**7. Iron acquisition systems for UPEC**

Bacteria and the host compete for available iron, which is needed for oxygen transport and storage, DNA synthesis, electron transport, and metabolism of peroxides [57, 58]. Pathogenic bacteria, including UPEC, have devised ways of accessing iron by producing siderophoremediated iron transport systems. UPEC exhibit multiple mechanisms for extracting iron from the host, mainly siderophore-siderophore receptor systems, but also heme uptake [59–62]. Siderophores, which are secreted low molecular weight molecules, have a high affinity for ferric (Fe3+) iron, which is insoluble as a free cation. UPEC retrieve iron-bound siderophores through receptors that facilitate the transportation of siderophore-iron complexes through the bacterial membrane and into the cytosol where the iron is concentrated and utilized. While all *E. coli* can produce the siderophore enterobactin, production of alternative siderophores has been shown to increase virulence of strains causing bacteremia [10].

Finally, UPEC produce salmochelins in order to access iron during invasion of the host. The salmochelin siderophore system, so named because it was first shown to be characteristic of *Salmonella* strains [74], is also present in UPEC. This siderophore system is encoded by *iroA* gene cluster, which is made up of five genes, *iroB, iroC, iroD, iroE*, and *iroN. iroN* gene encodes an outer membrane siderophore receptor which transports different catechol siderophores, including N-(2,3-dihydroxybenzoy)-L-serine and enterochelin. *iroB* encodes a glucosyltransferase that glucosylates enterobactin, *iroC* encodes an ABC transporter required for transport of salmochelins, whilst *iroD* and *iroE* encode a cytoplasmic esterase, and a periplasmic hydrolase, respectively [75]. The salmochelin receptor iroN may play a dual role as an iron uptake receptor as well as an internalization factor [10]. Using a neonatal rat model, it was shown that *iroN* plays a major role during the bacteremic step of the disease [76]. These findings suggest that *iroN* is associated with increased virulence. Studies by Bauer et al. showed that *iroN*

The Pathogenesis of *Escherichia Coli* Urinary Tract Infection

http://dx.doi.org/10.5772/intechopen.69030

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occurred 2.1–4 times more frequently in UTI isolates than in rectal isolates [77].

in contrast to the plasmid location among animal strains [32].

lation of inflammatory signaling pathways [87, 88].

Most hemolytic UPEC strains secrete a heat-labile cytolytic protein toxin known as alpha hemolysin [78], which is encoded by a polycistronic operon, consisting of four genes arranged in the order of hly-CADB [79]. The product of hlyC is important in the activation of the hemolytic toxin, which is the product of the *hlyA* gene. The gene products of *hlyB* and *hlyD* together with TolC are involved in secretion of the hemolysin through the bacterial cell wall [80]. The hemolysin determinants are located on the bacterial chromosome in human isolates of *E. coli*,

Alpha hemolysin lyses red cells of all mammals and even of fish [81], and is toxic to host cells resulting in inflammation, tissue injury, and impaired host defenses. Hemolysin stimulates super-oxide anion and hydrogen peroxide release from and oxygen consumption by renal tubular cells, including histamine release from mast cells and basophils [82, 83]. Hemolytic uropathogenic strains almost always also express P fimbriae [84]. Hemolysin production is found most commonly in UPEC strains from patients with pyelonephritis (49%), followed by cystitis isolates and ASB [85]. These data demonstrate an association of hemolysin production with invasive uropathogenic strains. UPEC strains that produce increased amounts of alpha hemolysin are also more resistant to the complement action of human serum when compared

UPEC also produce a toxin referred to as cytotoxic necrotising factor type 1 (CNF-1). CNF-1 is a chromosomally encoded UPEC toxin that catalyzes the glutamine deamination of the small GTPases RhoA, Rac, and Cdc 42 [86], leading to the disturbance of numerous eukaryotic cellular functions including formation of actin stress fibers, lamellipodia, filopodia, and modu-

Yamamoto et al. showed that 61% of UTI isolates and 38% of bacteremia isolates produced CNF-1 as opposed to only 10% of commensal fecal isolates [89]. Of these isolates, approximately 98% that produced CNF-1 also produced hemolysin. Studies by Mitsumori et al. showed a

to strains that are nonhemolytic or produce reduced amounts of hemolysin [81].

