Bacterial Flagella: Definition, Structure, Types, Functions, Rotation, Examples.

Bacterial Flagella Definition

• Most of the motile bacteria locomote by using threadlike appendages which is extending outward from the plasma membrane and cell wall is known as flagella. Flagella also known as flagellum (Singular form).
• The Latin meaning of the term flagellum is “ whip”, just because often flagella uses whipping motion for locomotion.
• Flagella helps in locomotion which is the primary function of flagella but also they helps in attachment to surfaces, and in some bacteria, they function as a sensory organ that can sense alterations in pH and temperature.
• Those cells contain flagella are known as flagellates. A flagellate can contain one or several flagella.
• The flagella can be found in bacteria, archaea, and eukaryotes. The function of flagella in these three domains is similar but they are different in protein composition, structure, and mechanism of propulsion. 
• The archaeal flagella is termed as archaellum, to indicate its distinction from bacterial flagella.’
• The flagellum of archaea is nonhomologous.
• The flagellum of bacterial cells are coiled, thread-like structure, sharp bent, consisting of a rotary motor at its base, and are made of the protein flagellin. Between the hook and a basal body a shaft is located which passes through the protein rings in the cell membrane.
• The flagella of eukaryotic cells are made up of tubulin protein. These flagella are pummeled backward and forward and are seen in protist cells, gametes of plants, and animals. 

Bacterial Flagella Types/Pattern of Distribution

There are five types of pattern in bacterial flagella such as;

1. Atrichous

• Atrichous bacteria are those that do not possess flagella or any other similar structure that would allow for motility. “Atrichous” comes from the Greek word “a,” meaning “without,” and “trichos,” meaning “hair.”
• Some examples of atrichous bacteria include Streptococcus and Lactobacillus species. These bacteria are typically non-motile and rely on other mechanisms to move around, such as passive diffusion or active transport.
• Despite not having flagella, atrichous bacteria are still able to survive and thrive in a variety of environments. For example, Lactobacillus species are commonly found in the human gut and play an important role in maintaining gut health.
• The absence of flagella in atrichous bacteria is thought to be an adaptation to their specific lifestyle and environment. For example, bacteria that are not exposed to a lot of external stimuli may not need to move around in order to survive, while bacteria that live in very specific environments may have evolved other means of movement that are better suited to their particular surroundings.
• Overall, while atrichous bacteria may not have flagella, they are still important and diverse members of the microbial world.
• Example: Lactobacillus lactis

2. Monotrichous

When the cell contains a single flagellum at one end of the cell is known as Monotrichous. If the flagellum is located at an end, it is called as a polar flagellum.

• Monotrichous flagella are a type of bacterial flagella characterized by a single flagellum or hair-like structure located at one end of the bacterial cell. “Mono” means “one,” and “trichous” refers to “hair.”
• Bacteria with monotrichous flagella are able to move in a unidirectional manner, using the rotation of their flagellum to propel themselves forward. Examples of bacteria with monotrichous flagella include Vibrio cholerae, which causes cholera, and Pseudomonas aeruginosa, which can cause infections in humans.
• The arrangement of the flagella on a bacterial cell can provide important information about the bacterium’s morphology and physiology. Monotrichous flagella are just one of several possible arrangements, which also include peritrichous (multiple flagella around the entire cell) and amphitrichous (a single flagellum at each end of the cell) arrangements.
• Understanding the structure and function of bacterial flagella is an important area of research, as it can help us to better understand bacterial motility and the ways in which bacteria interact with their environment.
• This type of flagellum can rotate clockwise and anti-clockwise. To move forward it rotate clockwise and to move backward it rotate the flagellum anti-clockwise.
• Example: Vibrio choleriae

3. Lophotrichous

When the cells contain a cluster of flagella at one or both ends are known as Lophotrichous. This type of flagellum is known as polar flagellum and they also rotate clockwise and anti-clockwise.

• Lophotrichous flagella are a type of bacterial flagella characterized by a tuft or cluster of flagella located at one or both ends of the bacterial cell. “Lophos” means “crest” or “tuft,” and “trichous” refers to “hair.”
• Bacteria with lophotrichous flagella are able to move rapidly and change direction quickly, using the coordinated action of their multiple flagella to propel themselves through their environment. Examples of bacteria with lophotrichous flagella include Vibrio parahaemolyticus, which is a marine bacterium that can cause gastrointestinal illness in humans, and Pseudomonas fluorescens, which is a common soil bacterium.
• The arrangement of flagella on a bacterial cell can provide important information about the bacterium’s morphology and physiology. Lophotrichous flagella are just one of several possible arrangements, which also include monotrichous (a single flagellum at one end of the cell), peritrichous (multiple flagella around the entire cell), and amphitrichous (a single flagellum at each end of the cell) arrangements.
• Understanding the structure and function of bacterial flagella is an important area of research, as it can help us to better understand bacterial motility and the ways in which bacteria interact with their environment.
• Example: Pseudomonas fluorescents

4. Amphitrichous

When the cell contains a single flagellum at each pole is known as Amphitrichous. These flagella also rotate clockwise and anti-clockwise.

