Artificial bacterial flagella are simply nonbiological structures whose design depends on that of bacterial flagella. They are mainly metallic and comprise two parts: a square, flathead and a helical tail. The head is magnetic and is composed of thin metal layers of chromium, nickel, and gold. The tail is usually a flat ribbon-like (nonmagnetic) metal helix. Applying a rotating magnetic field makes the head rotate, and the attached helical tail follows suit. This creates a spiral motion like that of a bacterial flagellum and generates thrust.

Flagella represents a complex filamentous cytoplasmic structure protruding through a cell wall. They are unbranched, long, thread-like structures mainly made up of the protein flagellin, intricately embedded in the cell envelope.  They are approximately 12-30 nm in diameter and 5-16 µm in length. They mainly function for bacterial motility. Motility plays a vital role in the survival and the ability of some bacteria to cause disease.

Structure of Flagellum

The size structure and number of flagella greatly vary in prokaryotes and eukaryotes. Even within prokaryotes, the bacterial flagellum is entirely different from the archaeal flagellum. Likewise, the composition and mechanism of flagella formation are also different and diverse. Moreover, the basic structure of a flagellum comprises some structures that are mainly common in all domains of life.

The basic structure of a flagellum involves the following structure:

Filament

• The filament is the prominent part of a flagellum that represents approximately 98% of all the structures of a flagellum.

• The filaments extend from the hook-like structure located within the cytoplasm of the cell with an average length of 18 nm. The length of the filament varies in different groups of living beings such as the filaments of bacterial flagella is somewhat 20 nm while that of archaea is 10-14 nm.

• Filaments can also be observed outside the cell via specific flagellar staining methods. The movement of the filaments is maintained by the motor located in the cytoplasm.

• Filaments are self-assembling macromolecular structures made up of hook proteins and flagellins, and the number of flagellin and hook protein subunits might vary in different cells.

Hook or anchoring structures

• The flagellar hook represents a short and curved tubular structure that mainly connects the basal body or the flagellar motor to the long filament.

• The most significant function of the hook is to transmit the motor torque to the helical filament which helps it to move in a different orientation for different functions. Besides, it also plays an essential role in the assembly of the flagellum.

• The hook is made up of numerous hook protein subunits forming polymorphic supercoil structures.

• This structure is situated near the cell membrane in all types of cells, but the shape and exact composition of the structure might vary between cells.

Basal body or motor device

• The basal body of a flagellum is the only structure of the flagellum that is located within the cell membrane. It is attached to the hook of the flagellum which further connects it to the long filament.

• The basal body structure is usually rod-shaped along with a system of rings of microtubules. The component of the basal body generally varies in different types of cells.

• The rods located in the basal body serve as a reversible motor that propels the filament in a different orientation for their specific functions.

• The basal body is also important for the transfer of flagellar proteins from the cytoplasm to the hook and filament part of the flagellum at the time of flagellar assembly.

Bacterial flagella arrangement 

Monotrichous

• The monotrichous arrangement of flagella represents the presence of a single flagellum in each cell. If the flagellum is situated at the polar end, it is known as a monotrichous polar distribution.

• The mechanism of movement of monotrichous flagella is usually simple and is coordinated via different types of chemoreceptors that help in the motility of the cell.

• Different sensory receptors help to sense changes in the environment thereby, resulting in a transmembrane electrochemical gradient of ions that powers the bacteria flagella motor.

• The thrust created in the motor is transferred to the hook and the filament, leading to a counterclockwise rotation of the flagella.

• The counterclockwise movement of the flagella leads the cell to move forward or ‘run’. 

• The change in the direction of the cell in a monotrichous type of arrangement is mainly due to the counterclockwise movement of the flagella. This movement generally pulls the bacteria backwards and allows its reorientation.

• Examples of a monotrichous arrangement of flagella can be observed in bacteria such as Vibrio cholerae, Campylobacter spp., Caulobacter crescentus, etc.

Functions of Monotrichous Flagella

Flagella are filamentous protein structures that are mainly found in bacteria, archaea, and eukaryotes, however, they are most commonly found in bacteria. They are used to propel a cell through the liquid (i.e. bacteria and sperm). Although, flagella possess many different specialized functions. 

Some eukaryotic cells use flagellum to increase their reproduction rates. Other eukaryotic and bacterial flagella are used to sense changes in the environment, like temperature or pH disturbances. Recent work on the green alga Chlamydomonas reinhardtii has shown that flagellum can also be used as a secretory organelle, but this discovery needs more time to be fully understood.

Conclusion

Monotrichous, amphitrichous, and lophotrichous flagellum are regarded as polar flagellum since the flagellum is strictly located on the ends of the organism. These flagella can rotate both in a clockwise and counterclockwise direction. A clockwise movement generally propels the organism (or cell) forward, whereas a counterclockwise movement pulls the organism backwards.

If any flagellum stops rotating regardless of polarity the organism will change its direction. This results from Brownian motion (constant movement of liquid particles) and fluid currents occurring in the organism and spinning it around. Some organisms that are unable to change direction on their own, depend upon Brownian motion and fluid currents to do it for them.