts in fresh undiluted serum but is absent in platelet extracts. A correlation was observed between the virulence of Listeria strains and their relative ability to survive in serum. The bactericidal titers obtained for plasma and plasma serum indicate that clotting must occur for optimum expression of antilisterial activity. The lethal action is not elevated after immunization with viable Listeria nor does it appear to depend on heat-labile components of complement. The active factor was removed from serum by filtration through a cellulose asbestos filter pad and further purified by carboxymethyl cellulose chromatography. Iron significantly diminishes serum lethality and completely abolishes the action of the purified component. The listericidal factor resembles beta-lysin but may be a distinct part of a multiple system of similar bactericidins.