Development of intracellular bacterial communities of uropathogenic Escherichia coli depends on type 1 pili

Summary Uropathogenic Escherichia coli , the predominant causative agent of urinary tract infections, use type 1 pili to bind and invade bladder epithelial cells. Upon entry, the bacteria rapidly replicate and enter a complex developmental pathway ultimately forming intracellular bacterial communities (IBCs), a niche with biofilm‐like properties protected from innate defences and antibiotics. Paradoxically, bacteria within IBCs produce type 1 pili, an organelle thought only to be an extracellular colonization factor. Thus, we investigated the function of type 1 pili in IBC development. The cystitis isolate, UTI89, was genetically manipulated for conditional fim expression under control of the tet promoter. In this strain, UTI89‐ tetR /P tet fim , piliation is constitutively inhibited by the tetracycline repressor, TetR. Repression is relieved by anhydrotetracycline (AHT) treatment. UTI89‐ tetR /P tet fim and the isogenic control strain, UTI89‐ tetR , grown in the presence of AHT, colonized the bladder and invaded the superficial umbrella cells at similar levels at early times in a murine model of infection. However, after invasion UTI89‐ tetR /P tet fim became non‐piliated and was unable to form typical IBCs comprised of tightly packed, coccoid‐shaped bacteria in contrast to the control strain, UTI89‐ tetR . Thus, this work changes the extracellular colonization functional paradigm of pili by demonstrating their intracellular role in biofilm formation.