Reduction of ferric iron by Listeria monocytogenes and other species of Listeria.

One mechanism by which Listeria monocytogenes is thought to obtain iron required for growth is through the extracellular reduction of a ferric iron source to the ferrous form. To better characterize this reductase activity we have developed a simple plate assay that allows detection of colonies of Listeria species able to reduce ferric iron. Cells are plated on an agar base medium containing a ferric iron source and ethylenediamine dihydroxyphenylacetic acid. Colonies are then overlain with soft agarose containing NADH, flavin mononucleotide, and Ferrozine, a chelator of ferrous iron. Colonies able to reduce the ferric iron source form a red-purple color as the ferrous iron is complexed with ferrozine. Using this qualitative assay we have shown that all species of Listeria are able to reduce ferric iron when presented as ferric ammonium citrate whereas most other species of Gram-positive and Gram-negative bacteria are not. Only Clostridium perfringens was able to reduce ferric iron to the same extent as L...