Pili in Gram - positive pathogens

| Most bacterial pathogens have long filamentous structures known as pili or fimbriae extending from their surface. These structures are often involved in the initial adhesion of the bacteria to host tissues during colonization. In Gram-negative bacteria, pili are typically formed by non-covalent interactions between pilin subunits. By contrast, the recently discovered pili in Gram-positive pathogens are formed by covalent polymerization of adhesive pilin subunits. Evidence from studies of pili in the three principal streptococcal pathogens of humans indicates that the genes that encode the pilin subunits and the enzymes that are required for the assembly of these subunits into pili have been acquired en bloc by the horizontal transfer of a pathogenicity island. R E V I E W S NATURE REVIEWS | MICROBIOLOGY VOLUME 4 | JULY 2006 | 509 © 2006 Nature Publishing Group Type II secretion systems Systems that are responsible for the extracellular secretion of toxins and hydrolytic enzymes, many of which contribute to pathogenesis in both plants and animals. Secretion through this type of pathway differs from most other membrane-transport systems in that the substrates of this pathway are folded proteins. This pathway shares many features with the type-IVpilus biogenesis system, including the ability to assemble a pilus-like structure. Nucleation-dependent polymerization A process that is involved in the assembly of curli pili in Escherichia coli. At least five proteins are dedicated to the assembly of these structures on the cell surface. There are two key proteins: CsgA (the major subunit), which can polymerize into curli pili when it comes into contact with CsgB (the minor subunit). It has been proposed that CsgB induces a conformational change in CsgA that nucleates its assembly into pili. Polymerized CsgA could then induce a similar change in the next incoming soluble CsgA and continue the