Walters et al. infected mice with ten slightly different clones of E. coli and followed their progress through the urinary tract and into the bloodstream. Looking at the presence and relative abundances of the various strains in the urethra, bladder, ureter, kidneys, and spleen (which is only linked to the kidneys through the bloodstream and, hence, serves as an indicator of bacteremia), they discerned that different lineages dominated different sites at different times.
These observations run counter to a simplistic stepwise model in which one strain moves at the forefront of infection and establishes itself at each site along the way. Rather, kidney infection and bacteremia associated with UTI are apparently the result of multiple rounds of ascension and dissemination and not necessarily the expansion of a single clonal UPEC lineage. So bacteria ascend the urinary tract using their flagella, they're washed back down by the flow of urine, more ascend, more get washed down, etc., and what you have in the end is a complicated picture of how E. coli gets around and lingers in the various areas of the anatomy. Passage from the kidneys to the bloodstream, on the other hand, is a big bottleneck for E. coli.
The authors write that getting a grasp on the ins and outs of E. coli populations in the urinary tract is more than just an academic pursuit: knowing how different bacterial populations become dominant over time while others are lost could point the way to new therapeutics that target specific strategies E. coli uses for success in this habitat.
