Urinary tract infections (UTIs) are one of the most common bacterial infections and can be divided into two categories based on the mechanism of acquisition, community and nosocomial acquired UTI. Community-acquired UTIs often occur in otherwise healthy women and studies have shown that approximately 50% of women will have at least one UTI in their lifetime 1. Additionally, recurrence is a major problem. A woman who has an initial acute infection has a 25-40% chance of having a second infection within six months despite appropriate antibiotic treatment and many women continue to have frequent recurrences 2. The bacteria that cause these infections are also becoming increasingly antibiotic resistant further confounding treatment protocols 3-6. UTI affect millions of individuals every year costing approximately 2.5 billion dollars in health care related expenses in the US, underscoring the impact and prevalence of the disease 1,7.Nosocomial acquired UTIs are mainly associated with the presence of foreign bodies such as indwelling catheters. Catheter-associated UTIs (CAUTI) remain the most common nosocomial acquired UTI, accounting for ~70-80% of such infections 8. Furthermore, CAUTI is associated with increased morbidity and mortality and it is the most common cause of secondary bloodstream infections 9.
UPEC associated community acquired UTIs are thought to be caused by the introduction of bacteria into the bladder from reservoirs in the gastrointestinal tract via mechanical manipulation during sexual intercourse, poor hygiene or other microbial population dynamics between different host niches 10. Once inside the bladder, UPEC employ numerous virulence factors, including capsule, iron acquisition systems, toxins, a virulence plasmid, tRNAs, pathogenicity islands and colonization factors that have been shown to play a role in pathogenesis 11-14. Critical to the establishment of UPEC colonization, UPEC also encode multiple types of adhesive chaperone usher pathway (CUP) pili that recognize receptors with stereochemical specificity 15. Type 1 pili, tipped with the FimH adhesin, are expressed by UPEC and bind mannosylated uroplakins 16 and α-1, β-3 integrins 17, which are expressed on the luminal surface of both human and mouse bladders 18. These FimH-mediated interactions facilitate bacterial colonization and invasion of the superficial epithelial cells19,20. Once inside the cell, UPEC can escape into the cytoplasm where a single bacterium can rapidly divide to form an intracellular bacterial community (IBC), which upon maturation, can contain ~104 bacteria 21. IBC formation has been demonstrated in at least six different mouse strains, C3H/HeN, C3H/HeJ, C57Bl/6, CBA, FVB/NJ and BALB/c, and with a wide variety of different UPEC strains and other Enterobacteriaceae 22-24. However temporal and spatial differences of IBC formation can vary depending upon the mouse background and the infecting UPEC strain. In C3H/HeN mice infected with the prototypical UPEC strains UTI89 or CFT073, IBC formation can be visualized as small biomasses of bacteria as early as 3 hpi (hr post infection). This community continues to expand and reaches a “midpoint” of development approximately 6 hpi when the rod shaped bacteria occupy a large percentage of the cytoplasmic space of terminally differentiated superficial umbrella cells These early IBCs form in a relatively synchronous manner with the majority displaying similar dimensions and morphologies. ~8 hpi the bacteria in the IBC change from a bacilli to cocci morphology. IBCs are transient in nature. Thus, IBC maturation from 12-18 hpi results in continued expansion of the bacterial population, followed by their filamentation and dispersal out of the cell with subsequent spread to neighboring cells 23. Thus, the IBC niche allows for rapid bacterial growth in an environment protected from host immune responses and antibiotics 25. The distinct stages of UPEC infection that are seen in mice are also observed in humans, such as IBCs and filamentation, supporting the mouse model of UTI as a beneficial tool that can be used to model UTI in humans 22,26-28.
