Siderophores are secreted ferric ion
chelators used to obtain
iron in nutrient-limited environmental niches, including human hosts. While all E. coli encode the
enterobactin (Ent)
siderophore system, isolates from patients with
urinary tract infections additionally encode the genetically distinct
yersiniabactin (Ybt)
siderophore system. To determine whether the Ent and Ybt systems are functionally redundant for
iron uptake, we compared growth of different isogenic
siderophore biosynthesis mutants in the presence of
transferrin, a human
iron-binding protein. We observed that the Ybt system does not compensate for loss of the Ent system during
siderophore-dependent, low density growth. Using transcriptional and product analysis, we found that this non-redundancy is attributable to a density-dependent transcriptional stimulation cycle in which Ybt assume an additional autoinducer function. These results distinguish the Ybt system as a combined quorum-sensing and
siderophore system. These functions may reflect Ybt as a public good within bacterial communities or as an adaptation to confined, subcellular compartments in infected hosts. The efficiency of this arrangement may contribute to the extraintestinal pathogenic potential of E. coli and related Enterobacterales.
IMPORTANCE:
Urinary tract infections (UTIs) are one of the most common human
bacterial infections encountered by physicians. Adaptations that increase the pathogenic potential of commensal microbes such as E.coli are of great interest. One potential adaptation observed in clinical isolates is accumulation of multiple
siderophore systems, which scavenge
iron for nutritional use. While
iron uptake is important for bacterial growth, the increased metabolic costs of
siderophore production could diminish bacterial fitness during
infections. In a
siderophore-dependent growth conditions, we show that the virulence-associated
yersiniabactin siderophore system in uropathogenic E. coli is not redundant with the ubiquitous E. coli
enterobactin system. This arises not from differences in
iron scavenging activity but because
yersiniabactin is preferentially expressed during bacterial crowding, leaving bacteria dependent upon
enterobactin for growth at low cell density. Notably, this regulatory mode arises because
yersiniabactin stimulates its own expression, acting as an autoinducer in a previously unappreciated quorum-sensing system. This unexpected result connects quorum-sensing with pathogenic potential in E. coli and related Enterobacterales.