Molecular Pathogens 2024, Vol.15, No.1, 17-29 http://microbescipublisher.com/index.php/mp 19 although non-toxin-producing cheaters can outcompete toxin producers, leading to a less virulent population (Raymond et al., 2012). 3.2 Host invasion and colonization Host invasion and colonization are essential steps for pathogens to establish infection. Marine Gram-positive pathogens, such as Renibacterium salmoninarumand Mycobacterium marinum, utilize various virulence factors to adhere to and invade host tissues. These factors include adhesins, hemagglutination activity, and the type VII secretion system, which facilitate the initial attachment and penetration of host cells (Gnanagobal and Santander, 2022). Enteric pathogens like Escherichia coli and Salmonella species also deploy type III secreted effector proteins to interfere with host defense mechanisms and promote colonization (Kitamoto et al., 2016). 3.3 Immune evasion strategies Successful pathogens have evolved sophisticated strategies to evade the host immune system. Staphylococcus epidermidis, for example, forms biofilms on implanted materials, which protect it from immune responses. The biofilm exopolysaccharide polysaccharide intercellular adhesion (PIA) and other proteins modulate effector cell-mediated killing, allowing the pathogen to maintain a chronic infection (Le et al., 2018). Additionally, pathogens like Citrobacter rodentium can downregulate their virulence gene expression in response to specific antibody responses, thereby avoiding immune detection and elimination (Kamada et al., 2015). 3.4 Virulence factors Virulence factors are molecules produced by pathogens that enhance their ability to cause disease. These factors can include toxins, enzymes, and other proteins that disrupt host cellular processes (Figure 1). For example, marine Gram-positive pathogens possess unique virulence factors such as hemolysins, Cytolysins, and immune-suppressive proteins that facilitate their survival and proliferation within the host (Gnanagobal and Santander, 2022). Enteric bacterial pathogens also utilize a wide array of virulence factors to outcompete commensal microbiota and establish infection in the gastrointestinal tract (Kamada et al., 2015). Marine pathogens employ a diverse array of mechanisms to infect their hosts and evade immune responses. These mechanisms include toxin production, host invasion and colonization, immune evasion strategies, and the expression of various virulence factors. Understanding these pathogenic mechanisms is essential for developing effective strategies to combat marine pathogen infections. Figure 1 Schematic representation of host-pathogen interactions between marine fish and opportunistic Streptococcus spp. (Adopted from Gnanagobal et al., 2022)
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