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  1. Salmonella causes many infections in the host. It must invade the immune system to cause the infection. Salmonella has many strategies to invade the immune response of the host. Some of the strategies are as follows.

    Endotoxin:

    Gram-negative bacteria are responsible for the production of endotoxins in the human. The outer membrane is present in gram-negative bacteria. It surrounds the cell wall of bacteria and creates a new membrane around a cell. This outer membrane is composed of lipo-protein, lipopolysaccharides, and phospholipids. The lipopolysaccharide is composed of two portions i.e. polysaccharides and lipid A portion. Lipid A portion of this membrane is an endotoxin in nature. This endotoxin is the distinguishing feature of gram-negative bacteria. Lipid A portion is considered as endotoxin. These endotoxins released when the bacteria die. The death of bacteria released lipopolysaccharide in the body. These lipopolysaccharides break and released endotoxin in the blood.

    They are also called pyogenic because they increase the temperature of body. These endotoxins initiate the non-specific immune responses of body. These non-specific immune responses increase the body temperature and vasodilation of body. This vasodilation results in the lowering of blood pressure that leads to death and shock in organism. These endotoxins are also responsible for leukopenia in host body. Leukopenia is the condition when the blood leukocytes i.e. white blood cells level become low. Endotoxin induces the early refractory state to temperature change and thus results in level of white blood cells. Endotoxin is also responsible for leukocytosis by increasing the adjuvant activity of host body. Leukocytosis is the condition in which the white blood cell level in the body increases from its normal values.

    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 fimbriae are composed of 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. There are many functions of fimbriae that are discussed below.

    1. 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.

    1. Virulence:

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

    • It increases the attachment of the bacteria to the target site. Better attachment leads to the developing of disease. Salmonella causes the disease when they attach to the surface. They are fimbriae bacteria and fimbriae help in the attachment with surfaces.
    • The fimbriae also increase the replication rate of bacteria. The better attachment leads to the better division of the bacteria. The division rate is directly proportioned to the rate of disease caused. When bacteria divide exponentially, it produces the metabolites and pathogenic factors that cause the severity of disease.
    • It helps the bacteria to interact with the host organism. The fimbriae identify the target host cell and then attaches to them. The receptors are identified by fimbriae and then bind with them. The binding induces the formation of micro-colonies and then produce the toxin.

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