Bacteria may have one, two, many, or no flagella at all. Bacteria that can swim are said to be motile. Bacteria that lack flagella are non-motile. They are also known as attachment pili. These fibers are used to attach bacteria to surfaces.
Many pathogenic bacteria use pili to adhere to animal tissues.
For example , E. coli adheres to urinary tracts via pili. The tight binding by the pili to the urinary tract cells prevents the bacteria from being washed away when the bladder is voided. A pilus very long and is used to transfer DNA from one bacterium to another. These structures are hollow tubes. The protein fiber winds itself into a tight helix that forms a tube.