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    Salmonella is a gram-negative rod-shaped bacterium. They are motile and non-spore forming. They are facultative anaerobes that can survive in the absence of oxygen. These bacteria grow best at 37ᴼC but the temperature range is 5-45ᴼC. These bacteria grow at 5.5 – 9 pH ranges and grow best at neutral pH.

    They don’t produce any toxins. Bacteria are present in the food that goes into the intestine by crossing the stomach.

    1. Fimbriae:

    Fimbriae are the smallest appendages that are present in the outside of the bacterial cell. Like flagella, it also protrudes out from the cell. It is also called as attachment pilli and that helps in the attachment of bacteria to the surfaces. The characteristics of fimbriae are as follows:

    • The fimbriae are composed of a protein called fimbrin.
    • They may either straight or flexible filament.
    • It is present in mostly gram-negative bacteria.
    • It is less common in gram-positive bacteria.
    • It is the small appendages that are present outside the cell.
    • The size ranges from 3-10 nm in thickness and length can be different from bacteria to bacteria.
    • The number of fimbriae varies from bacteria to bacteria.
    • A bacterial cell may have thousands of fimbriae.
    • This may be present either on one pole of bacteria or the whole cell.

    Functions of fimbriae:

    There are many functions of fimbriae that are discussed below.

    • Attachment:

    Fimbriae play a major role in the attachment of bacteria. It is most common in gram-negative bacteria that help the bacteria to attach to the surfaces. These are called as attachment pilli.  There are three pathways that help bacteria to attach to the surfaces.

    • Lectin pathway:

    They have lectin molecules that attach to the oligosaccharides of the target molecule. Oligosaccharides have the affinity to bind with the lectin molecule. Their binding affinity allows the bacteria to attach with the surfaces.

    • Covalent bonding:

    Some bacterial fimbriae bind to the extracellular matrix. They form a covalent bond with the extracellular matrix and then attach with them. This bonding may either be strong that allows the bacteria to attach to surfaces for long time or be weak that helps bacteria to attach with surfaces for a very short time.

    • Pellicle:

    This is the basic pathway that is used for aerobic bacteria. The aerobic gram-negative bacteria form a pellicle. A pellicle is a thin layer that is formed by the bacteria on the surfaces of broth. When bacteria grow on the broth, it adheres to the surface by using pellicle. This pellicle also helps in taking the nutrient from the broth culture.

    • Virulence:

    Fimbriae also play an important role in the pathogenicity of the bacteria. It acts as a virulence factor for the bacteria.

    1. Type 3 secretory system:

    Type 3 secretary system helps the bacteria to invade the cell. Bacteria cross the stomach and reach intestine. In the intestine, it adheres to enterocyte and invades in the cell. Bacteria secrete many proteins from type 3 secretary system that helps the bacteria to move in the cell. Salmonella is an intracellular pathogen that increases the adenylate cyclase activity. This increases the cAMP level of the intestine that leads to diarrhea. The inflammation of intestine starts when cell lysis occurs. This inflammation is called gastroenteritis. From intestine, it adheres to the blood vessel and causes bacteremia. Bacteria affect many organs when it enters the blood. There are many factors that help salmonella to invade the cell.


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