**8. Toxins produced by UPEC**

Several enterobacteria contain a gene cluster called the high pathogenicity island (HPI), which encodes proteins for biosynthesis of the yersiniabactin siderophore and its uptake system [63, 64]. The HPI is widespread among members of the *Enterobacteriaceae* family, and is essential for virulence in *Yersinia* and certain pathotypes of *E. coli* [63]. One of the important genes residing on the HPI is *fyuA* encoding the 71 kDa outer membrane protein *FyuA* (ferric siderophore uptake), which act as a receptor for Fe-yersiniabactin uptake [65]. FyuA, which was first described in *Yersinia species*, is associated with virulence in many members of the *Enterobacteriaceae* family [65]. Studies by Hancock and Klemm have confirmed that the ferric yersiniabactin receptor (FyuA) is required by UPEC for efficient biofilm formation [66].

Aerobactin is another important hydroxamate siderophore synthesized from the condensation of two lysine and one citrate molecules. In UPEC, the aerobactin system is encoded by a five-gene operon with four genes encoding the enzymes needed for aerobactin synthesis and a fifth encoding the outer membrane receptor protein [67, 68]. The synthesis genes are designated *iuc*, for iron uptake and chelation and the receptor gene is *iut*, for iron uptake and transport [69]. Successive steps in the biosynthesis of aerobactin are catalyzed by the iuc genes and involve hydroxylation of lysine and acetylation of the hydroxyl group to form hydroxamic acid molecules which react with citrate to form aerobactin [69].

Previous studies have shown that the aerobactin system and P fimbriae are commonly found together in UPEC isolates from patients with UTI and urosepsis [70, 71]. However, among urosepsis patient isolates, this association is only true for chromosomally encoded aerobactin [71]. An association of chromosomally encoded aerobactin with hemolysin among urosepsis or UTI patient isolates has also been confirmed [71]. These observations suggest that the association of aerobactin with other VFs differs between plasmid and chromosomal aerobactin. Plasmids carrying the aerobactin region sometimes also carry antimicrobial resistance genes [71–73]. The aerobactin system is found more commonly among UPEC strains from patients with pyelonephritis (73%), cystitis (49%), or bacteremia (58%) than among ASB patient isolates (38%) or fecal strains (41%), which suggest that aerobactin contributes to virulence both within and outside of the urinary tract. The association of aerobactin with more serious forms of UTI is seen specifically in infants, girls, and women [26, 9].

Finally, UPEC produce salmochelins in order to access iron during invasion of the host. The salmochelin siderophore system, so named because it was first shown to be characteristic of *Salmonella* strains [74], is also present in UPEC. This siderophore system is encoded by *iroA* gene cluster, which is made up of five genes, *iroB, iroC, iroD, iroE*, and *iroN. iroN* gene encodes an outer membrane siderophore receptor which transports different catechol siderophores, including N-(2,3-dihydroxybenzoy)-L-serine and enterochelin. *iroB* encodes a glucosyltransferase that glucosylates enterobactin, *iroC* encodes an ABC transporter required for transport of salmochelins, whilst *iroD* and *iroE* encode a cytoplasmic esterase, and a periplasmic hydrolase, respectively [75]. The salmochelin receptor iroN may play a dual role as an iron uptake receptor as well as an internalization factor [10]. Using a neonatal rat model, it was shown that *iroN* plays a major role during the bacteremic step of the disease [76]. These findings suggest that *iroN* is associated with increased virulence. Studies by Bauer et al. showed that *iroN* occurred 2.1–4 times more frequently in UTI isolates than in rectal isolates [77].