While a majority of women experience a UTI in their lifetime, the outcome of these infections can range from acute self-limiting infection with no recurrence, to frequent recurrent cystitis. Further, studies have shown a strong familial occurrence of UTI, suggesting a genetic component contributes to UTI susceptibility 29. We have found that the differing UTI outcomes seen in clinics can be mirrored by the differing outcomes of experimental UPEC infection among inbred mouse strains 30. For example, C3H/HeN, CBA, DBA, and C3H/HeOuJ mice are susceptible, in an infectious dose-dependent manner, to long-lasting, chronic cystitis characterized by persistent, high titer bacteria (>104 colony forming units (CFU)/ml), high titer bacterial bladder burdens at sacrifice >4 weeks post-infection (wpi), chronic inflammation, and urothelial necrosis . These mice also display elevated serum levels of IL-6, G-CSF, KC, and IL-5 within the first 24 hpi that serve as biomarkers for the development of chronic cystitis. This may accurately represent the natural course of UTI in some women, as placebo studies have shown that a large percentage of women experiencing UTI will retain high levels of bacteria in their urine for several weeks after the first symptoms of cystitis if not given antibiotic treatment 31,32. Further, using C3H/HeN mice, we found that a history of chronic cystitis is a significant risk factor for subsequent severe recurrent infections. Recurrent UTI is the most significant clinical manifestation of UTI and the C3H/HeN mouse is currently the only studied model that recapitulates an increased predisposition after previous exposure. A second UTI outcome is recapitulated in C57Bl/6 mice where acute UPEC infection is self-limiting, with resolution of bladder inflammation and bacteriuria within approximately a week. Interestingly, in this model, UPEC readily form quiescent intracellular reservoirs within the bladder tissue from which UPEC are capable of emerging from a dormant state to reinitiate an active UTI, potentially explaining one mechanism for same strain recurrent UTI in humans 33,34.
In addition to genetic influences on UTI susceptibility, introduction of a catheter into the bladder greatly increases the likelihood of having an infection as well as increasing the range of bacteria able to cause an infection. It has been demonstrated that human urinary catheterization causes histological and immunological changes in the bladder due to mechanical stress that results in a robust inflammatory response, exfoliation, edema of the lamina propria and submucusa, urothelial thinning, and mucosal lesion of the urothelium and kidney 35,36. Additionally, the catheter provides a surface for bacterial attachment thereby creating an environment utilized by several species to cause CAUTI. While UPEC are still a major contributor, Enterococcus faecalis accounts for 15% of these CAUTI 37. E. faecalis is becoming increasingly resistant to antibiotics with vancomycin resistance emergence, posing a serious health concern 38. E. faecalis possess numerous virulence factors including toxins and adhesins necessary for attachment to both the catheter and epithelium 38. During urinary catheterization, the host is vulnerable to microbial adhesion, multiplication and dissemination in the urinary tract 39,40. E. faecalis forms a biofilm on the catheter as part of a mechanism to persist in the bladder and disseminate to the kidneys, which is reproduced in a mouse CAUTI model41. Recently, it has been shown during urinary catheterization, fibrinogen (Fg) is released into the bladder as part of the inflammatory response. Fg accumulates in the bladder, coats the catheter and is essential for E. faecalis biofilm formation, functioning as an attachment scaffold. In a C57BL/6 mouse model of CAUTI, we discovered that E. faecalis biofilm formation on the catheter, and thus persistence in the bladder, was dependent on the Ebp pilus, specifically its tip adhesin EbpA. We found that the N-terminal domain of EbpA specifically binds to Fg coating the catheter. Additionally, it was found that E. faecalis utilizes Fg as metabolite source during infection, thus enhancing biofilm formation 42.
Mouse models have proven critical to understanding as well as predicting clinical manifestations of UTI and CAUTI 41. In this article we demonstrate inoculum preparation of the cystitis UPEC isolate UTI89 and transurethral inoculation of C3H/HeN mice. Additionally, we demonstrate a protocol for catheter insertion in C57BL/6 mice and inoculation of the E. faecalis OG1RF strain. Both of these techniques lead to consistent and reliable UTI or CAUTI in mice. We also display techniques used to observe IBC formation during acute cystitis and urine collection for the analysis of chronic or recurrent cystitis. C3H/HeN mice have been used to study aspects of UPEC pathogenesis including initial bacterial invasion, IBC formation, filamentation and the development of chronic cystitis 23,33,43. These virulence parameters have also been studied in a variety of other mouse backgrounds 22,33. For CAUTI, the C57BL/6 model allows for foreign body implantation into the bladder with subsequent bacterial colonization, which can be maintained for 7 days post infection 41. These models have been useful for assessing bacterial virulence mechanisms, host responses to UTI and mechanisms to subvert host responses, much of which has been subsequently recapitulated or observed in clinical human populations.