Definition of Pili:
Some gram-negative bacteria have numerous fine, thread-like appendages called pili or fimbriae on their outer surface. They are more numerous, shorter and stiffer than flagella. They are made up of a protein called pilin. The presence of pili or fimbriae is determined by genes located on plasmids. Their length is 0.2 – 20µm and their diameter is usually 250Å.
Types of Pili:
Pili are generally of two types, namely-
1. Simple pili: Simple pili help bacterial cells to cling to each other or to the host.
2. Sex pili: These pili are used by one bacterium (F*) to establish a connection with another bacterium (F-) and through this, genetic material is transferred. The tip of the sex pili of the F* cell of E. coli can recognize the F-cell by touching it.
Pili usually intertwine with each other in a liquid medium to form a pellicle and help in oxygen uptake. Sex pili act as conjugation tubes. Pili help in hemagglutination due to antigenic properties.