Molecular Pathogens 2024, Vol.15, No.1, 17-29 http://microbescipublisher.com/index.php/mp 20 Image caption: Gram-positive streptococci infect immunocompromised fish with a decreased immune response that lives in a conducive environment (e.g., polluted marine environment) that facilitates such an infection (Adopted from Gnanagobal et al., 2022) Figure 1 illustrates host-pathogen interactions between marine fish and opportunistic streptococcus species. In contaminated marine environments, immunocompromised fish are more susceptible to gram-positive streptococcal infections. The infection process consists of several stages: bacteria first invade the epithelial barrier by adhesion and colonization, and then by transcellular action; Antiphagocytic factors such as capsules and M proteins of streptococci help them survive during phagocytosis. Cytolysin secreted by streptococci also inhibits phagocytosis and induces apoptosis in phagocytic cells. Infected macrophages act as "Trojan horses" that carry streptococci across the blood-brain barrier and into the central nervous system. This process illustrates how streptococcus bacteria are rapidly transferred to the circulatory system through a brief period of intracellular survival and ultimately affect the central nervous system of fish. 4 Environmental Factors Influencing Pathogenicity 4.1 Water temperature Water temperature is a critical factor influencing the pathogenicity of marine pathogens. Elevated seawater temperatures have been shown to enhance the growth and virulence of various marine bacteria. For instance, the abundance of Vibrio species, including Vibrio vulnificus, Vibrio parahaemolyticus, and Vibrio cholerae, increases in warm, low-salinity waters, correlating with higher ambient temperatures (Baker-Austin et al., 2017). This relationship is particularly evident during heatwaves and extreme weather events, which have been linked to outbreaks of Vibrio-related diseases in temperate regions (Baker-Austin et al., 2017). Similarly, the fish pathogen Photobacterium damselae subsp. damselae exhibits enhanced growth and upregulation of virulence factors at higher temperatures, suggesting that elevated seawater temperatures contribute to disease outbreaks in aquaculture (Matanza and Osorio, 2018). Additionally, increased seawater temperatures have been shown to upregulate the expression of virulence factors in Vibrio parahaemolyticus, facilitating adhesion and biofilm formation, which are critical for infection (Billaud et al., 2022). 4.2 Salinity and pH Salinity and pH are also significant environmental factors that influence the distribution and pathogenicity of marine pathogens. Vibrio cholerae, for example, thrives in specific salinity ranges and pH levels, which are critical for its survival and proliferation in marine environments (Thurber, 2020). Changes in these parameters can affect the ecological niche of V. cholerae, potentially expanding its geographical distribution under future climate scenarios (Escobar et al., 2015). The interplay between salinity, pH, and other environmental factors such as chlorophyll-a and sea surface temperature further complicates the dynamics of pathogen distribution and disease transmission (Escobar et al., 2015). 4.3 Nutrient availability Nutrient availability plays a crucial role in the growth and virulence of marine pathogens. The presence of nutrients such as iron can significantly influence the proliferation of opportunistic organisms and the virulence of pathogenic microbes. For instance, increased deposition of iron-rich eolian dust to typically iron-poor marine regions has been linked to the growth of opportunistic organisms and the virulence of pathogenic microbes (Issifu et al., 2022). This phenomenon is particularly evident in macronutrient-rich coastal systems, where the increased iron supply alters the micronutrient factors limiting the growth of these organisms. Additionally, nutrient uptake mechanisms are upregulated in pathogens like Photobacterium damselae subsp. damselae at higher temperatures, further contributing to their pathogenicity (Kibria et al., 2021). 4.4 Pollution and contaminants Pollution and contaminants are significant environmental stressors that can influence the pathogenicity of marine pathogens. Pollutants such as plastics can serve as vectors for harmful pathogens, facilitating their dissemination in marine environments (Billaud et al., 2022). Elevated seawater temperatures, combined with plastic pollution,
RkJQdWJsaXNoZXIy MjQ4ODYzNA==