Identification of Multiple Iron Uptake Mechanisms inEnterococcus faecalisand Their Relationship to Virulence

Among the unfavorable conditions bacteria encounter within the host is restricted access to essential trace metals such as iron. To overcome iron deficiency, bacteria deploy multiple strategies to scavenge iron from host tissues with abundant examples of iron acquisition systems being implicated in bacterial pathogenesis. Yet, the mechanisms utilized by the major nosocomial pathogenEnterococcus faecalisto maintain intracellular iron balance are poorly understood. In this report, we conducted a systematic investigation to identify and characterize the iron acquisition mechanisms ofE. faecalisand to determine their contribution to virulence. Bioinformatic analysis and literature surveys revealed thatE. faecalispossesses three conserved iron uptake systems. Through transcriptomics, we discovered two novel ABC-type transporters that mediate iron uptake. While inactivation of a single transporter had minimal impact on the ability ofE. faecalisto maintain iron homeostasis, inactivation of all five systems (Δ5Fe strain) disrupted intracellular iron homeostasis and considerably impaired cell growth under iron-deficiency. Virulence of the Δ5Fe strain was generally impaired in different animal models but showed niche-specific variations in mouse models, leading us to suspect that heme can serve as an iron source toE. faecalisduring mammalian infections. Indeed, heme supplementation restored growth of Δ5Fe under iron-depletion and virulence in an invertebrate infection model. Collectively, this study reveals that the collective contribution of five iron transporters promotesE. faecalisvirulence and that the ability to acquire and utilize heme as an iron source is critical to the systemic dissemination ofE. faecalis.