The sequence of events that take place during the progression of the establishment of a urinary-tract infection (UTI). The infection is cyclical in nature and the events that occur in one cycle of the acute infection are depicted. Uropathogenic Escherichia coli (UPEC) bind to, and invade, superficial bladder epithelial cells. At 'early' stages, the average bacterial length is 3 mum and the community doubles in size every 30 minutes10. Transition to the 'middle' intracellular bacterial-community stage is hallmarked by a decrease in the average bacterial length to 1 mum and a doubling time of 45 minutes. In 'late' stages, the bacteria return to the typical rod shape (3 mum)10. Filamentous and late-stage rod-shaped bacteria detach from the community, 'escape' from epithelial cells and either attach to naive epithelial cells or exit the host during micturition. Filamentous UPEC are induced intracellularly in the urothelial cells that are capable of recognizing bacterial lipopolysaccharide11. The acute infection can progress through additional cycles (indicated by arrows). This figure is modified, with permission, from Ref. 10 © (2004) National Academy of Sciences.