In vitroassays for clinical isolates of sequence type 131Escherichia colido not recapitulatein vivoinfectivity using a murine model of urinary tract infection

Sequence Type 131 isolates are a major cause of cystitis and pyelonephritis. Many studies rely solely on in vitro assays to screen for bacterial virulence factors associated with the pathogenicity of clinical isolates of E. coli . Few studies have compared in vitro findings to in vivo infectivity of clinical isolates. The purpose of this study was to evaluate the correlation between in vitro assays with the ability to cause cystitis and pyelonephritis in a murine model of urinary tract infection. In vitro assays were conducted according to published protocols and included: motility assays, biofilm formation, epithelial cell adhesion and invasion, and curli production. Twenty-one UPEC isolates of E. coli ST131 and non-ST131 were used for both in vivo and in vitro studies. Six mice per isolate were inoculated via urethral catheterization. CFUs were determined from bladder and kidneys. In vitro and in vivo correlations were evaluated by multiple linear regression analysis. Pairwise linear regressions showed trendlines with weak positive correlations for motility, adhesion, and invasion and weak negative correlations for hemagglutination, biofilm and curli production. The ability of E. coli ST131 and non-ST131 clinical isolates to cause cystitis and pyelonephritis varies among strains. The R 2 Pearson Correlation value was less than ±0.5 for any pair, indicating little to no statistical association between in vitro and in vivo findings. These data show in vitro data are not predictive of the ability of ST131 E. coli to infect and/or cause disease in a mouse model. Author summary Urinary tract infections affect 150 million people annually and E. coli ST131 have become the pandemic strain responsible for a majority of UTI, cystitis, and pyelonephritis cases. How ST131 E. coli have become such prolific strain still remains to be elucidated. When evaluating bacterial pathogenicity, it is customary practice to use in vitro assays to predict isolate virulence and mechanisms of fitness, due to the lower cost, and relative ease of experimentation compared to more costly and complicated in vivo models. It is also common to use model organisms like pathogenic E. coli CFT073 or non-pathogenic lab strains such as BW25113 as representatives for the entire species. However, our research has shown that not only are model organisms substantially different from clinical isolates of ST131 E. coli , but in vitro assays are poor predictors of clinical isolates’ ability to cause infection in a murine model of UTI. As such, research into the mechanisms of fitness for ST131 infectivity need to veer away from studying only model organisms and focus on utilizing pathogenic clinical isolates in conditions that more closely recapitulate urinary tract environmental niches.