The Types And Function Of Pili And Fimbriae

What Are Pili And Fimbriae?

Both pili and fimbriae are hair-like structures or appendages that are usually constructed from protein. They usually extend off of the cytoplasmic membrane from a variety of different types of bacteria.

They are often compared to flagella, but both pili and fimbriae are thinner and shorter than these. In spite of this, they are both different from each other and have some shared and some exclusive properties.

They are both occasionally found on some gram-positive bacteria, however they are most commonly found on almost all gram-negative bacteria being one of their most common features.

If you want to know more about the functions of pili and fimbriae, what their different types are, and what makes them unique from each other and other proteinaceous cell structures, keep reading to find all of this out!

Different Types Of Pili

The plural for pili is pilus and compared to fimbriae they tend to be longer in length as well as thicker.

As previously mentioned, they are sometime found on gram-positive types of bacteria, but are found on almost all gram-negative bacteria, and archaea as well. When talking about pili, there are 2 main varieties worth talking about.

Conjugative Pili

This type of pili is sometimes also referred to as sex pili, and these conjugative pili are one of the most common types of bacterial pili. As the name implies, this type of pili is most commonly used in the process of transferring the genetic material of one cell onto another.

In comparison to other types of Pili, the F pilus (being the F sex pilus) has generally received more attention, and because of this, there is much more information surrounding it.

This is encoded by the F plasmid, and the F pilus tends to be seen in “male” gram-negative bacteria. F-pili tend to be close to 20um in length.

There have been microscopic studies made on these pili using incredibly powerful electron microscopes and the appendages have been recognized to have dynamic movement.

They have been recognized to be able to retract as well as be able to elongate on a continuous basis.

When considering their structure, pili are a polymer which is made of a protein called pilin (VirB2) and is considered to be quite similar to the F-like pilus pED208.

This is because they both have been measured at 87Å in diameter on average and have both shown to have an internal lumen which is around 28Å in diameter as well.

Different types of pili have been shown to have its protein building blocks either arranged in a helical manner or a pentamer layer depending on the type of pili. A helical structure means there are 5-start helical filaments involved.

When considered that bacterial pili tend to originate from the membrane, they also are comprised of phospholipids as well as proteins, and these all form a protein-phopholipid complex.

In spite of this, the lipid which is present in pilus has shown that it is quite different from the lipid that is present in the membrane. This is attributed to binding the TraA to a section of the phospholipid during the pili formation. 

What Are The Functions Of A Conjugative Pili?

As has been previously mentioned, the conjugative pili are mainly used to transfer DNA from a bacterial cell to another one (male F+ to F-). And this is why they are often called sex pili.

However, this process is not fully understood, it can be understood that the single strands of DNA are able to pass through the pilus’ hollow lumen and this is what is utilized to transfer the genetic data from the giver to the recipient. 

While some do not agree with this interpretation of the process, there have been studies made which display that the genetic material that is being transferred is able to be transferred over a sizeable distance in which the donor as well as the recipient are not actually in direct contact with each other through the use of the F-pili.

The F-pilus are also understood to have an important role when it comes to the identification of the recipient cell and are also maybe responsible for the contact that is between the 2 cells. 

Type IV Pili

This is a type of pili which is found on most of the gram-negative bacteria as well as some gram-positive bacteria like clostridia. This type of pili has quite a few important functions, this can range from a role in DNA exchange, to a role in locomotion.

Similarly to conjugative pilus, these type IV pili are a tube-like proteinaceous structure that starts from the membrane. They are strong and also flexible which makes them perfect for their use and in performing their functions.

As well as this, structurally these type IV pili are polymers which means that they are made pilin protein. This is where many copies of pilin (usually thousands) will be polymerized from different subunits in the membrane and will then construct a filament of the type IV pilus.

While the polymerization of the pilin copies is not fully understood, there is a general understanding that this process involves assembly of the ATPase on top of the core protein as well as the inner membrane.

There are pilins that are involved with the formation of type IV pili and these are split into 2 categories.

These categories are type IVa pilins (that are defined by having about 5 to 7 amino acid signal peptides as well as phenylalanine) and then type IVb pilins and these consist of longer peptides as well as having a hydrophobic residue.

