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    Salmonella has the organ for locomotion that helps the bacteria to move freely without any support of another organism. The bacteria that can move are called motile bacteria. The organ for locomotion that makes the bacteria motile is called flagella. Flagella are present in the outer surface of bacteria. Flagella are a Latin word that means whip. It is the extracellular component of bacteria that are present outside the cell. Flagella allow the bacteria to move in all 360ᴼ angles. Bacteria move in the form of run and tumble.

    How do bacteria move?

    Bacteria move in the response of chemotaxis. Chemotaxis is the process in which bacteria move in response to the chemicals. Chemicals always diffuse from the high gradient to the low gradient. Bacteria move in the response of chemicals. When bacteria tumble, it moves away from the chemicals. When bacteria run, it moves towards the chemicals.

    There are two types of chemicals that are faced by bacteria in their environment.


    The attractant is the chemicals that attract the bacteria towards themselves. They may be sugars, amino acids, fatty acids, etc. Bacteria move towards attractants to get nutrition and to fulfill their other basic requirements.  When bacteria move towards attractants; bacteria run fast and delay their tumbling movement. When bacteria move away from attractants, bacteria tumble fast and delay its run movement.


    Repellent is the chemicals that are toxic to the growth of bacteria. These may be antibiotics, toxins, and bacteriocins. These chemicals are produced when bacteria are present in a competitive environment. Bacteria produce different bactericidal and bacteriostatic components for survival in a tough environment. When bacteria move towards repellent; bacteria tumble fast and delay its run movement. When bacteria move away from repellent, bacteria run fast and delay its tumbling movement


    Flagella are protein in nature. The flagella composition varies from eubacteria and Archaea bacteria. Flagella are the hollow tube that protrudes out from the cell. It is composed of flagellin proteins. Its size varies from bacteria to bacteria and ranges from 20-30 nm. Their flagella are evolved from a type 3 secretary system. There are three parts to flagella.

    • Basal body
    • Hook
    • Filament


    • Basal body:

    The basal body is responsible for originating the structure that protrudes out from the cell. It is present inside the cell wall. It is a rigid component that consists of the shafts or motor. Shaft allows bacteria to move. A motor moves in the clockwise and anticlockwise direction. When shaft moves in clockwise direction, bacteria tumble. When shaft moves in anticlockwise direction, bacteria start run.

    The basal body consists of the rings that are present in the membrane of the cell wall. Two basal rings are present in gram-positive bacteria. These basal rings are named as M that is present in the plasma membrane and S ring that is present in the membrane space. Four basal rings are present in the gram-negative bacteria. The two rings are the same as gram-positive bacteria and two rings are different. The one ring is L ring that is present in lipopolysaccharide layer and the other ring is P ring that is towards the Periplasmic space of the bacteria.

    As salmonella is gram-negative bacteria, it contains four basal rings.

    • Hook:

    The hook is the component that allows the filament to move outside from the cell. It forms the bridge between the filament and the basal body. It bonds the filament and basal body and forms a connection between them.

    • Filament:

    The filament is the part that is composed of flagellin protein. It is helical part of the flagella. The protein amino acids arrange to form a helical structure. This helical structure makes the flagella a hollow structure. The ribosomes that are responsible for synthesis of the flagellin protein are present in the cytosol of cell. These proteins are transported to the outside of the cell with the help of the transporter proteins. The amino acid adds into the growing tip of the hollow core and synthesizes the filament of flagella.

    Energy for flagella movement:

    Flagella move with the help of energy. The shaft requires ATP for movement. This ATP is provided to the bacteria by proton motive force (PMF). The protons H+ ions move across the cell membrane against the concentration gradient. This movement generates ATP that is used by the shaft of bacteria. The shaft gets energy and starts moving. The hollow filament starts moving and bacteria glide

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