Overexpression of PoCD9.3 and its effect on antibacterial infection in fish. Flounders were administered pCN3 (lane 1) and pCNPoCD9.3 (lanes 2–4). Muscle tissues were taken at 5 days post-administration and used for DNA extraction, and then, PCR was performed using primers specific to pCN3, PoCD9.3, and β-actin (internal control) (A). RNA was extracted from the spleen and used for RT-qPCR with primers specific to PoCD9.3. Data are shown as the mean gene expression relative to the expression of the endogenous control β-actin (B). Flounders were administered pCN3 and pCNPoCD9.3, and at 5 days post-administration, the fish were infected with E. piscicida. Bacterial amounts in the spleen were determined at 24 h and 48 h post-infection (C). Data are presented as the mean ± SEM (N = 3). N, the number of times the experiment was performed. *, P < 0.05; **, P < 0.01.