Structure and synthesis
Many gram-negative bacteria have short, fine, hair-like surface appendages called fimbriae or pili depending on their function. They are shorter and thinner than flagella (0.1 to 1.5 μm in length and uniform width between 4 and 8 nm) and emerge from the cell wall. Single cells have been seen to be covered with as few as 10 fimbriae to as many as 1000. They originate in the cytoplasmic membrane and are composed of structural protein subunits termed pilins like flagella. They occur in non-motile, as well as in motile strains. Fimbriae are antigenic. It is necessary to ensure that the bacterial antigens employed for serological tests and preparation of antisera are devoid of fimbriae as members of different genera may possess the same fimbrial antigen.
Demonstration of Fimbriae
1.Electron microscopy: They cannot be seen with the light microscope but are only visible in an electron microscope due to their smaller size.
2.Hemagglutination: Most fimbriate bacteria bear fimbriae of a type that enables them to adhere to, among other kinds of tissue cells, the red blood cells of many animal tissues. They adhere to guinea pig, fowl, horse and pig red cells very strongly, to human cells moderately strongly, to sheep cells weakly and to ox cells scarcely at all. The adherence functions can be detected by the ability of bacteria to adhere to epithelial cells or to cause hemagglutination. Therefore, a simple hemagglutination test can be used to determine whether a culture contains fimbriate bacilli or not. The adherence of bacteria to red cells, cultured cells, or tissue surfaces can be competitively blocked by the fimbriae.
Types of Fimbriae
Different types of fimbriae have different adhesive properties and can be divided into four types:
Type 1 fimbriae (mannose-sensitive)
Type 2 fimbriae
Type 3 fimbriae (mannose-resistant)
Type 4 fimbriae.
Functions of Pili
Two classes can be distinguished on the basis of their function:
ordinary (common) pili and sex pili.
1.Ordinary (common) pili: Fimbriae probably function as organs of adhesions that allow attachment of a bacterial cell to other cells or surfaces. The adhesive property may be of value to the bacteria in holding them in the nutritionally favourable microenvironment.
2.Sex pili: Sex pili are similar to fimbriae, about 1-10 per cell, but are functionally different. These are longer and fewer in number than other fimbriae. They are genetically determined by sex factors or conjugative plasmids and appear to be involved in the transfer of DNA during conjugation. They are found on ‘male’ bacteria and help in the attachment of those cells to ‘female’ bacteria, forming hollow conjugation tubes through which, it is assumed, genetic material is transferred from the donor to the recipient cell. Some bacterial viruses attach specifically to receptors on sex pili at the start of their reproductive cycle.
Pili are classified into different types (for example, F, I) based on their susceptibility to specific bacteriophage.