Over 50% of women will experience a urinary tract infection (UTI) in their lifetime. Uropathogenic Escherichia coli (UPEC) is the etiological agent of ~80% of UTIs. Rates of antibiotic resistance in UPEC are on the rise, rendering many previous treatments ineffective. Therefore, it is of the utmost importance to understand how these currently circulating strains of UPEC are causing disease to find novel methods for treatment of UTI. One potential therapeutic target is the iron acquisition systems that UPEC employ to survive in the iron-limited host environment. Indeed, current vaccine candidates involve the use of iron receptors as potential antigens to elicit host immune responses and protection. UPEC use heme receptors and synthesize low molecular weight siderophores, with high iron affinities, to scavenge iron from the host. There are multiple functional redundancies in these iron and heme acquisition systems, underlining their importance. However, a UPEC strain recently isolated from a female patient with symptoms of acute UTI, HM7, lacks all but one of the six major iron acquisition systems ordinarily associated with UPEC strains: It encodes a singular siderophore, enterobactin. Interrogating the online resource PATRIC revealed several recently isolated UPEC strains without redundancy in their iron acquisition systems. These, like HM7, potentially represent a distinct population of presently circulating UPEC strains. To determine if HM7 possesses previously unidentified iron acquisition systems, we performed RNA-sequencing under iron-limiting conditions and found the ferric citrate uptake system (fecABCDEIR) highly upregulated. There are high levels of citrate within urine; therefore, HM7 could be using this uptake system as an alternative means of iron acquisition within the host. Deletion of both enterobactin biosynthesis and ferric citrate uptake (ΔentB/ΔfecA) abrogated the strain’s ability to use ferric citrate as an iron source. fecA does not provide an advantage in in vitro in pooled human urine when competed against WT. However, in the absence of enterobactin, fecA is an in vitro fitness factor. This finding suggests that siderophore iron acquisition is preferred, but in certain environmental conditions, ferric citrate uptake can be used instead. Finally, fecA alone is a fitness factor in the CBA/J mouse model. The competitive advantage is abrogated in a Lcn-/- knockout mouse line, indicating that the advantage may be dependent on the host protein, lipocalin-2, which binds enterobactin. These findings further support the hypothesis that ferric citrate uptake is used as an iron source when siderophore efficacy is limited, such as in the host during UTI. Defining these novel compensatory mechanisms, and understanding the nutritional hierarchy for survival within the urinary tract will be important in describing new approaches to combat these common infections.
