Flagella

The motility of the bacteria is due to flagella (singular flagellum). They are found in both Gram positive and Gram negative bacteria. Flagella are long, filamentous surface appendages that are 15 to 20 μm long and about 20 nm thick.

Bacterial species are characterized on the basis of the number and pattern of flagella distribution on their surface. Based on this, bacteria may be one of the following:

Monotrichous (mono means single; trichous means hair): They have one flagellum for example Vibrio cholera. If flagellum is located at an end, it is called polar flagellum.

Amphitrichous (amphi means on both sides): They have a single flagellum attached at each end for example Alkaligens faecalis.

Lophotrichous (lopho means tuft): They have cluster of flagella at one or both the ends for example Spirillum.

 

Peritrichous (peri means around): Flagella are spread all over the surface for example Salmonella typhi.

Atrichous: Having no flagella for example Diptheriae bacillus.

Structure of flagella

A bacterial flagellum is composed of three parts: filament, basal body and hook. The long filament lies external to the cell surface; basal body is embedded in the cell; and hook connects filament and basal body.

The flagellar filament is a rigid cylindrical structure with a hollow core and is made up of a single protein called flagellin. A capping protein is found at an end of filament. In some bacteria, the flagella are surrounded by sheath like Vibrio cholera has lipopolysaccharide sheath.

The hook are short, curved and are flexible unlike filament. The basal body comprises of four rings, which are connected to a central rod as seen in E.coli and other Gram negative bacteria. These four rings are M (for membrane), S (for supramembranous), P (for peptidoglycan layer) and L (for lipopolysaccharide). But later study has shown that M and S ring contains different domains of same protein and they function as a unit (MS ring or M ring). A fourth ring is present inside the cell i.e. C ring (for cytoplasmic). Gram positive bacteria contains only two rings; one inner ring that is connected to the plasma membrane and an outer ring that is attached to the peptidoglycan.

Mechanism of Flagella movement

Several studies have shown that flagella acts like a propeller on a boat. Bacteria generally moves when the rigid filament rotates and the kind of bacterial movement is determined by the flagellar motion. Flagella can rotate clockwise or counterclockwise. When flagella rotates counterclockwise, bacterial cell will slowly rotate clockwise and they run forward. Whereas, when flagella rotates clockwise, cells tumble and forward motion ceases.

When flagella rotates counterclockwise, they basically bends at their hooks and forms rotating bundle that help them to propel forward. On the contrary, clockwise motion disrupts the bundle and leads to the tumbling of cell.

A motor is present at the base that helps in the rotation of rigid flagella. A rod can rotate freely in the plasma membrane and it extends from the hook and ends in the M ring. The rotor portion of the motor consists of a rod, M ring and C ring. C ring is joined to the rotor on the cytoplasmic side of the basal body. These two rings are made up of several proteins; in particular Fli G. Fli G is involved in flagellar rotation. Mot A and Mot B are two membrane proteins that are present at the stator part of the motor. Mot A and Mot B forms a proton channel through plasma membrane. Mot B also attaches the Mot complex to the peptidoglycan. The free energy that is required for flagellar rotation is not derived directly from ATP but is derived from proton or sodium gradient across the plasma membrane. ATP is used as an energy source in eukaryotic flagella. 

The S ring does not rotate and is attached to the cell wall in Gram positive bacteria. P and L rings acts as bearings for the rotating rod in Gram negative bacteria. 

In this way, flagella helps in the movement of bacteria and bacteria can swim from about 20 to 90 m per second. Besides flagella rotation, there are other mechanisms also that can aid in bacterial movement. In spirochetes, flagella reside inside the cell within periplasmic space and are called as periplasmic flagella. Spirochetes travel in viscous media by flexing and spinning movements, caused by special axial filament. Another type of non-flagellar movement is gliding motility; which is generally seen in Cyanobacteria, myxobacteria and some mycoplasmas and helps bacteria to move on solid surfaces.