Amphitrichous

The bacterial flagellum is generally a helical filamentous organelle which is  responsible for motility. In bacterial species possessing flagella at the cell exterior, the long helical flagellar filament serves as a molecular screw to create thrust. Meanwhile, the flagella of spirochetes remains within the periplasmic space and it not only serves as a cytoskeleton in determining the helicity of the cell body, and also rotates or undulates the helical cell body for propulsion. Despite structural diversity of the flagella among bacterial species, flagellated bacteria possess a common rotary nanomachine, which is known as the flagellar motor, which is located at the base of the filament. The flagellar motor is made up of a rotor ring complex and multiple transmembrane stator units that converts the ion flux through an ion channel of each stator unit into the mechanical work needed for motor rotation. Intracellular chemotactic signalling pathways help to regulate the direction of flagella-driven motility in response to changes in the environments, thereby allowing the bacteria to migrate towards more suitable environments for their survival. Recent experimental and theoretical studies have deepened our understanding of the molecular mechanisms of the flagellar motor. In this review article, we have tried to describe the current understanding of the structure and uses of the amphitrichous bacterial flagellum.

Flagella

A flagellum or flagella is mainly a lash or hair-like structure located on the cell body that is significant for different physiological functions of the cell.

• The term ‘flagellum’ is the Latin term for ‘whip’ meaning the long slender structure of the flagellum that represents a whip.

• Flagella are a unique characteristic of the members of the protozoan group such as Mastigophora, but it also occurs in different microscopic and macroscopic animals such as bacteria, fungi, algae, and animals.

• Flagella mainly acts as an important organelle of locomotion in different organisms along with helping in gathering food and circulation.

• Flagella that occurs in a variety of living beings are different in structure, mechanism, movement, and even functions. A flagellum occurring in archaea is known as the archaellum since it is quite different from the flagellum occurring in bacteria.

• Flagella are quite similar in structure with other hair-like protrusions known as cilia but differ in number, occurrence, movement, and even, functions.

• These appendages have been studied in various groups of animals for their functions of both movement and sensation as they are able to detect changes in the environmental composition and pH.

• The most commonly known flagellum is the flagellum that occurs in Chlamydomonas reinhardtii as a result of its size.

• However most of the flagella arise at the polar ends of the cells, their number and position varies as their composition and functions remain the same within a species.

Types of Flagella

The three types of flagella are:

• Bacterial flagella

• Archaeal flagella

• Eukaryotic flagella

Amphitrichous Bacterial Flagellum

• Amphitrichous distribution of flagella represents the presence of either a single flagellum or multiple flagella at either polar ends of the cell.

• In the case of multiple flagella situated at the ends, the flagella are usually present at a region of the cell membrane known as the polar organelle.

• As compared to the mechanism of movement of other arrangements of flagella, today not much is known about the working of the amphitrichous arrangement.

• Different research studies have suggested that the two flagella operate differently as each of the flagella is able to move in only one direction in a monotrichous or lophotrichous manner.

• Another study had indicated that the two flagella though work simultaneously but rotate mainly in opposite directions.

• The mechanism of movement is quite similar to that of other arrangements.

• Examples of bacteria with amphitrichous flagella are given as follows: Campylobacter jejuni, Rhodospirillum rubrum, and Magnetospirillum.

Conclusion

Many bacteria are preferably motile and use flagella to swim through liquid environments. The basal body of a bacterial flagellum functions like a rotary molecular motor, that helps the flagellum to rotate and propel the bacterium across the surrounding fluid. Bacterial flagella can be seen in several arrangements, each unique to a particular organism. Motility helps to keep bacteria in an optimum environment by taxis.

Taxis represents a motile response to an environmental stimulus that enables the net movement of bacteria towards some useful attractant or mainly away from some harmful repellent. Most bacterial flagella are able to rotate both clockwise and counterclockwise , thus enabling them to stop and change direction. The protein flagellin that forms the filament of bacterial flagella acts as a pathogen-associated molecular pattern or PAMP that binds to pattern-recognition receptors or PRRs on a variety of defence cells of the body to activate the innate immune defenses.