The regulation of CgDM9CP-7 for maintaining the homeostasis of hemolymph microbiota in oyster Crassostrea gigas

• The number of bacterial colonies decreased significantly in r Cg DM9CP-7 group. • Cg DM9CP-7 inhibited the abundances of Vibrio, Arcobacter , and Acinetobacter . • Cg DM9CP-7 regulated the expressions of immune effectors in hemocytes. The DM9-containing protein (DM9CP) identified as pattern recognition receptor (PRR) in invertebrates plays an important role in the innate immune response. In previous study, Cg DM9CP-7 had been demonstrated to possess the capability for recognizing various bacteria. In the present study, the effects of Cg DM9CP-7 on maintaining the homeostasis of hemolymph microbiota were explored. In r Cg DM9CP-7-treated oysters, the number of bacterial colonies in the hemolymph cultured on 2216E agar plates was significantly reduced. The composition of hemolymph microbiota rather than the alpha diversity indices changed significantly in r Cg DM9CP-7-treated oysters. At the phylum level, the relative abundance of Proteobacteria significantly decreased, whereas that of Firmicutes increased. At the genus level, Vibrio, Arcobacter and Acinetobacter were significantly decreased in hemolymph of r Cg DM9CP-7-treated oysters. The pathways associated with bacterial proliferation, including biotin biosynthesis pathway II, TCA cycle VII (acetate generation) and glycolysis III (from glucose), were significantly enriched in r Cg DM9CP-7-treated oysters. r Cg DM9CP-7 exhibited inhibitory activity against the growth of Vibrio anguillarum, Vibrio alginolyticus , and Vibrio vulnificus . In r Cg DM9CP-7-treated oysters, the mRNA expressions of Cg IL17-3, Cg IL17-4, Cg IL17-5 and Cg Defh2 were significantly upregulated in hemocytes. Collectively, these results suggested that Cg DM9CP-7 could reduce the abundance of potentially pathogenic bacteria in the hemolymph microbiota and enhance the mRNA expressions of inflammatory factors and antibacterial peptide.