Expression of flagella is coincident with uropathogenic Escherichia coli ascension to the upper urinary tract

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Abstract
Uropathogenic Escherichia coli (UPEC) cause most uncomplicated urinary tract infections (UTIs) in humans. Because UTIs are considered to occur in an ascending manner, flagellum-mediated motility has been suggested to contribute to virulence by enabling UPEC to disseminate to the upper urinary tract. Previous studies from our laboratory and others have demonstrated a modest yet important role for flagella during ascending UTI. To better understand the role of flagella in vivo, we used biophotonic imaging to monitor UPEC infection and temporospatial flagellin gene expression during ascending UTI. Using em7-lux (constitutive) and fliC-lux transcriptional fusions, we show that flagellin expression by UPEC coincides with ascension of the ureters and colonization of the kidney. The patterns of fliC luminescence observed in vitro and in vivo were also validated by comparative quantitative PCR. Because fliC expression appeared coincident during ascension, we reassessed the contribution of fliC to ascending UTI using a low-dose intraurethral model of ascending UTI. Although wild-type UPEC were able to establish infection in the bladder and kidneys by 6 hours postinoculation, fliC mutant bacteria were able to colonize the bladder but were significantly attenuated in the kidneys at this early time point. By 48 hours postinoculation, the fliC mutant bacteria were attenuated in the bladder and kidneys and were not detectable in the spleen. These data provide compelling evidence that wild-type UPEC express flagellin and presumably utilize flagellum-mediated motility during UTI to ascend to the upper urinary tract and disseminate within the host.