Bacterial Flagella, Fimbriae and Pili

Cell Surface Appendages of Bacterial Flagella (Flagella vs Fimbriae vs Pili of Bacteria). Flagella are long whip-like filamentous structures.

Bacterial Capsule

  • Some bacteria have a layer of material lying outside the cell wall which is well organized and not easily washed off is called a capsule.
  • The capsule contains 90% water. Hence difficult to stain.
  • Capsules are clearly visible on a light microscope when negative stains or special capsule stains are used.
  • They can also be read with an electron microscope.
  • Capsules are not required for bacterial growth and reproduction Capsules confer several advantages when bacteria grow in their normal habitats.
  • These bacteria are resistant to phagocytosis by phagocytic host cells.
  • Example – Streptococcus pneumoniae. In the absence of a capsule, it is easily destroyed and will not lead to diseases, while the closed option is a quick kill in the mice.
  • In human pathogens and capsules for all ports of antigen vi (virulence).
  • Capsules contain a lot of water and bacteria and protect them from drying out.
  • They exclude bacterial viruses and most hydrophobic toxic materials such as detergents.
  • Glycocalyx also helps in attaching bacteria to the surface of solid objects in an aquatic environment or to the surface of tissue in plant and animal hosts.
  • Gliding bacteria tend to produce mucus, which probably contributes to their motility.

Bacterial S-layer

  • The S-layer is a regular structure layer.
  • To participate in many gram-positive, due to peptidoglycan and gram-negative bacteria, is located directly on the outer surface of the membrane.
  • The S-layer is also very popular among archaea, where it is the only wall structure outside the membrane.
  • The S-layer has a pattern similar to a tile on the floor and consists of proteins and glycoproteins.

What are the functions of bacterial S-layers?

  1. To protect cells from ion and pH fluctuations, osmotic stress, enzymes, or predatory bacteria Bdellovibrio.
  2. Helps maintain the shape and stiffness of the shell of at least some bacterial cells.
  3. It is able to promote cell adhesion to the surface.
  4. It must protect itself from pathogens by attacking complement and phagocytosis, and thus contribute to their virulence.

Bacterial Flagella and its Motility

Most motile bacteria move around using flagella (s. flagellum).

Filamentous locomotor appendages extending beyond the membrane and cell wall.

They are Thin, rigid structures, about 20 nm long and up to 15-20 meters long.

So thin that they can be directly observed with a light field microscope, but they need to be colored with special methods designed to increase the thickness.

You can see the detailed structure of the beacons in a scanning electron microscope.

Bacterial species often have differences in the distribution of flagella.

The monotonous bacterium tristichous means for it to have flagella; if it is found at the end, then it is said to be Poland’s plague.

Amphitrichid bacteria (Amphi-means “on both sides”) and have one flagellum at each pole.

Conversely, lophotrichous bacteria (loophole mean small) are a collection of cells at one end or both ends. Spire.

The flagellates were relatively evenly distributed over the entire surface. (peri means ” around”) bacteria. The working forms of Proteus Vulgaris are very useful for identifying bacteria.

Non-motile sperm cells called spermatia and they lack flagella and therefore they cannot swim.

Flagellar Ultrastructure

Transmission electron microscope studies have shown that the bacterial flagellum is composed of three parts,

  • The largest and most attractive part of the filament, which stretches from the surface of the cell to the end.
  • A basal body of flagella is embedded in the cell.
  • The short, curved part, hook, connects the cord to its basic body and acts as a flexible joint.
  • The filament is a hollow, rigid cylinder constructed of a single protein called flagellin, which ranges in molecular weight from 30,000 to 60,000.
  • The filament ends with a capping protein.
  • Some bacteria have sheaths surrounding their flagella.
  • For example, Bdellovibrio has a membranous structure surrounding the filament.
  • Vibrio cholerae has a lipopolysaccharide sheath.
  • The hook and basal bodies are quite different from the filaments.
  • The hook is slightly wider than the filament and is made of different protein subunits.
  • The basal body is the most complex part of the ultrastructure of the flagellum.
  • In E. coli and most gram-negative bacteria in the body, four rings are connected by a central rod.
  • The outer rings L and P are connected to the lipopolysaccharide and peptidoglycan layers, respectively.
  • The inner M ring contacts the plasma membrane. Gram-positive bacteria have only two basal body rings, an inner ring connected to the plasma membrane and an outer one probably attached to the peptidoglycan.

What is Flagellar Synthesis?

  1. Flagellar synthesis is a complex process involving at least 20-30 genes.
  2. In addition to the flagellin gene, a code of 10 or more genes for proteins and basal bodies.
  3. Other genes are concerned with the control of function or the construction of flagella.

Which are Various patterns of motility?

  1. When a bacterium moves in a certain direction for a long time, the movement is called a “run” or “swim”.
  2. “Runs” are interrupted by repeated, sudden, random changes in the direction called “tumbles.”
  3. Then, a “run” resumes.
  4. “Tumbles” are caused by a change in the rotation of flagellar rotation. i.e. Reverse flagellar rotation.
  5. Some types of bacteria have multiple flagella, for example, they can be a “swarm” or a manifestation of fast-flowing movement on a hard surface.

