Pili

The pili are the plural form of the Latin word Pilus, which means hair. It is a hair-like appendage found on the surface of many bacteria and archaea. The term pili can also be interchangeably used with the term fimbria, which is Latin for ‘fringe’. In some bacteria, pili are used for bacterial conjugation. The hair-like appendages pili are primarily composed of pilin proteins, which are oligomeric. Oligomeric proteins are composed of more than one polypeptide chain. The meaning of the word pili in Hawaiian also reflects its nature, which is to touch, join, cling, stick, adhere, associate with, be with, be close, adjoin, cleave to, or adjacent.

Pili

Pili is antigenic, which means they show immune responses triggered by the antigens. They are also fragile and replaced constantly, sometimes with pili of different compositions, resulting in altered antigenicity. Specific hosts give responses to the older pili structure and aren’t effective on the new structure. Recombination genes for the pili code of the variable (V) and constant (C) regions of the pili or similar to immunoglobulin diversity. 

Brinton received the credit for the detailed study of bacterial pili. He showed that pili has two distinct phases in a single strain of bacteria: pileated and non-pileated.

Pili prevents the attack of white blood cells on bacteria. Streptococcus pyogenes prevent themselves from the host’s phagocytosis with the help of certain proteins and pili as they act as resistance and adhere to the bacteria. Pili have a hollow core, which allows the transfer of genetic material. This process is known as conjugation. 

Pili are most commonly involved in adherence to surfaces. Just like the host cell bacteria, which requires adherence for colonising surfaces and pathogenicity, pili are negatively charged. This causes a net repulsive force. To overcome this problem, the pili extend with adhesins located on their tip. The adherence can be specific to certain surfaces, for example, as it is seen in Neisseria gonorrhoeae. Neisseria gonorrhoeae specifically adheres to the cervical or urethral epithelium with the help of its pili.

Pili Diagram

The pilus shaft is the name of the top part of the pilus. Pilin protein is the constituent of pilus shaft. 

This pilus shaft is made up of repeating proteins called pilin. Different species of bacteria show a variety of pili, and their protein structure also differs.

Consider an example, in type Iva pili, three different major subunits are present in the species of Pseudomonas, Neisseria, and Vibrio cholerae. These subunits are named PilA, PilE, and MshA. As per the conditions, bacteria have different pili.

 Gram-negative bacteria consist of four varieties of pili whose categorisation is based on the pathways of their assembly. As far as we know, the variations in the pili present in Gram-positive bacteria are lesser. They are categorised into one of two groups, the short, thin rods, and the longer, flexible pili. In Streptococcus species, the short and thin pili have been found, whereas, in Corynebacterium species and pathogenic streptococci, the longer and flexible pili are found.

The assembling of Pili in Gram-negative and Gram-positive bacteria are different. In Gram-negative pili, there are differences in the mechanism of assembling pili. In Gram-negative pili, excretion and assembly of subunits occur near to the developing pilus, and the mechanism of assembly is different in Gram-positive bacteria.

Conclusion

Pili are evenly distributed around the surface of the cell or localised to one or both of the poles. The size of pili is small and is thinner than the flagella. These are the characteristic features of Gram-negative bacteria. They can also be observed in Gram-positives as well as archaea. 

Dozens of these structures can exist on bacterial and archaeal surfaces. Some bacteria, viruses, or bacteriophages attach to receptors present on pili during the onset of their reproductive cycle.