Molecular Microbiology Research 2024, Vol.14, No.1, 1-9 http://microbescipublisher.com/index.php/mmr 5 After reproducing within the cell for a period of time, Salmonella leaves the host cell and further spreads to other host cells or tissues. They can escape by disrupting the host cell membrane or be phagocytosed by host cells and survive within them. Salmonella also employs several strategies to evade surveillance and attack by the host immune system. Through the action of virulence factors, they interfere with the signal transduction pathway of host cells, reducing the resistance of the host immune system. At the same time, Salmonella can also produce various molecules, such as surface proteins and lipopolysaccharides, to activate the inflammatory response of the host immune system, thereby forming inflammatory foci (Sahler et al., 2018). The survival and reproduction of Salmonella within the host are also influenced by the host immune system. When the host immune system senses the presence of Salmonella, a series of immune responses will be initiated, such as inflammation, phagocytosis by macrophages, etc. Salmonella needs to evade these immune responses in order to survive and multiply within the host. To achieve this, Salmonella can utilize its transport proteins, toxins, etc., to evade attacks by the host immune system. 2.3 The host immune response after Salmonella infection After Salmonella infection, the host immune system initiates a series of immune responses. The most important of these is the inflammatory response. Inflammation is a protective response of the host immune system that can clear Salmonella and other pathogens, and restore the function of damaged tissues. The characteristics of inflammation include redness, swelling, heat, pain, and tissue damage. During the inflammatory response, the host immune system releases a variety of inflammatory cytokines such as TNF-α, IL-1β, and IL-6 to induce inflammation. The inflammatory response involves vasodilation, increased blood flow, and aggregation of white blood cells to direct immune cells and inflammatory mediators to the site of infection to prevent further spread of the pathogen. In addition, immune cells such as macrophages, dendritic cells, and neutrophils are activated, which eliminate the infection by recognizing and destroying Salmonella, releasing cytotoxins, and producing cytokines. In addition to the inflammatory response, the host immune system also initiates a macrophage phagocytic response. Macrophages are important immune cells that can phagocytose and kill Salmonella and other pathogens. Macrophages can also release a variety of inflammatory cytokines such as TNF-α, IL-1β, and IL-12 to induce inflammation and promote immune responses. After Salmonella infection, the host immune system also initiates adaptive immune responses. Adaptive immune responses include cellular immunity and humoral immunity. Cellular immunity mainly clears Salmonella and other pathogens through T cells and macrophages. Humoral immunity mainly clears Salmonella and other pathogens through antibodies. These antibodies can neutralize the virulence factors of Salmonella, prevent its invasion into host cells, and promote its phagocytosis and destruction. In addition, cellular immune responses also play an important role, including cell-mediated immune responses and cytotoxic effects. Activated T cells and natural killer cells release cytotoxins that directly cause the death of infected cells. After successfully fighting Salmonella infection, the immune system forms immunological memory. This means that the host immune system can quickly recognize and respond to reinfection by rapidly activating immune responses to prevent the reproduction and spread of Salmonella. 3 The Interaction of Salmonella with the Host Immune System 3.1 The role of Salmonella in infecting host cells Before infecting host cells, Salmonella must first adhere to the surface of the host cell. To do this, Salmonella can utilize specific proteins on its surface, such as the FimH protein, to bind to receptors on the surface of the host cell, forming a strong connection. This adhesion allows Salmonella to more effectively invade the host cell interior. After Salmonella infects the host cell, it enters the interior of the host cell. This process typically relies on Salmonella's type three secretion system (T3SS). The T3SS injects Salmonella proteins directly into the host cell, altering its biological activity. These proteins include those encoded by the pathogenicity island, such as SPI-1 and
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