Bacterial flagella – Its structure and mechanism of movement

Most prokaryotes can move by using thread-like structures called flagella, which attach to the plasma membrane and cell wall.

The bacterial flagella are thin and simpler than eukaryotic flagella, rigid structures that are about 20 nm wide and up to 20 um long. It is so thin that it can’t be seen directly through a bright-field microscope. Instead, they need to be stained in a special way to make them thicker so they can be seen. Only with an electron microscope, you can see how a flagellum looks alike in detail.

Types of bacterial flagella

Bacterial species often have different patterns according to their distribution of flagella, and these patterns can be useful in bacterial identification. 

Monotrichous bacteria (the word “trichous” means “hairy”) only have one flagellum. A polar flagellum is one that is at the end of the cell.

Amphitrichous bacteria, (amphi means on both ends), only have one flagellum at each end.  lophotrichous bacteria, whose name comes from the Greek word for “tuft,” have a group of flagella at one or both ends. 

Peritrichous (peri means all around) bacteria have flagella on every part of their surface.

Parts of bacterial flagella 

Studies with a transmission electron microscope have shown that the flagellum of a bacterium is made up of three parts. 

(1) The flagellar filament is the longest and most obvious part. It goes from the surface of the cell to the tip. 

(2) The base of the filament is inside the cell. 

(3) The flagellar hook is a short, curved piece that connects the filament to the base of the filament and acts as a flexible link.

The flagellar filament is a hollow, rigid cylinder made of subunits of the protein flagellin. Depending on the type of bacteria, the molecular weight of these subunits ranges from 30,000 to 60,000 daltons. At the end of the filament, there is a capping protein. Some bacteria’s flagella are wrapped in sheaths. Vibrio cholerae has something called a lipopolysaccharide sheath.

The hook and basal body are quite different from the filament. The hook is made up of different protein subunits that are just a little bit wider than the filament. The part of a flagellum with the most moving parts is the base. 

In transmission electron micrographs of E. coli and most other gram negative bacteria, the basal bodies look like they are made up of four rings connected by a central rod. These rings are called the L ring, P ring, S ring, and M ring. The S ring and the M ring are now known to be different portions of the same protein. Together, they are called the MS ring. The C ring was found after the MS ring. It is on the cytoplasmic side of the MS ring.

 Gram-positive bacteria only have two rings. The inner ring is connected to the plasma membrane, and the outer ring is probably connected to the peptidoglycan.

Flagella structure
Cited from Tortora, G. J., Funke, B. R., & Case, C. L. (2013). Microbiology: An introduction. Pearson.

Synthesis of bacterial flagella 

Bacterial flagella are made through a complicated process that involves at least 20 to 30 genes. In addition to the flagellin gene, there are at least 10 genes that code for hook and basal body proteins. Other genes control how the flagellum is built or how it works. 

Many flagellum components are outside the cell and must be transported outside the cell to assemble. The basal body is interesting because it is a version of the type III protein secretion system that is usually found in Gram-negative bacteria. Proteins are secreted through a structure that looks like a needle in type III secretion systems. In the flagellar type III secretion system, the needle is replaced by the filament. Through the hollow filament, individual flagellin proteins are moved. When the subunits reach the tip, they stick together on their own, guided by a protein called the filament cap. This means that the filament grows at its tip instead of its base.

Flagellar movement

The filament of a bacterial flagellum is shaped like a rigid helix, and when this helix rotates, it moves the cell like a boat’s propeller. The motor in the flagellum can turn very rapidly. For many bacteria living in water, flagellar rotation leads to two kinds of movement: a smooth swimming motion called a “run,” which actually moves the cell from one place to another, and a “tumble,” which helps the cell change its position.

The motor that makes the flagellum rotate is at the base of the flagellum. It makes torque, which is sent to the hook and filament. The motor has two parts: the rotor ( a moving part) and the stator (a stationary part). It is thought to work like an electric motor, where the rotor spins in the middle of a ring of electromagnets, and the stator holds it all together.

Flagella movement
Cited from Tortora, G. J., Funke, B. R., & Case, C. L. (2013). Microbiology: An introduction. Pearson.

How do flagella rotate?

In gram negative bacteria, The central rod and all four rings make up the rotor. FliG, a protein in the C ring, is very important because it works with the stator. The stator is made up of the proteins MotA and MotB, which form a channel through the plasma membrane. MotB also holds MotA to the peptidoglycan in the cell wall.

As with all motors, the flagellar motor needs a way to get power so it can make torque and turn the flagellum. 

Most flagellar motors get their power from a difference in pH and charge across the plasma membrane. The difference is called the proton motive force (PMF). PMF is mostly made by the metabolic processes of living things.

How can PMF power the motor in the flagella?

The chain of electron carriers that helps move electrons from an electron donor to an electron acceptor at the end of a metabolic process is called the electron transport chain  (ETC). Most components of the ETC in bacterial cells are found in the plasma membrane. As electrons are moved down the ETC, protons are moved from the cytoplasm to the outside of the cell. Because there are more protons outside the cell than inside, there are more positively charged ions (the protons) and a lower pH outside the cell. PMF is a type of potential energy that can be used to do work. It can be used to do mechanical work, like turning the flagellum, transport work, like moving materials into or out of the cell, or chemical work, like ATP synthesis, the cell’s main source of energy.

Through the channels made by the MotA and MotB proteins, protons can move from the outside to the inside of the cell. So, the protons move down the charge and pH gradient. This movement gives off energy that turns the flagellum. In essence, a proton going into the channel is like a person going through a door that spins around. Torque is made by the “power” of the proton, which is like a person pushing on a revolving door. In fact, the PMF has a direct relationship with how fast the flagellum spins.

how flagella rotate
Cited from Willey, J. M., Sandman, K. M., & Wood, D. H. (2020). Prescott’s microbiology (11th ed.). McGraw-Hill Education.


  • Willey, J. M., Sandman, K. M., & Wood, D. H. (2020). Prescott’s microbiology (11th ed.). McGraw-Hill Education.
  • Tortora, G. J., Funke, B. R., & Case, C. L. (2013). Microbiology: An introduction. Pearson.
Mubashir Iqbal
Mubashir Iqbal

Mubashir Iqbal is a highly dedicated and motivated Microbiologist with an MPhil in Microbiology from the University of Veterinary and Animal Sciences. Currently, he is researching the efficacy of commercially available SARS Cov-2 vaccines to neutralize the omicron variant in Pakistan. He holds a Bachelor's degree in Microbiology and has experience in chemical and microbiological analysis of water samples, managing SOPs and documents according to standard ISO 17025. Additionally, he has worked as an internee in BSL 3, Institute of Microbiology, UVAS, where he gained experience in RNA extraction, sample processing, and microscopy.

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