Antibacterial Activity of Rainbow Trout Plasma: In Vitro Assays and Proteomic Analysis

SIMPLE SUMMARY: The blood plasma of fish can acquire antibacterial activity against a wide range of microorganisms. In our study, when we exposed the pathogenic bacterium Aeromonas hydrophila in vitro to blood plasma from naturally infected rainbow trout, we observed agglutination, changes in cell morphology, growth inhibition, and an arrest of protein synthesis in the bacterial cells. Importantly, these effects occurred without causing significant damage to the cell surface. Proteomic analysis revealed that known immune proteins were both up- and down-regulated in the plasma of infected trout compared to healthy fish. A closer examination of the fish proteins that could be retained on the bacterial cells revealed that plasma with high antimicrobial activity contained specific proteins with different biological activities, including antibacterial properties, such as immunoglobulins and ladderlectins. Interestingly, the natural immune defence mechanism in trout plasma, known as the membrane attack complex of the complement system, did not appear to assemble efficiently on the bacteria in the experiment. These findings contribute to our understanding of how plasma proteins defend against pathogenic bacteria. ABSTRACT: The objective of this study was to investigate the bactericidal activity of blood plasma from cultured rainbow trout obtained from two different fish farms. Plasma from trout naturally infected with the bacterial pathogen Flavobacterium psychrophilum was found to inhibit the growth of Aeromonas hydrophila in vitro. Incubation of A. hydrophila in bacteriostatic trout plasma resulted in agglutination and growth retardation, without causing massive damage to the cell membrane. The proteome of the plasma with high antimicrobial activity revealed an abundance of high-density apolipoproteins, some isoforms of immunoglobulins, complement components C1q and C4, coagulation factors, lectins, periostin, and hemoglobin. Analysis of trout proteins retained on A. hydrophila cells revealed the presence of fish immunoglobulins, lectins, and complement components on bacteria whose growth was inhibited, although the native membrane attack complex of immunised trout plasma did not assemble effectively, resulting in a weak bactericidal effect. Furthermore, this study examined the bacterial response to trout plasma and suggested that the protein synthesis pathway was the target of antimicrobial proteins from fish blood. Taken together, these findings illustrate the advantages of the affinity approach for understanding the role of plasma proteins in host defence against pathogens.