Fimbriae

August 11, 2018

These are thin short filamentous appendages which measure about 1-1.5 micron in length and less than 4 to 8 nm in width.
They extrude from the cytoplasmic membrane and are also called pili.
They are found in certain gram-negative bacilli, including saprophytic, intestinal commensals, and pathogenic species in the family
These are different kind from flagella which may occur in some motile and non-motile forms.
They are far numerous than flagella e. 100-500 being borne peritrichously by each cell.
They are best developed in freshly isolated strains from liquid culture.
They tend to disappear when subcultures are made on solid media.
They are composed of protein subunits called pilin.
They cannot be seen with the light microscope but are clearly seen with the electron microscope.
They are visible only in preparations that have been metal-shadowed or negatively stained with phosphotungstic acid.

Image source: quora

Occurrence

They occur mostly in Enterobacteriaceae like Proteus, Shigella, Salmonella, Serratia,

Types

There are 3 main types of fimbriae, common pili, sex or F (fertility) pili and col I pili (colicin).
Common pili are of 6 types which are based on the morphology, number per cell, adhesive properties and antigenic nature.
The virulence in pathogenic bacteria is due to the toxins as well as ‘colonization antigens’ i.e. pili which provides adhesive properties.

Functions

a) Adhesin

Pili are organs of adhesion on cells.

b) Sex pili

They are specialized fimbriae which are fewer in number, possessed by male bacteria.
They are longer (18-20 micron) and 1-4 in number and hollow tubes.
They help the male cells to attach with non-male (female) cells.
They help in the formation of ‘conjugation tubes’ through which genetic material is believed to be transferred from the donor to the recipient cell.
They also act as receptor sites for certain bacteriophages described as being ‘male specific’.

c) Haemagglutination

The majority of fimbriate bacteria bear fimbriae of a type that helps them in adhesion.
They can adhere in different kinds of tissues cells, the red blood cells of many animal species like guinea-pig, horse and pig red blood cells very strongly, to human cells moderately strongly, to sheep cells weakly and to ox cells scarcely at all.
A drop of the concentrated suspension of fimbriate bacilli can form clumps visible to the naked eye if mixed to a drop of a suspension of red cells.
A simple haemagglutination test can be used to determine the fimbriate bacilli in culture.
There exist different types of fimbriae having different adhesive properties.
The commonest type is kind type 1, which occurs in Escherichia, Klebsiella, Serrtia, Salmonella and Shigella which is about 8 nm in width and their adhesive properties are affected by a small amount of D-mannose.
Some Klebsiella and Serratia organisms may possess type 3 fimbriae about 5nm in width.
These do not agglutinate the red cells unless they are heated or tanned and the adhesive properties are not affected by
Proteus organisms possess a third kind of fimbriae (type 4) which have mannose resistant haemagglutinating activity against untreated red cells of certain species.
A few Salmonella species consist of a fourth kind of fimbriae (type 2) apparently devoid of all haemagglutinating and adhesive properties.
In some bacteria i.e. many strains of Escherichia coli haemagglutinating factors are present but not associated with fimbriae.