pili

Pili are thin, hair-like surface appendages that extend from the cell envelope of many bacteria and some archaea. They are composed of protein subunits called pilins and can range from a few hundred nanometers to several micrometers in length. Pili serve a variety of functions, including adhesion to surfaces and host tissues, DNA uptake, and, in some systems, genetic exchange.

There are several classes of pili. Fimbriae (adhesive pili) are usually short and numerous and mediate attachment by recognizing specific receptors on surfaces or cells. Sex pili are longer, less numerous, and function to connect donor and recipient cells during conjugation, enabling horizontal transfer of plasmid DNA. Type IV pili are versatile and dynamic, capable of extension and retraction; they mediate twitching motility, surface attachment, biofilm formation, and, in some organisms, uptake of DNA during transformation.

Assembly of pili occurs through distinct secretion pathways. Fimbriae are often assembled by the chaperone-usher pathway in Gram-negative bacteria, producing rigid, ordered filaments. Type IV and sex pili are assembled by specialized secretion systems powered by ATPases, and type IV pili are retractable through the action of specific ATPases like PilT, enabling movement and DNA uptake.

Functions of pili are central to microbial ecology and pathogenesis. By mediating adhesion, pili promote colonization and biofilm formation. Sex pili enable sharing of genetic material, spreading traits such as antibiotic resistance. Type IV pili contribute to surface exploration and host interactions and can participate in natural competence for DNA uptake.

Pili are a focus of research due to their role in infection, bioengineering, and as potential targets for vaccines or anti-adhesion therapies.