Urinary tract infections: microbial pathogenesis, host-pathogen interactions and new treatment strategies

Abstract

Urinary tract infections (UTIs) are common, recurrent infections that can be mild to life-threatening. The continued emergence of antibiotic resistance, together with our increasing understanding of the detrimental effects conferred by broad-spectrum antibiotic use on the health of the beneficial microbiota of the host, has underscored the weaknesses in our current treatment paradigm for UTIs. In this Review, we discuss how recent microbiological, structural, genetic and immunological studies have expanded our understanding of host-pathogen interactions during UTI pathogenesis. These basic scientific findings have the potential to shift the strategy for UTI treatment away from broad-spectrum antibiotics targeting conserved aspects of bacterial replication towards pathogen-specific antibiotic-sparing therapeutics that target core determinants of bacterial virulence at the host-pathogen interface.

Figure 1:. Overview of urinary tract infection pathogenesis by UPEC.

A) Uropathogenic Escherichia coli (UPEC) colonizing the gastrointestinal tract, perineum or vagina inoculate the urethra and ascend into the bladder. Isolated infections of the bladder are termed cystitis, and result in the classic urinary tract infection (UTI) symptoms, such as urinary frequency, urinary urgency, dysuria, and suprapubic tenderness. Bacteria can also ascend the ureters to the kidneys, where they cause a kidney infection termed pyelonephritis that can result in fever, chills and flank pain. Finally, bacteria can invade the bloodstream, causing bacteremia that can eventually lead to septic shock. B) In the bladder, individual bacteria adhere to and invade superficial urothelial umbrella cells in a FimH-dependent manner. There, they undergo clonal expansion to form biofilm-like intracellular bacterial communities (IBCs), where they can evade mechanical and immunological clearance mechanisms by the host. Bacteria then form filaments that flux out of the umbrella cells into the bladder lumen, where they can subsequently bind to neighboring cells and begin a new infection cycle.

Figure 2:. Bacterial urovirulence factors.

A) Four chaperone-usher pathway pili, including type 1 pili (also known as Fim pili), F17-like pili (also known as Ucl pili), Fim-like pili (also known as Fml pili) and P pili (also known as Pap pili), are involved in urinary tract infections (UTIs) through various mechanisms (left panel). Type 1 pili bind mannosylated uroplakins in the bladder and mannosylated mucous components in the gastrointestinal tract using the two-domain FimH adhesin. F17-like pili bind O-glycans in the gastrointestinal tract, helping to establish a reservoir for recurrent UTIs. Fim-like pili bind GalNAc moieties found in the inflamed bladder and kidneys, which promotes chronic UTIs. P pili bind globosides in the kidney and are essential for progression to pyelonephritis. Biophysical studies performed on type 1 pili revealed the importance of conformational dynamics in the adhesin and pilus rod during infection (right panel). More specifically, positive selection at positions 27, 62, and 163 influencesconformational changes in the FimH adhesion that modulate the affinity of the binding pocket for mannose residues on the bladder epithelium. The FimHA62S variant preferentially adopts a ‘tense’ conformation with a low affinity for mannose, whereas the FimHA27V/V163A variant preferentially adopts a ‘relaxed’ conformation with high affinity for mannose. In the wild type protein, it is thought that an equilibrium between these species is necessary for maximum virulence. These adhesin subunits are noncovalently linked to the pilus tip and rod via a process known as donor strand exchange, wherein a donor strand from the preceding pilus completes the beta strand of the immunoglobulin-like fold. Furthermore, interactions between neighboring subunits within the Fim pilus rod when it adopts its native helical conformation modulate the force-length characteristics of the type 1 pilus in response to external force. Alterations to these interactions lead to an attenuation of virulence in mouse models of cystitis. B) Uropathogenic Escherichia coli (UPEC) preferentially express a subset of siderophores, including yersiniabactin (Ybt), during infection of the bladder. Ybt binds extracellular iron and copper ions and delivers them to the FyuA outer membrane receptor for internalization. The YbtP-YbtQ complex then transports the copper-Ybt complexes to the cytoplasm. The presence of depressed cytoplasmic metal levels upregulates transcription of the HWMP1 and HWMP2, which subsequently synthesize Ybt. A metabolic byproduct of the HMWP1/2-mediated Ybt synthesis pathway, escherichelin (Esc), engages in interbacterial competition by disrupting prochelin (Pyo)-mediated iron uptake through the FptA outer membrane transporter in Pseudomonas aeruginosa. C) Periplasmic CpxA (the kinase of the CpxA-CpxR two component system) can sense protein misfolding (periplasmic stress) in E. coli during UTI, leading to the induction of genes responsible for the production of the pore-forming toxin α-hemolysin (HlyA) by the CpxR response regulator. Approximately 40-50% of UPEC strains encode HlyA, which contributes to activation of the NRLP3 inflammasome, caspase-1 and caspase-4, interleukin-1β (IL-1β) and IL-1α, leading to urothelial cell death in the host. Deletion of the CpxR in UPEC results in overexpression of HlyA and loss of bacterial fitness in mouse models of acute and chronic cystitis by triggering the caspase-mediated inflammatory cell death pathway and early vigorous urothelial exfoliation. Such vigorous exfoliation overcomes the ability of wild type UPEC to subvert the exfoliation response and invade underlying epithelium. D) Mechanisms of gastrointestinal colonization and curli biofilm formation. In the gastrointestinal environment, E. coli can embed themselves in a complex extracellular matrix containing curli fibers, cellulose and DNA fragments to promote persistence and prevent clearance. Curli comprise repeating amyloidogenic CsgA subunits that are assembled into extracellular fibers. The periplasmic chaperone CsgE interacts with unfolded CsgA to facilitate the transport of CsgA to and through the nonameric CsgG outer membrane pore. CsgE and the chaperone-like protein CsgC prevent premature CsgA polymerization in the periplasm. Following transport to the CsgG pore, CsgA is secreted across the outer membrane where curli fiber formation is facilitated by CsgB, which is held to the outer membrane by CsgF. CsgB nucleates the folding of CsgA resulting fiber polymerization. The FimH adhesin at the tip of type 1 pili and UclD adhesin at the tip of F17-like pili also mediate binding to the intestinal crypts, helping to establish a stable gastrointestinal reservoir.

Figure 3:

Pathogenesis of catheter-associated urinary tract infections. Introduction of a urinary catheter into the bladder environment results in inflammation and the generation of fibrinogen, which is deposited on the urinary catheter. Urinary pathogens exploit this fibrinogen deposition in various ways. Enterococci utilize Ebp pili to bind directly to the fibrinogen, and the GelE and SprE proteases to promote fibrinogen cleavage and urinary growth using fibrinogen as a food source. Staphylococci use clumping factor B (ClfB) to adhere to the fibrinogen-coated catheter and potentiates bladder inflammation to promote persistence. Proteus species use various virulence factors, including mannose-resistant Proteus-like (MR/P) fimbriae, flagella and urease to promote the formation of bladder stones and crystalline biofilms that permit bacterial persistence in the catheterized bladder. Finally, Escherichia coli use the FimH adhesin to bind fibrinogen coating the catheters.