Pili are proteic retractile filaments up to 20 micrometer long that protrude from gram-negative bacteria. Some RNA and DNA bacteriophages use pili to attach to the host cell.
There are many types of pili and each bacterial virus binds specifically to a precise type. Some filamentous viruses like bacteriophage fd adsorb onto the tip of F-pili and enter the cell at the pili basal pore. Other filamentous phages can target I-pili (phage If1), N-pili (phage IKe), or type IV pili (e.g. bacteriophages Pf1 and Pf3) . Icosahedral virus usually adsorb onto the sides the pili.