- 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.
- They occur mostly in Enterobacteriaceae like Proteus, Shigella, Salmonella, Serratia,
- 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.
- Pili are organs of adhesion on cells.
- 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’.
- 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.