In vivo dynamics of type 1 fimbria regulation in uropathogenic Escherichia coli during experimental urinary tract infection.

Escherichia coli is the primary cause of uncomplicated infections of the urinary tract including cystitis. More serious infections, characterized as acute pyelonephritis, can also develop. Type 1 fimbriae of E. coli contribute to virulence in the urinary tract; however, only recently has the expression of the type 1 fimbriae been investigated in vivo using molecular techniques. Transcription of type 1 fimbrial genes is controlled by a promoter that resides on a 314-bp invertible element capable of two orientations. One places the promoter in the ON orientation, allowing for transcription; the other places the promoter in the OFF orientation, preventing transcription. A PCR-based assay was developed to measure the orientation of the invertible element during an experimental urinary tract infection in mice. Using this assay, it was found that the percentage of the population ON in urine samples correlated with the respective CFU per gram of bladder (P = 0.0006) but not with CFU per gram of kidney (P > 0.069). Cystitis isolates present in the urine of mice during the course of infection had a higher percentage of their invertible elements in the ON orientation than did pyelonephritis isolates (85 and 34%, respectively, at 24 h; P < 0.0001). In general, cystitis isolates, unlike pyelonephritis isolates, were more likely to maintain their invertible elements in the ON orientation for the entire period of infection. E. coli cells expressing type 1 fimbriae, expelled in urine, were shown by scanning electron microscopy to be densely packed on the surface of uroepithelial cells. These results suggest that expression of type 1 fimbriae is more critical for cystitis strains than for pyelonephritis strains in the early stages of an infection during bladder colonization.