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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.

    Types of bacteria according to flagella:

    There are five types of bacteria according to the flagella.

    1. Atrichous bacteria:

    These bacteria don’t contain any flagellum. They are non-motile bacteria.

    1. Monotrichous bacteria:

    These bacteria have only one flagellum for movement. This flagellum is present on either side of cell.

    1. Lophotrichous bacteria:

    They have the tuft of flagella on one pole of the cell. Multiple flagella allow the bacteria to move fast in 1 direction.

    1. Amphitrichous bacteria:

    They have a tuft of flagella on both poles of the bacterial cell. But at one time, only one side activates and allows the bacteria to move.

    1. Peritrichous bacteria:

    These are the bacteria that the whole cell is covered with flagella. They have the flagella on every side of the cell. salmonella have peritrichous flagella.


    Flagella are protein in nature. The flagella composition varies from eubacteria and Archaea bacteria. Flagella are 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 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 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. Motor moves in 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 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 form 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.

    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

    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.

    Flagella staining:

    To visualize the flagella, staining is done. Due to its small size, flagella can’t be properly stained. Tannin compound is used to thicken the filament of flagella.

    Wet mount method:

    The wet mount method is used to identify the number and location of flagella in the cells. The water droplet is added that allows the bacteria to move. The colonies are mixed with water and coverslip is placed. Add 2 drops of RYU flagella stain into the edge of coverslips.  It will stain the bacterial flagella. Then, after 5-10 minutes, observe the smear into the microscope.

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