REDUCTION OF EXOGENOUS FERRIC IRON BY A SURFACE-ASSOCIATED FERRIC REDUCTASE OF LISTERIA SPP

The reduction of exogenous ferric iron by Listeria monocytogenes, a Gr am-positive food-borne pathogen, was investigated. Using an assay inco rporating the ferrous iron chelator ferrozine, we showed that intact c ells of L. monocytogenes, when exposed to ferric iron, were able to ra pidly reduce and solubilize the iron to the ferrous form. Reduction oc curred only after direct contact between the bacteria and the iron sou rce. A number of different ferric iron chelates, including transferrin and lactoferrin-bound iron, haemoglobin, ferritin, and iron complexed to siderophores, could be reduced. The ferric reductase activity was expressed by both reference strains and clinical isolates of L. monocy togenes and by all other species of Listeria, although significant qua ntitative differences were observed. In L. monocytogenes, the expressi on of ferric reductase was not affected by the growth phase of the bac teria nor by the presence or absence of iron in the growth medium. How ever, expression was greatly reduced in bacteria grown anaerobically a nd when cultured in media of reduced pH. In addition, bacteria grown a t a cold temperature displayed greater ferric reductase activity than cells grown at higher temperatures. A surface-associated ferric reduct ase system may be one component of a general iron scavenging mechanism which can be used by Listeria growing in a variety of environments.