Flagella are filamentous protein structures attached to the cell surface that provide swimming movement for most motile bacterial cells. The diameter of a bacterial flagellum is about 20 nanometers, well-below the resolving power of the light microscope. The flagellar filament is rotated by a motor apparatus in the plasma membrane allowing the cell to swim in fluid environments. During aerotaxis , bacteria swim toward or away from O2. For a few pathogens motility is known to be a determinant of virulence .
Flagella are antigenic , and therefore, vulnerable to attack by host antibody molecules .
Fimbriae and Pili are interchangeable terms used to designate short, hair-like structures on the surfaces of bacterial cells. Fimbriae are shorter and stiffer than flagella, and slightly smaller in diameter. Like flagella, they are composed of protein. Inasmuch as many bacteria are able to exchange genes for virulence by means of conjugation, the sex pilus which confers the ability to conjugate, may well play a role in the their assembly of virulence determinants. Fimbriae and flagella on the surface of a bacterium.