Recurrent Urinary Tract Infection: A Mystery in Search of Better Model Systems

Abstract

Urinary tract infections (UTIs) are among the most common infectious diseases worldwide but are significantly understudied. Uropathogenic E. coli (UPEC) accounts for a significant proportion of UTI, but a large number of other species can infect the urinary tract, each of which will have unique host-pathogen interactions with the bladder environment. Given the substantial economic burden of UTI and its increasing antibiotic resistance, there is an urgent need to better understand UTI pathophysiology - especially its tendency to relapse and recur. Most models developed to date use murine infection; few human-relevant models exist. Of these, the majority of in vitro UTI models have utilized cells in static culture, but UTI needs to be studied in the context of the unique aspects of the bladder's biophysical environment (e.g., tissue architecture, urine, fluid flow, and stretch). In this review, we summarize the complexities of recurrent UTI, critically assess current infection models and discuss potential improvements. More advanced human cell-based in vitro models have the potential to enable a better understanding of the etiology of UTI disease and to provide a complementary platform alongside animals for drug screening and the search for better treatments.

Keywords: in vitro infection model systems; microphysiological systems; mouse models; organ-on-chip; organoid; urinary tract infection (UTI); uropathogenic E. coli (UPEC); urothelium.

Figure 1

Comparison between human versus mouse urothelium. In human bladder, basal cells have higher expression of CK5, 13, 14 and 17, intermediate cells are stacked in 5-7 layers (vs 3-4 in mice), and umbrella cells have higher expression of CK8, 7, 18 and 20, as well as a more cationic MD-2 protein associated with the TLR4, while mice have a high expression of TLR11 (which is not present in humans). Urine is less concentrated in humans, which also have higher storage capacity and lower urination frequency compared with mice.

Figure 2

Platforms with fluid flow and/or mechanical stretch used for in vitro bladder studies. (A) The Cellix Vena8 Fluoro+ Biochip was used by Feenstra et al., 2017 to study E. coli adhesion to human microvascular endothelial cells and bladder epithelial cell lines. Figures modified from Cellix company website, with permission. (B) Custom flow chambers were used by Andersen et al., 2012 and Zalewska-Piatek et al., 2020 to study the role of flow in E. coli adhesion to human bladder epithelial cells. Example shown is from Andersen et al., 2012 reproduced with permission from the American Society for Microbiology. (C) The CellASIC ONIX platform was used by Iosifidis and Duggin, 2020 to study the role of urine composition and pH on UPEC infection of a bladder epithelial cell line. Top schematic of plate from Lee et al., reproduced with permission from Springer Nature. Bottom panel from Iosifidis and Duggin, 2020 reproduced with permission from the American Society for Microbiology. (D) Truschel et al., 2002 used modified Ussing chambers to induce stretch of excised rabbit bladder tissue. Figure reproduced in compliance with the Creative Commons Non-Commercial Share Alike 3.0 Uported license agreement. (E) FlexCell systems were used by Sun et al., 2001 and studies to investigate ATP release from primary human bladder urothelial cells from healthy and IC patients. Author schematic depicting the platform’s function. (F) Recently, Sharma et al., 2021 used Emulate’s organ-on-chip platform that incorporates both fluid flow and stretch in a bladder chip model of infection. Figure modified from Sharma et al., 2021 pre-print in compliance with the Creative Commons CC-BY-NC-ND 4.0 International License.

Figure 3

Key features required for advanced human urothelial models and their importance for UTI research.