• Amphitrichous flagella are a type of bacterial flagella characterized by a single flagellum or hair-like structure located at both ends of the bacterial cell. “Amphi” means “both,” and “trichous” refers to “hair.”
• Bacteria with amphitrichous flagella are able to move in multiple directions, using the rotation of their flagella to propel themselves forward or backward. Examples of bacteria with amphitrichous flagella include Alcaligenes faecalis, which is a soil bacterium that can degrade a variety of organic compounds, and Acetobacter xylinum, which is a bacterium that produces cellulose.
• The arrangement of flagella on a bacterial cell can provide important information about the bacterium’s morphology and physiology. Amphitrichous flagella are just one of several possible arrangements, which also include monotrichous (a single flagellum at one end of the cell), peritrichous (multiple flagella around the entire cell), and lophotrichous (a tuft or cluster of flagella at one or both ends of the cell) arrangements.
• Understanding the structure and function of bacterial flagella is an important area of research, as it can help us to better understand bacterial motility and the ways in which bacteria interact with their environment.
• Example: Aquaspirillum serpens

5. Peritrichous

In this type, the flagella are spread evenly over the whole surface of the cell. The term “peri” means “around”. To rotate in one direction they rotate the flagella in anti-clockwise and form a bundle. If any of the flagella occur and begin wheeling clockwise, the organism does not go in any direction and starts tumbling.

• Peritrichous flagella are a type of bacterial flagella characterized by multiple flagella or hair-like structures distributed around the entire bacterial cell. “Peri” means “around,” and “trichous” refers to “hair.”
• Bacteria with peritrichous flagella are able to move in multiple directions, using the coordinated action of their multiple flagella to propel themselves through their environment. Examples of bacteria with peritrichous flagella include Escherichia coli, which is a common gut bacterium in humans and other animals, and Salmonella enterica, which can cause food poisoning in humans.
• The arrangement of flagella on a bacterial cell can provide important information about the bacterium’s morphology and physiology. Peritrichous flagella are just one of several possible arrangements, which also include monotrichous (a single flagellum at one end of the cell), amphitrichous (a single flagellum at both ends of the cell), and lophotrichous (a tuft or cluster of flagella at one or both ends of the cell) arrangements.
• Understanding the structure and function of bacterial flagella is an important area of research, as it can help us to better understand bacterial motility and the ways in which bacteria interact with their environment.
• Example: Salmonella typhie

Bacterial Flagella Structure

Bacterial flagella are composed of flagellin protein. These are 20-30 nm in diameter and about 15µm long. Flagellum is made of three important part such as;

1. Basal Body
2. Hook
3. Filament

1. Basal Body

• M.L. De Pamphilis and J. Alder first Isolated the basal body from E. coli and Bacillus subtilis and studied its fine structure and arrangement.
• The basal body helps in the attachment of flagellum to the cell wall and plasma membrane.
• It is made of a small central rod that contains a series of rings. The types and numbers of these rings vary in gram-positive and gram-negative bacteria. These rings are inserted into the central rod.
• In gram-negative bacteria, the basal body contains four rings such as L, P, MS, and C. These rings are connected to a central rod.
• The L, P, and MS rings are inserted within the cell envelope, and the C ring is on the cytoplasmic side of the MS ring.
• In gram-positive bacteria, the basal body contains only two rings such as S (Super membrane) ring and M (Membrane) ring.
• The inner ring in gram-positive bacteria connected to the plasma membrane and an outer one probably attached to the peptidoglycan.

2. Hook

• The hook is located at the outside of the cell wall and connects filaments to the basal body.
• It is a short and curved segment and acts as a flexible coupling.
• The hook is composed of different protein subunits.
• The hook of gram-positive bacteria is slightly larger than the gram-negative bacteria.

3. Filament

• The longest and most obvious portion of the flagellum is known as filament. It extends from the cell surface to the tip.
• It is composed of a globular protein known as flagellin, which varies in molecular mass from 30,000 to 60,000 daltons, depending on the bacterial species.
• The flagellins are arranged in several chains that inter-twist and form a helix around a hollow core.
• At the end, the filament contains a capping protein. Some bacteria contain sheaths surrounding their flagella.i.e, Vibrio cholerae flagella contain lipopolysaccharide sheaths.