Advantage of motility –

  • This allows the bacteria to move in the direction of a favorable environment or away from a negative one.
  •  The movement of bacteria in the direction or away from a particular stimulus is called taxis.
  • Such stimuli include chemicals (chemotaxis) and light (phototaxis).
  • Motile bacterial-containing receptors in various places, for example, on the cell wall or directly below it.
  • These receptors also perceive chemical stimuli, such as oxygen, ribose, and galactose.
  • In response to stimuli, that is, information is transmitted to the cellular level.
  • Because the chemotactic signal is positive, however with the bait for the bacteria to move in the direction of the stimulus, with a lot of meters, and some drops.
  • Since the chemotactic signal is negative, it is called a water-repellent agent, and the frequency of drops increases as the microbes are on the toy.
  • The flagellar protein called H antigen is useful for distinguishing among serovars, or variations within a species, of Gram-negative bacteria.
  • For example, there are at least 50 different N antigens, as well as for Escherichia coli. These serovars, which have been identified as E. coli 0157: H7, are associated with medical products and epidemics.


Fimbriae, bacterial Fimbriae
  • Fimbriae and pili are filamentous structures composed of protein that extend from the surface of a cell and can have many functions.
  • Fimbriae are short, thin, hair-like performances that are thinner than flagella and should not participate in the movement.
  • A cell that can be covered with up to 1000 fimbriae.
  • They are only visible under an electron microscope due to their small size.
  • These are thin tubes consisting of helically arranged protein subunits, with a diameter of 3 to 10 nm and a length of up to several micrometers.
  • Fimbriae allow cells to adhere to surfaces, including for animals, tissue, and in the case of pathogenic bacteria or pellicles (a thin sheet of surface cells in liquid or biofilms on the surface).
  • The most well-known human pathogens in these fimbriae to help with the disease are the species Salmonella( salmonella), Neisseria gonorrhoeae (gonorrhea) and Bordetella pertussis (whooping cough).

Fimbriae vs Flagella

StructureBristle-like filamentous structure on surface of some bacteria.Whip-like filamentous structure on surface of some bacteria.
OriginThey are external in origin.They are internal in origin.
FunctionAttachment of surfaces is the main function of fimbriae.Locomotion is the main function of flagella.
LengthThey are shorter in length than flagella.They are longer in length as compared to fimbriae.


Pili, bacterial Pili
  1. Pili are similar to fimbriae but are typically longer.
  2. Only one or a few pili are present on the surface of a cell.
  3. Because pili can be receptors on certain types of viruses, they can be seen under a microscope where they are coated with particles.
  4. Many classes of pili are known, distinguished by their structure and function.

Two very important functions of pili include –

1. To simplify the genetic exchange between cells in a process called F-pili conjugation. (from 1 to 10 per cell). In this process, the conjugation tablets a bacterium called a ‘ C to the cell and binds to receptors on the surface of other bacteria, from their own species or other species.

Two cells are in physical contact, and the DNA (F+ cells) must be transferred to the other cell. By trading the BOTTOM, you can add it to a new position in the host cell, such as antibiotic resistance or the ability to digest the environment more efficiently.

2. connecting pathogen-specific host tissues and subsequent invasion. (Gram-negative pathogens, such as Neisseria spp., which cause gonorrhea and meningitis)
Saws they are also present in some gram-positive pathogens, such as Streptococcus polygenes, which cause sore throats and scarlet fever

Pili differ from fimbriae in the following ways

StructureBristle-like filamentous structure on surface of some bacteria.Whip-like filamentous structure on surface of some bacteria.
OriginThey are external in origin.They are internal in origin.
FunctionAttachment of surfaces is the main function of fimbriae.Locomotion is the main function of flagella.
LengthThey are shorter in length than flagella.They are longer in length as compared to fimbriae.
  • Pili often are larger than fimbriae (around 9 to 10 nm in diameter).
  • They are genetically determined by gender, factors, or conjugative plasmids and are required to coat the bacteria.
  • Some bacteria, viruses, specifically attach to the receptors located on the genital pili at the beginning of their reproductive cycle.

One important class of pili – (Type IV pili)

  1. This also helps the cells attached, but also provides high cell mobility, called twitching mobility, type IV pili have a diameter of 6 nm, which is present only at the ends of these rod-shaped cells containing them.
  2. Twitching motility is a type of gliding motility, movement along a solid surface.
  3. In twitching motility, an extension of pili followed by their retraction drags the cell along a solid surface, with energy supplied by ATP.
  4. Certain species of Pseudomonas and Moraxella are well known for their twitching motility
  5. Type IV pili are key colonization factors for certain human pathogens, including Vibrio cholerae (cholera) and Neisseria gonorrhoeae (gonorrhea).
  6. The twitching motility of these pathogens presumably assists the organism to locate specific sites for attachment to initiate the disease process.
  7. Type IV pili are also thought to mediate genetic transfer by the process of transformation in some bacteria.



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FAQs :

Q1. How bacterial capsule helps bacteria to escape from phagocytosis?

Ans – Capsule can protect bacterial cell from the process of phagocytosis. It may occurs because capsule makes bacterial cell surface component more slippery, results in bacterial cell escape from phagocytosis process.