**8. Toxins produced by UPEC**

**7. Iron acquisition systems for UPEC**

been shown to increase virulence of strains causing bacteremia [10].

50 *Escherichia coli* Escherichia coli - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications

acid molecules which react with citrate to form aerobactin [69].

forms of UTI is seen specifically in infants, girls, and women [26, 9].

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

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

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

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

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 modulation of inflammatory signaling pathways [87, 88].

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 CNF-1 prevalence of 64% among UPEC isolates from prostatitis and 36% from pyelonephritis. These results suggest that CNF-1 is associated with increased virulence in UTI pathogenesis. CNF-1 production may also increase the inflammatory response of the host [90]. Specifically, Elliott et al. reported that CNF-1 evokes edema and necrosis and is associated with inflammation in the intestines of rabbits in a diarrhea model of infection [90]. Human neutrophils have been observed to be less effective at killing CNF-1 positive, than CNF-1 negative bacteria.

a strong association between Usp and bacteremia of urinary tract origin, suggesting that Usp is important in the migration of UPEC from the urogenital tract to the blood stream [107]. Other studies have shown comparable prevalences of Usp in cystitis, pyelonephritis, and prostatitis isolates [108]. Furthermore, Usp has (frequently) been associated with all common

The Pathogenesis of *Escherichia Coli* Urinary Tract Infection

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

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Biofilm production by *E. coli* is an important VF which may also protect bacteria from antibiotic action and so contribute to resistance [110–113]. Recent studies have shown that biofilm production in *E. coli*, mediated by co-expression of curli and cellulose, supports long-term survival of UPEC in the urinary tract by surrounding the organism with an inert, hydrophobic extracellular matrix [110, 113, 114]. Most studies of biofilm formation in UTI have

Curli belong to a class of fibers known as amyloids and are involved in adhesion to surfaces, cell aggregation and, finally, biofilm development. Curli fibers are encoded in the curling subunit gene (*csg*) gene cluster, made up of two differently transcribed operons. One operon codes for *csgB, csgA*, and *csgC*, and the other one for *csgD, csgE*, and *csgG* [115]. Expression of both curli operons is important for curli fiber assembly. Curli fibers are also essential for

Co-expression of curli and cellulose tends to decrease in prevalence from more severe to less severe UTI, then to commensal isolates, suggesting that biofilm may facilitate progression from the lower to upper urinary tract [110]. Furthermore, co-expression of both biofilm components is associated with a high prevalence of individual VF genes, high VF scores, and phylogenetic group B2, consistent with heightened urovirulence of such strains. Notably, there may be an interaction between classic VFs and biofilm formation. For example, in one study, all isolates that co-expressed both biofilm components also harbored fyuA, implying that iron uptake via the yersiniabactin system may play a significant role in biofilm growth [64]. Additionally, recent studies have shown that the biofilm components, curli fimbriae and cellulose, also play important roles in adhesion, invasion, and long-term survival of

**11. Phylogenetic group and VF distribution among patient groups and** 

*E. coli* is commonly classified into four main phylogenetic groups namely A, B1, B2, and D [116] as defined by multilocus enzyme electrophoresis and multilocus sequence typing [117]. Several studies have shown that *E. coli* pathogenic strains from extraintestinal infections mostly derive from group B2, and to a less extent group D [21, 118, 119]. Most studies quote prevalence rates around 63–65% for group B2 in pathogenic strains, and 10–15% for group D [21]. Commensal

serotypes of UPEC [109].

**10. Biofilm production by UPEC**

addressed its role in recurrences.

internalization of bacteria during an infection [30].

UPEC within the host urinary tract [110, 111].

**clinical syndromes**
