Flagella Overview

Flagella, a thin hair-like appendage, is mainly used for motility. Some bacteria use flagella as a sensory organ and a motility organ. Flagella can be of three types – bacterial, archaeal and eukaryotic – with varying protein compositions. Organisms that have flagella are called flagellates.

Cilia and flagella are largely the same, with the main difference associated with movement. Flagella have a planar movement, whereas cilia are a back and forth beating-like movement. A bacterial flagellum is a hollow cylindrical body with mainly three parts – basal body, hook and filament. Flagella are of different types – atrichous, monotrichous, lophotrichous, amphitrichous and peritrichous.

What are flagella?

• They are thin hair-like appendages used for motility. Some bacteria use flagella as a sensory organ besides a motility organ.
• Flagella can be of three types – bacterial, archaeal and eukaryotic – with varying protein composition.
• They are 12 – 30 mm in thickness and 15 – 20 mm long.
• Bacterial flagella are similar to archaeal flagella but not homologous.
• Eukaryotic flagella are complex protrusions with back and forth movements.

Flagella types

The flagellum is used for motility and as a sensory organ. For example, bacteria are highly responsive to their environment; they can be anaerobic, chemical tolerant, thermo phobic or halo phobic. Flagella helps bacteria to move from an unfavourable environment to a safe one. The speed of flagellum is 200-1000 rotations per minute. They move their helical flagella anti-clockwise for a forward movement. Different types of bacterial flagella are:

• Atrichous – no flagella present.

Examples include Spirochetes, Lactobacillus and Pasteurella.

• Monotrichous – One polar flagellum is present.

Examples include Pseudomonas aeruginosa, Vibrio cholera and Campylobacter.

• Lophotrichous – A group of polar flagella at one end or both ends.

Examples include Spirillum and Pseudomonas fluorescens.

• Amphitrichous – A single flagellum at both ends.

Examples include Alcaligens faecalis and Aquaspirillum serpens. Studies differ on whether flagella on both ends work independently or in coordination.

• Peritrichous – Flagella all over the body of bacteria

Examples include Escherichia coli, Bacillus subtilis, and Salmonella enteric. Some relevant studies show a peritrichous flagella movement is done by elastohydrodynamic instability.

Flagella structure

A flagellum normally has three parts – basal body, hook and filament.

Basal body

Flagella’s basal body is made of centrioles and other proteins. The flagella basal body is known as the microtubule-organising centre (MTOC). The basal body has protein rings with it, which act like a bearing embedded throughout the membrane layers. The number of protein rings will vary with bacterial classification.

Hook

Flagella hook is an extremely short, highly curved tubular structure, comprising 120 copies of a single protein named ‘FIgE.’ The hook connects the basal body to the filament.

Filament 

The filament of a flagellum is composed of thousands of copies of a single protein, flagellin. Flagellin is used as an adjuvant in vaccines due to its ability to provoke an immune response. The filament is arranged helically with a cap at the end. The filament is important for bacteria in reproduction and survival.

Protozoan flagellum

Gram-positive bacteria possess only two basal body rings, whereas gram-negative bacteria possess four. Gram-positive bacteria have one ring in peptidoglycan and another ring in the plasma membrane. The four rings in Gram-negative bacteria are described as follows,

• L ring- seen in lipopolysaccharide
• P ring- seen in peptidoglycan
• M ring- seen as embedded in the plasma membrane.
• S ring- seen as directly connected to the plasma membrane.

Suppose the whole flagella structure is a machine, M ring is the motor, and S ring is the stator, while P and L rings are the bearings.

Archaeal flagellum

• It is a hair-like cell membrane appendage of several types of flagellin. 
• They are more similar to type (IV) bacterial pili than bacterial flagellum.
• They are thinner in size when compared to bacterial flagellum.
• The flagellum with a hook helps to propel the cells through any liquid medium.

Eukaryotic flagellum

• The eukaryotic cilia are often called eukaryotic flagellum.
• Cilia are extremely functional structures performing many tasks.
• They act as a sensory organ for pressure, temperature, light and fluid flow in mammals.
• Failed functioning of cilia leads to hydrocephalus, infertility and blindness.

Conclusion

Flagella are thin hair-like appendages acting as a motility and sensory organ. It is of many types – atrichous, monotrichous, lophotrichous, amphitrichous and peritrichous. Flagella in three main domains seem different in composition and structure. There are bacterial, archaeal and eukaryotic flagella in nature. Eukaryotic flagella are just another name for eukaryotic cilia.

 Flagella have a planar movement, whereas cilia are a back and forth beating-like movement. A bacterial flagellum is a hollow cylindrical body with mainly three parts – basal body, hook and filament. Some gram-negative bacteria also have a structure called ‘sheath’ surrounding the flagellum.