The extracellular nucleation-precipitation (ENP) pathway or Type VIII secretion system (T8SS) in Gram-negative (diderm) bacteria is responsible for the secretion and assembly of prepilins for fimbiae biogenesis, the prototypical curli. Besides the T2SS that can be involved in the assembly of prototypical Type 4 pilus, the T4SS that can be involved in the biogenesis of the prototypical pilus T, the T3SS involved in the assembly of the injectisome and the T7SS involved in the formation of the prototypical Type 1 pilus, the T8SS differs in that fibre-growth occurs extracellularly. The curli, also called thin aggregative fimbriae (Tafi), are the only fimbriae dependent on the T8SS. Tafi were first identified in Salmonella spp and the controlling operon termed agf; however subsequent isolation of the homologous operon in E coli led to its being called csg. In the absence of extracellular polysaccharides Tafi appear curled, although when expressed with such polysaccharides their morphology appears as a tangled amorphous matrix. CsgF is one of three putative curli assembly factors appearing to act as a nucleator protein. Unlike eukaryotic amyloid formation, curli biogenesis is a productive pathway requiring a specific assembly machinery.