A hollow, thread-like structure, many of which are present on some bacteria;
pili are shorter than flagella and very
thin (about 10 nanometers in diameter). They function in the adherence of
bacteria to one another and to animal cells, as tools for grabbing onto
and manipulating objects at a distance, and as tactile and sensory structures.
They also serve in the formation of cytoplasmic bridges along which DNA
can pass between bacteria during conjugation.
This is a rare, but important event with profound implications for bacterial
evolution. Bacteria are not very fastidious about who they exchange DNA
with. Thus genes can be acquired from unrelated
bacteria, and even from non-bacteria. For this reason, "lateral inheritance"
of genes from unrelated organisms is quite frequently observed in bacteria.
Pili are as diverse in structure as they are in function. Typically the
backbone of the pilus is made up of a long chain protein or polysaccharide
chain with some type of functionally specific arrangement at the tip. One
function of clinical interest is cell-to-cell recognition. The complex array
of carbohydrates in and on the pili are the method by which bacteria recognize
other cells, and are recognized by them.