Mot Protein

• Besides, there is another protein called Mot-protein which controls the rotation of flegella.
• This Mot-protein is anchored in the cytoplasm membrane and cell-wall.
• The Motor conceit of a small central rod that passes through a system of rings.

Motor

• The bacterial flagellum contains a rotary engine (Mot complex) at the flagellum’s anchor point on the inner cell membrane. It controls the rotation of flagella.
• It is made up of protein which is known as Mot-protein.
• The proton motive force powered this rotary engine. In proton motive force, the hydrogen ions or protons move across the bacterial cell membrane due to a concentration gradient which is set up by the cell’s metabolism.
• The rotor carries protons crossed the membrane, and is used in the process. The rotor solely can spin at 6,000 to 17,000 rpm, but besides the flagellar filament attached normally only gives 200 to 1000 rpm.
• The direction of rotation can be altered by the flagellar motor switch almost immediately, affected by a slight alteration in the state of a protein, FliG, in the rotor.
• The flagellum uses very little energy, which means it is highly energy-efficient.
• The definite mechanism for torque formation is yet badly understood. Because there are no on-off switch for the flagellar motor, the protein epsE is utilized as a mechanical link to release the motor from the rotor, therefore stopping the flagellum and subtracting the bacterium to settle in one place.
• The rotational velocity of flagella changes in response to the strength of the proton motive force, some bacteria achieve roughly 60 cell lengths per second. At this speed, a bacterium would take about 245 days to cover 1 km.

Bacterial Flagella Functions

• Movement: Bacteria can use their flagella to move towards or away from environmental stimuli like nutrients or poisons. Chemical attraction is called chemotaxis.
• Colonization: Flagella aid colonisation by facilitating bacterial attachment to and colonisation of surfaces including the intestinal mucosa and the surface of plant roots.
• Protection: To hide from immune cells and antibiotics, certain bacteria use their flagella to create a protective coating.
• Sensing: Flagella can also function as sensors, allowing bacteria to detect changes in their surrounding conditions such as temperature, pH, and oxygen levels.
• Swarming: Some bacteria, such as Proteus mirabilis, use flagella to swarm across surfaces in a coordinated manner. Swarming allows bacteria to rapidly colonize new areas and outcompete other microorganisms.
• Biofilm formation: Bacteria can use flagella to move along surfaces and initiate the formation of biofilms, which are communities of bacteria surrounded by a protective matrix. Biofilms protect bacteria from environmental stressors and can also facilitate the exchange of genetic material between bacteria.
• Nutrient acquisition: Flagella can help bacteria locate and move towards sources of nutrients, such as sugars or amino acids. This is particularly important for bacteria that live in nutrient-poor environments.
• Virulence: Some pathogenic bacteria use flagella to invade host cells or tissues. For example, Helicobacter pylori, a bacterium that can cause stomach ulcers, uses its flagella to penetrate the protective mucous layer of the stomach lining.
• Swimming against currents: Some bacteria, such as marine Vibrio species, use their flagella to swim against ocean currents and locate nutrient-rich areas. This allows them to thrive in challenging environments where other microorganisms cannot survive.

Flagella formation mechanism

The convoluted and perplexing process of flagella formation and assembly commences with the creation of the FliF ring complex, which is an integral component of the basal body. This intricate mechanism occurs both inwards and outwards in the cytoplasmic membrane. While numerous studies have been conducted on bacteria, the process of flagellum formation and assembly remains enigmatic due to its complex nature, involving several proteins and their interactions.

The intricate process of flagella formation and assembly can be described as follows, albeit with great difficulty and obscurity:

1. Formation of the basal body: The process begins with the integration of FliF into the cytoplasm. FliF is a crucial integral protein that consists of MS-ring, which serves as the foundation for the assembly of all other structures in the flagellum. The FliF proteins come together to form a single ring that creates two adjacent loops across the cytoplasmic membrane. Subsequently, other proteins such as FliG, FliM, and FliN are integrated into the cytoplasmic face of the MS-ring. These proteins perform a variety of functions critical to flagellar motility and the export of other flagellar component proteins. The inward assembly of the basal body involves the formation of a C-ring in the cytoplasmic space. Within the C-ring, a flagellar export apparatus is formed to export flagella axial proteins through the channel. A flagellum-specific type III protein export system then binds and moves flagellar axial proteins into the central channel of the flagellum. The next step involves the creation of the rod, which is a significant component of the basal flagellar body, consisting of five proteins attached to the FliF ring at the proximal end and to the hook at the distal end.
2. Formation of the hook: Proteins are transported through the rod into the hook, resulting in the growth of the hook up to a length of 55 nm, which may vary in different types of cells. Hook formation is induced by a protein called FlgD, which is absent in completed flagella. As hook assembly begins, the rod cap protein is substituted by hook capping proteins. FlgD is necessary for the polymerization of subunits into an α-helical structural arrangement. The hook capping protein, FlgE, is exported from the cytoplasm through the central channel from base to tip.
3. Filament assembly: Filament assembly occurs in the presence of the HAP2 pentamer complex, which caps the distal end of the filaments as the filament monomers are assembled. The cap is essential to prevent the diffusion of subunits from the filament and to induce a conformational change to enable polymerization.

