The ferritin-like protein Frm is a target for the humoral immune response to Listeria monocytogenes and is required for efficient bacterial survival.

The identity and role of listerial antigens recognized by antibodies following listerial infection is largely unknown. We identified the listerial ferritin-like protein Frm as a target of the humoral response following infection of mice with pathogenic Listeria monocytogenes. Specific antibodies to Frm are detected in antisera of mice infected with a pathogenic strain but not in antisera of mice infected with a non-pathogenic L. innocua strain. Antibodies raised to purified listerial ferritin allowed us to demonstrate that the expression of Frm is both growth phase- and temperature-dependent. Using an isogenic Deltafrm mutant, we find that ferritin is essential for bacterial growth in chemically defined minimal media but not in complex media such as brain-heart infusion. Mutant bacteria also exhibit a defect in intracellular replication. The Deltafrm strain is hypersensitive to hydrogen peroxide indicating that the Frm is required for protection against reactive oxygen intermediates under various growth conditions. Animal studies show that Frm contributes to pathogenesis in mice, in particular, at early time points following infection.