While the pilus is made, a protein called secretin is used for the formation of an oligomeric gated channel on the outer membrane of some bacterial cells and this is so the pilus is able to move through this.

In this, PilN as well as PilO that are both pilus alignment subcomplex proteins which are used on the inner membrane, will come into contact with secretions and then channel the protein PilP which will result in forming the periplasmic conduit which allows pilus to be able to grow through.

These activities make it so the pilus which originates from this membrane will be able to extend to the outer environment.

Type IV polymers are characterized by having a helical structure and have an orientation of awesome between 6 nm to 1um for their length.

It is also worth noting that dependent on the species of the pilus, the pilin could be either glycosylated or it could be phosphorylated.

What Are The Functions Of Type IV Pili?

In comparison to the previous conjugative pilus, the type IV pili have a few unique functions to help them stand out from this type of pili.

Firstly their adhesion characteristics help them stand out. The adherence capabilities are on of the most important functions of Type IV pili as on top of making it so attaching to a bacterial cell or any other surfaces is easy.

They also make it easy for its cell to use the filament to adhere and attach to other bacteria which is a very useful skill.

This ability of the bacteria to be able to adhere to many surfaces is possible because of the diverse sequence of the pilin amino acids that are inside the pili.

This adhesion to bacterial cells which is achieved through the type IV pili contributes to the process of the conjugation process.

After type IV pili achieve adhesion, the pili is able to utilize its retraction capabilities which it uses to bring the two cells even closer together and then for ease of DNA exchange.

This activity is also shown to be important when it comes to uptake of viruses on top of invading host cells when it comes to use by parasitic bacteria.

Another important function of the type IV pili is its motility. Apart from the unique adhesion, this is one of the most important functions of type IV pili.

Due to the aforementioned ability to retract, it is possible for the bacteria using the pili to be able to move itself along surfaces and this process is known as twitching motility.

Generally speaking this method of motility is used in 3 main stages that are the extension, the tethering, and then the retraction.

After the type IV pili extends, it will then attach/adhere to a surface which is the tethering stage. Following this tethering it will then retract. This makes it, so the cell is able to easily glide over different surfaces and move directly in the direction it wants to.

This cycle of motility has been displayed to occur at the speed of 0.5um every second.

It is possible for a single bacterial cell to have multiple pili which are all independently working to help the bacterial cell with its motility.

The final important unique function of the type IV pili is the biofilm formation. This is in reference to the aggregation of microorganisms which is used by this type of pili for overcoming stressful circumstances.

As previously mentioned, pili are used in motility and this makes it, so cells have the ability to move to certain areas, and because of this they are proven to have a significant role when it comes to forming microcolonies and because of this, the maturation of biocolonies.

This means they do not just adhere to surfaces as well as cells, but also are used for promoting proximity from cells to biofilm formation.

There are some other functions of type IV pili worth mentioning. These include their electrical conductivity, their protein production, and finally their adhesion of their bacterial cells onto eukaryotic cells.

Type V Pili

It is also worth mentioning type V pili which are a specific style of pili which is found on the certain type of bacteria called Bacteroidia.

How these pili are formed is not fully understood, however researchers suggest that it could involve protease-mediated polymerization. Similarly to the type IV pili, this type of pilus is important for its function for adhesion as well as biofilm formation.

Different Types Of Fimbriae

Fimbriae are also sometimes referred to as attachment pili, they are characterized by being shorter than pili but compensate by being more numerous than they are with having anywhere between 100 and 600 present on any one cell.

Dependent on the type of bacteria, fimbriae could be found on just the poles of the cell, or they could have a complete even distribution over the bacteria surface. Because they are shorter, the fimbriae are much more stiff than pili.

Similarly to pili, fimbriae have multiple varieties which we will now go over.

Type I Fimbriae

These type I fimbriae are usually found on the surface of most gram-negative bacteria like E.coli, and it is usually used for adhesion. They are usually between 1 and 2 um in length and close to 7 nm in width. Similarly to most other fimbriae, they are stiff and rarely bend.

