**4. Siderophores**

Iron is a very important molecule for all living beings, and *E. coli* uses iron for transporting and storing oxygen, DNA synthesis, electron transport, and metabolism of peroxides. But the amount of iron availability is reduced in the host urinary tract during UTIs [49]. In response to this, *E. coli* possesses some multiple functionally redundant systems that mediate iron uptake by secreting low-molecular-weight Fe3+-chelating molecules which are known as siderophores [50]. Iron utilization, mediated by these siderophores, is critical for colonization of the urinary tract by UPEC [51]. There are four distinct siderophore systems found in *E. coli* such as yersiniabactin, aerobactin, enterobactin, and salmochelin [52]. These systems also include some genes such as ent genes encoding enterobactin, iuc genes encoding aerobactin, and iro genes encoding an ent-like system. However, all these systems are expressed under low-iron conditions and are negatively regulated by ferrous iron and the ferric uptake regulator Fur [53].
