Resumo

Uropathogenic Escherichia coli (UPEC) are responsible for the majority of urinary tract infections, some of the most common bacterial infections in humans worldwide. Though many virulence factors associated with UPEC pathogenicity are known, regulation of their expression is still not fully understood. FNR (fumarate and nitrate reduction), a well-known global regulator that works as an oxygen sensor, was found to control the expression of major UPEC virulence genes. Deletion of fnr significantly reduced the expression of type 1 fimbriae (fim operon) and affected operon’s switch between “phase on” and “off” by down-regulating expression of FimB. FNR also upregulated expression of P fimbriae (pap operon) by targeting the papA gene and increased UPEC’s motility by upregulating fliC (flagellar gene). The direct regulation of type 1 and P fimbriae and flagella by FNR was confirmed using a gel mobility shift assay. In addition, FNR protein binds to the promoter regions of two novel Two-Component Systems (TCSs). One TCS modulates the expression of hemolysin, a very important virulence factor of UPEC, and the other is involved in α-ketoglutarate utilization under anaerobic conditions, which mediates UPEC’s adaptation to renal physiology. Finally, comparative testing of wild-type and mutant UPEC in adherence and invasion assays demonstrated that deletion of fnr significantly decreases UPEC’s adhesion to and invasion of bladder (HTB-1) and kidney (HTM-44) cells. Furthermore, in a mouse model of urinary tract infection, the fnr mutant showed significantly reduced colonization in the bladder and kidney as compared to the wild type, while reintroduction of fnr back into the mutants restored wild-type levels of colonization in both bladder and kidney. Such results suggest that FNR is very important in the regulation of UPEC virulence.