**4.3 Iron acquisition by UPEC**

Iron, a necessary cofactor for enzymes found in all organisms, remains concealed by iron-binding proteins in humans. Iron is very crucial for the growth of the bacteria. Therefore, bacteria colonizing and causing infections in humans should have some systems to obtain it. Iron is present at a very low concentration at the infection site of the urinary tract and UPEC is known to have multiple systems for iron scavenging. One such potent way to hunt iron is the possession of siderophores by bacteria as it has got a very high affinity for Fe3+ that enables *E. coli* to escort iron back to the cell. It is documented that siderophores are usual in *E. coli* strains causing UTI as compared to fecal strains [34]. UPEC encodes a siderophore called enterobactin (which is also present in commensal bacterium) in addition it contains multiple other iron acquisition systems such as yersiniabactin, salmochelin and aerobactin [35]. The gene that encodes aerobactin has been identified to be *iutA.* Furthermore, there is fact that UPEC does not use siderophores alone for scavenging iron, it also employs other iron receptors that are found in the outer membrane which binds iron and takes it back inside the bacterial cell. CFT073 (prototypical UPEC strain) encodes for 14 such outer membrane iron receptors [36].
