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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. There are many factors that help the bacteria to invade the immune system.

    1. Capsule:

    The capsule is tightly or loosely to the bacteria cell. Capsule performs many functions in the bacterial cell. Some of the functions of capsule are as follows:

    • Prevent phagocytosis:

    The capsule is virulent in nature. It is responsible for the pathogenesis of bacteria. It invades the immune system and allows bacteria to produce disease. It prevents the phagocytosis of the cell. Phagocytosis is the process in the immune regulation of host. In this process, first the pathogen must enter the body and phagocytosis occurs. Phagocytosis is an important process that is performed by phagocytes. Phagocytes phagocytized the pathogens or particles. When a pathogen is engulfed by a phagocyte, an intracellular vesicle called a phagosome captured it. Phagosome fuses with the lysosome to form a phagolysosome. The lysosome contains many digestive enzymes and chemicals that produce respiratory burst. It releases free radicals into the phagolysosome that killed the pathogen.

     The capsule has many methods to avoid phagocytosis.

    • It remains confined to the surfaces where the macrophages are inaccessible.
    • It remains in many tissues and surface tissue has less accessibility of macrophages.
    • It provokes the inflammation that doesn’t allow the immune system to recognize the pathogen.
    • It inhibits the phagocytosis chemotaxis. For example; streptolysin of Streptococcus pneumoniae inhibits neutrophils. If neutrophils don’t come, no immune response occurs.
    • It covers the pathogen by the membranes. This membrane helps the bacteria to invade the immune system. The immune system doesn’t recognize the bacteria because it identifies it as self-molecule due to the presence of the membrane.


    • Attachment:

    Capsule helps bacteria to attach to the surfaces. The capsule has a sticky nature that attaches to the surface of host cell. The fruitful attachment is responsible for the development of the disease. Without the attachment, bacteria don’t cause disease. E.g. pneumonia is caused by the capsulated Streptococcus pneumoniae. If the capsule of bacteria removed by using heat or chemical, the non-capsulated bacteria don’t cause disease in the host.

    • Protection:

    The capsule is the polysaccharide layer that protects the bacterial cell from a harsh environment. It protects the cell from mechanical injury, heat and unfavorable environment. Capsule doesn’t allow water to move outside the cell and thus prevent cell from desiccation. In warm environment, bacteria water loss and bacteria.

    1. Catalase enzymes:

    It is an enzyme that is produced by microbes as well as humans when cells are exposed to oxygen. This enzyme protects the cells from oxidative damage during aerobiosis. When bacteria grow in the presence of oxygen, many oxygen radicals are produced. These radicals are lethal for the bacteria.  These radicals include superoxide and peroxides that denature the nucleic acid and protein of bacteria. For this, bacteria have catalase, peroxidase and superoxide dismutase enzyme. When oxygen accepts the free electrons from the electron transport chain, oxygen radicals are formed. These radicals are toxic to the growth of bacteria. Aerobic bacteria have catalase and superoxide dismutase enzymes. The catalase enzymes catalyze the hydrogen peroxide into water and oxygen. Superoxide dismutase breaks the superoxide radicals produced due to metabolism. This enzyme breaks oxide and superoxide radicals. When these radicals are broken down, the bacteria are free to grow and survive. This enzyme is very important as It is the virulence factor of bacteria. This enzyme destroys the oxygen radicals that are produced by neutrophils.

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