Covert operations of uropathogenic Escherichia coli within the urinary tract

Abstract

Entry into host cells is required for many bacterial pathogens to effectively disseminate within a host, avoid immune detection and cause disease. In recent years, many ostensibly extracellular bacteria have been shown to act as opportunistic intracellular pathogens. Among these are strains of uropathogenic Escherichia coli (UPEC), the primary causative agents of urinary tract infections (UTIs). UPEC are able to transiently invade, survive and multiply within the host cells and tissues constituting the urinary tract. Invasion of host cells by UPEC is promoted independently by distinct virulence factors, including cytotoxic necrotizing factor, Afa/Dr adhesins, and type 1 pili. Here we review the diverse mechanisms and consequences of host cell invasion by UPEC, focusing also on the impact of these processes on the persistence and recurrence of UTIs.

Figure 1. CNF1 effects on bladder cell motility and bacterial entry

In normal uninfected bladder epithelial cells, Rho GTPases like Rac cycle between active GTP-bound and inactive GDP-bound states, regulated by GAPs and GEFs (guanine nucleotide exchange factors). CNF1 renders Rho GTPases constitutively active by catalyzing deamidation of glutamine 63 in RhoA (or glutamine 61 in Rac and Cdc42). CNF1-modified constitutively active Rac, in this example, is shown as a black oval. Activation of Rac and other Rho GTPases leads to cytoskeletal rearrangements resulting in increased cell spreading and membrane ruffling (1–4 h post-CNF1 intoxication). By 4 h postintoxication, constitutively activated Rho GTPases are ubiquitinated and targeted for proteasomal degradation. The resulting depletion of Rho GTPases promotes host cytoskeletal rearrangements, resulting in enhanced filopodia formation and stimulating bacterial uptake. Bacterial internalization continues to increase up to 24 h post-CNF1 intoxication. Rho depletion also stimulates bladder cell migration. Theoretically, this could result in redistribution of host cells within a stratified three-dimensional tissue such as the bladder epithelium, allowing UPEC to more efficiently disseminate and colonize underlying cells. The model shows the migration of host cells in the upper (yellow) and lower (blue) layers of an idealized tissue bilayer as a consequence of CNF1-stimulated Rac depletion.

Figure 2. Pathway and consequences of FimH-mediated invasion

FimH, localized at the distal tips of type 1 pili, bind and cluster unspecified host receptors, possibly concentrated within lipid raft domains (green regions). This triggers a signaling cascade leading to localized rearrangements of F-actin (red), which in turn results in the envelopment and uptake of adherent UPEC. Once internalized, UPEC is trafficked into membrane-bound compartments. From here, undefined signals derived from the host environment (and possibly associated with the host cell differentiation state) trigger UPEC replication, resulting in the formation of intracellular bacterial communities (IBCs) and the eventual efflux of UPEC. Alternatively, UPEC enters a nonreplicating or viable but nonculturable (VBNC) state that can persist for days or weeks, possibly serving as a reservoir for subsequent recurrent acute infections. PIPs phosphoinositides.

Figure 3. Localization of UPEC within bladder epithelial cells

Transmission electron microscopy reveals that the human UPEC cystitis isolate UTI89 is trafficked into compartments with morphologic characteristics of MVBs (arrowheads). Images were taken 24 h postinfection of immortalized human bladder cells designated 5637. After an initial 2-h incubation, the host cell impermeable antibiotic gentamicin was added to the cell culture media to prevent extracellular bacterial growth. Scale bars, 5 μm. n, nucleus.

Figure 4. Efflux of filamentous UPEC from a bladder superficial epithelial cell

(A and B) Scanning electron microscopy shows filamentous UPEC (a cystitis isolate designated UTI89) emerging from and binding to mouse bladder superficial cells. Scale bars, (A) 5 μm and (B) 3 μm.