On top of this they are shown to be a structure that consists of a short thin tip of fibtillin which is located on the distal end of the rod. Similarly to pili, these fimbriae are characterized by their helical orientation.

Like this, the structure is distinct for a right-handed helical orientation which consists of 27 FimA subunits with close to 8 helical turns.

Functions Of Type I Fimbriae

These Type I fimbriae are one of the more common structures on the family of Enterobacteriaceae. In this they are usually used for the adherence of bacteria to its host.

In spite of this, they are shown to have specific glycoproteins which have one or N-linked with high mannose structures. Because of this, type I fimbriae are assumed to promote infection within the lower urinary tract as well as with some mucosal surfaces.

Type III Fimbriae

The type III fimbriae are more thin fimbriae which has a diameter between 5 nm and are between 0.5 and 2um in length. Similarly to type II fimbriae, these type III fimbriae are common in the family of Klebsiella ssp. as well as Enterobacteriaceae.

Similarly to other fimbriae they are assembled through the usher/chaperone pathway.

In this the subunits (or building blocks) will be transported into the periplasm with the general secretory pathway in which a chaperone which is encoded by the gene mrkB will promote development and then assembly of the final fimbriae.

Like the pili as well as fimbriae, these subunits MrKA of the Type III fimbriae are polymerized in the style fo a helical orientation. Te beta strands of the C-terminal region wihtin the subunits provide some strucutral support to its appendage.

Functions Of Type III Fimbriae

The type III fimbriae have an important role with the adhesion of its bacteria onto abiotic surfaces on top of formation of biofilm. With bacteria like K.pneimonia, the attachment to surfaces, for example catheters, will result in aggregation which will lead to biofilm formation.

This is good for the bacteria as it makes sure that pathogeneis is promoted and this ends up promoting antibiotic resistance within certain species of bacteria.

Curli Fimbriae

Curli is a type of fimbriae which similar to others is found mainly in gram-negative bacteria with some examples being Escherichia as well as some species of Salmonella.

Within E.coli, the biogeneis of fimbriae is often associated with about 6 proteins which will be encoded by csgBA as well as csgDEFG operons. Similarly to fimbriae as well as pili, curli fimbriae is able to extned from the outer membrane.

Functions Of Curli Fimbriae

The main role of curli fimbriae is their involvment in biofilm formation.

An example of this is the use of fimbria with whihc studies have displayed that the spcific bacterium Salmonella enteritis is able to adhere to surfaces like, for example, stainless steel and this leads to the formation of biofilm which leads to food materials getting contaminated.

The Differences Between Pili And Fimbriae

As mentioned back in the introduction, pili and fimbriae are often compared to each other due to their similar structure and the fact that they are both common appendages of gram-negative bacteria.

They are both also filaments which are made from different proteins and they both originate on the membrane of their cells and then extend outwards from it.

Because of this they are both easy to recognize using an electron microscope, and because they are both antigenic they are both able to create immune responses when in vivo.

But in spite of these similarities, there are some key differences which should be remembered to help differentiate the two filamentous appendages:

Firstly is their size, as has been previously mentioned, they are both smaller than flagella, however pili tend to be significantly larger than fimbriae with them resembling hair while pili are closer in appearance to bristles.

Secondly, the bacteria which they are found on, fimbriae are much more likely to appear on both gram-positive as well as gram-negative.

In comparison to this, pili are much less likely to appear on gram-positive, with most of the bacteria they appear on being gram-negative.

Also, their distribution sets them apart with fimbriae being much more numerous on a single cell with their often being an even distribution over a cell’s surface.

A bacterium could have anywhere up to around 400 fimbriae. In comparison, cells tend to have only somewhere between 5 and 10 pili.

Finally, and one of the most important factors to keep in mind is the difference in function. Fimbriae are mostly involved i adhesion and attachment which is used to promote the growth of biofilm.

Pili are also involved in attachment, but they are also involved in gene transfer, motility, and some other roles. The differing structure is also worth considering.

Fimbriae have a significantly more solid and less flexible structure to enable them to fulfill their function, however, pili, especially the sex pili, emphasize their hollow tubular design and are more flexible to make transferring genetic data a lot more simple and easy.

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