Bacterial flagella: types and structure

August 5, 2018

Introduction

Strains of bacteria which are motile possess filamentous, cytoplasmic appendages protruding through cell wall called flagella.
This effect screw like propulsive movements and act as organs of locomotion.
They are much thinner than the flagella or cilia of eukaryotes.
The flagellum is a long, thin filament, hair-like, un-branched, twisted spirally in an open, regular wave form.
It is about 0.02 micron thick and is usually several times the length of the bacterial cells.
It can be seen with the light microscope only after being stained with special stains that increase their diameter.
Electron microscope is also used to observe flagellum and when stained with phosphotungstic acid it appears as hollow tube.
It originates in the bacterial protoplasm and is composed entirely of protein called flagellin.
According to the species, flagella have the characteristics patterns of distribution in the bacterial cell.
There are four types of flagellar distribution on bacteria. They are:

Types of flagella

1.Monotrichous: single polar flagellum. E.g. Vibrio cholera Campylobacter spp., Caulobacter crescentus, etc.

2.Amphitrichous: single flagellum attached to each end. E.g. Alkaligenes faecalis, Magnetospirillum, etc.

3.Lophotrichous: tufts of flagella at one or both ends. E.g. spirilla, Pseudomonas fluorescens, etc.

4.Peritrichous: numerous flagella all over the bacterial body. E.g. typhoid bacilli, E. coli, Salmonella, Klebsiella, etc.

5.Atrichous: absence of flagella. E.g. Shigella.

Image source: wikipedia

Structure of flagellum

Each flagellum consists of various components and moves by rotation, much like a propeller of a boat motor.
It consists of three distinct parts. They are: helical, long filament, short hook and basal body.
The filament lies external to the cell and the hook-basal body portion is embedded in the cell envelope.
The basal body is attached to the cytoplasmic membrane by ring like structures.
The chemical composition of the basal body is unknown while the hook and filament are composed of protein sub-units (monomers) arranged in a helical fashion called flagellin.
Flagellin is highly conserved in amino acid sequences in species of bacteria, which suggests that flagellar motility has deep roots within this evolutionary domain.
The flagella are driven by the rotary action of the swivel-like basal-hook.
The motor portion is present at the base of filament which is anchored in the cytoplasmic membrane and cell wall.

Image source: biologybrain

The motor has a central rod that passes through a series of rings.
An outer ring called L-ring is anchored in the lipopolysaccharide layer in gram negative bacteria.
The second ring called the P-ring is anchored in the peptidoglycan layer of the cell wall.
MS and C rings, which are the third set of rings, are located within the cytoplasmic membrane and the cytoplasm respectively.
Only inner pair of rings is present in the case of gram positive bacteria as it lacks an outer membrane.
Mot proteins are present in series which surrounds the inner ring and are anchored in the cytoplasmic membrane.
The Fli proteins which are a final set of proteins function as the motor switch.
These proteins reverse the direction of rotation of flagella in response to intracellular signals.

References:

i) https://www.intechopen.com/chapters/39455

I am Shailesh Koirala