Escherichia coli curli fibers are amyloid-like structures that play a role in bacterial colonization and biofilm formation. This study shows that curli fibers, produced by E. coli, are amyloid-like and have properties similar to those of human amyloids. The curli fibers are composed of the CsgA curlin subunit, which, when purified and incubated, forms fibers indistinguishable from curli. In vivo, curli biogenesis requires a specific assembly machinery involving the CsgE and CsgF proteins. Unlike eukaryotic amyloid formation, curli biogenesis is a productive pathway that requires a specific assembly machinery. Curli are extracellular fibers expressed by E. coli and Salmonella spp. that are involved in colonization of inert surfaces and biofilm formation. They mediate binding to a variety of host proteins. Curli fibers appear as 4- to 7-nm-wide fibers under electron microscopy. They are rich in β-sheet secondary structure and can bind Congo red and thioflavin T, indicating their amyloid-like nature. The study also shows that curli biogenesis is dependent on the nucleation-precipitation machinery encoded by the csgAB and csgDEFG operons. CsgB is thought to nucleate CsgA fibers, while CsgE and CsgF are involved in the assembly process. A csgF- mutant showed aberrant CR binding properties, indicating a defect in curli assembly. However, the mutant could still act as a recipient and guide CsgA polymerization. The study also shows that CsgA can be secreted in a soluble, assembly-competent form by a csgB- mutant and can be assembled on the surface of a csgA- mutant. This process, called interbacterial complementation, indicates that CsgA is secreted from csgF- cells and assembled on the CsgB+ recipient cells. The study also shows that the nucleation-precipitation assembly machinery may be critical in preventing CsgA polymerization within the cell and accelerating it at the cell surface. The findings suggest that bacterial amyloids could play a role in certain human neurodegenerative and amyloid-related diseases. This study provides a useful model system to study the formation of amyloid fibers.Escherichia coli curli fibers are amyloid-like structures that play a role in bacterial colonization and biofilm formation. This study shows that curli fibers, produced by E. coli, are amyloid-like and have properties similar to those of human amyloids. The curli fibers are composed of the CsgA curlin subunit, which, when purified and incubated, forms fibers indistinguishable from curli. In vivo, curli biogenesis requires a specific assembly machinery involving the CsgE and CsgF proteins. Unlike eukaryotic amyloid formation, curli biogenesis is a productive pathway that requires a specific assembly machinery. Curli are extracellular fibers expressed by E. coli and Salmonella spp. that are involved in colonization of inert surfaces and biofilm formation. They mediate binding to a variety of host proteins. Curli fibers appear as 4- to 7-nm-wide fibers under electron microscopy. They are rich in β-sheet secondary structure and can bind Congo red and thioflavin T, indicating their amyloid-like nature. The study also shows that curli biogenesis is dependent on the nucleation-precipitation machinery encoded by the csgAB and csgDEFG operons. CsgB is thought to nucleate CsgA fibers, while CsgE and CsgF are involved in the assembly process. A csgF- mutant showed aberrant CR binding properties, indicating a defect in curli assembly. However, the mutant could still act as a recipient and guide CsgA polymerization. The study also shows that CsgA can be secreted in a soluble, assembly-competent form by a csgB- mutant and can be assembled on the surface of a csgA- mutant. This process, called interbacterial complementation, indicates that CsgA is secreted from csgF- cells and assembled on the CsgB+ recipient cells. The study also shows that the nucleation-precipitation assembly machinery may be critical in preventing CsgA polymerization within the cell and accelerating it at the cell surface. The findings suggest that bacterial amyloids could play a role in certain human neurodegenerative and amyloid-related diseases. This study provides a useful model system to study the formation of amyloid fibers.