Endoflagella

• The flagella-like organelles which are found in periplasmic space and encased by the outer membrane are known as Endoflagella.
• The endoflagella start at each edge of the organism and wind throughout it, spreading to and overlapping at the midpoint. 
• Inside the endoflagella is the interior membrane (cytoplasmic membrane) that provides osmotic balance and includes the protoplasmic cylinder. 

Flagella Example

• Escherichia coli: Gram-negative Escherichia coli, or E. coli, is a common intestinal microbe. It can swim to the food it needs in the intestines thanks to its many flagella.
• Salmonella enterica: Salmonella enterica is a gram-negative bacteria that can make humans sick from eating contaminated food. It may enter host cells and swim through the intestines thanks to its numerous flagella.
• Vibrio cholerae: Cholera, a severe form of diarrhoea, is caused by the gram-negative bacterium Vibrio cholerae. It swims by using its solitary, polar flagellum, which also helps it find and colonise the small intestine.
• Pseudomonas aeruginosa: Pseudomonas aeruginosa is a gram-negative bacteria that is widely recognised as a significant contributor to healthcare-associated illnesses. Since it possesses several polar flagella, it may enter host tissues and swim through mucus.
• Borrelia burgdorferi: B. burgdorferi, or Borrelia burgdorferi, is a gram-negative bacterium that is spread by ticks and is responsible for Lyme disease. It may penetrate various tissues thanks to its several flagella, which let it swim through the bloodstream.

There are many different kinds of flagellated bacteria, and these are only a few examples. Flagella are an integral part of bacteria and can be found almost everywhere.

FAQ

References

• Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 19.4, Cilia and Flagella: Structure and Movement. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21698/
• Nikhil A. Thomas, Sonia L. Bardy, Ken F. Jarrell, The archaeal flagellum: a different kind of prokaryotic motility structure, FEMS Microbiology Reviews, Volume 25, Issue 2, April 2001, Pages 147–174, https://doi.org/10.1111/j.1574-6976.2001.tb00575.x
• Vonderviszt F, Namba K. Structure, Function and Assembly of Flagellar Axial Proteins. In: Madame Curie Bioscience Database [Internet]. Austin (TX): Landes Bioscience; 2000-2013. Available from: https://www.ncbi.nlm.nih.gov/books/NBK6250/
• Samatey, F., Matsunami, H., Imada, K. et al. Structure of the bacterial flagellar hook and implication for the molecular universal joint mechanism. Nature 431, 1062–1068 (2004). https://doi.org/10.1038/nature02997
• Ng SY, Chaban B, Jarrell KF. Archaeal flagella, bacterial flagella and type IV pili: a comparison of genes and posttranslational modifications. J Mol Microbiol Biotechnol. 2006;11(3-5):167-91. doi: 10.1159/000094053. PMID: 16983194.
• Dmitry Apel, Michael G. Surette. Bringing order to a complex molecular machine: The assembly of the bacterial flagella. Biochimica et Biophysica Acta (BBA) – Biomembranes. Volume 1778, Issue 9. 2008. Pages 1851-1858. https://doi.org/10.1016/j.bbamem.2007.07.005.
• Nakamura, Shuichi, and Tohru Minamino. “Flagella-Driven Motility of Bacteria.” Biomolecules vol. 9,7 279. 14 Jul. 2019, doi:10.3390/biom9070279
• Tohru Minamino, Katsumi Imada. The bacterial flagellar motor and its structural diversity. Trends in Microbiology. Volume 23, Issue 5. 2015. Pages 267-274. https://doi.org/10.1016/j.tim.2014.12.011.
• https://www.slideshare.net/amjadkhanafridi4all/flagella-54587839
• https://biologydictionary.net/flagellum/
• https://byjus.com/biology/flagella/#:~:text=Flagella%20are%20microscopic%20hair%2Dlike,changes%20in%20pH%20and%20temperature.
• https://en.wikipedia.org/wiki/Flagellum