Fimbriae, Pili, and Axial Filaments

Fimbriae and pili are thin, hairlike appendages, usually 2-10 nanometers in diameter, made of pilin protein and extend from the bacterial cell surface.

Fimbriae enable bacterial cells to adhere to the available surfaces, enabling the formation of biofilms.

They also help bacteria adhere to their host cells, facilitating colonization and establishing infections, as observed in pathogens like Neisseria gonorrhoeae.

Pili are generally longer and fewer in number than fimbriae.

Pili also contribute to adhesion, allowing pathogenic bacteria like Pseudomonas aeruginosa to attach to their host cells.

Pili also help in bacterial conjugation, where specialized pili, known as sex pili, or conjugative pili, facilitate the transfer of genetic material between bacterial cells.

Type IV pili, another specialized pilus, are involved in twitching motility, which allows bacteria to move along surfaces.

Axial filaments, or endoflagella, found in spirochetes, such as Treponema pallidum are located within the periplasmic space.

These axial filaments enable spirochetes to move with a corkscrew-like motion, which allows them to move efficiently through viscous environments such as bodily fluids.