Blood bacterial loads in ducklings infected with the WT SX, M2, M18, and complemented strains. The bacterial loads in blood of ducklings infected with the WT SX, M2, M18, M2 (pRES-Riean_0790), and M18 (pRES-Riean_0653) strains were measured at 24 h post-inoculation. The results showed that the bacterial loads of blood of ducklings infected with the complemented strain M2 (pRES-Riean_0790) or M18 (pRES-Riean_0653) were significantly restored as compared to those infected with M2 or M18. The error bars represent means ± standard deviations of eight ducklings. Asterisks indicate statistically significant differences between two groups (***p < 0.0001).