DctR is required for the virulence and colonization of APEC during systemic infection. A Determination of bacterial virulence. Seven-day-old ducks were infected with 106 CFU of the APEC strains or negative control PBS. Survival was monitored for 7 days and the percent survival is shown. The virulence of the mutant strain was attenuated compared to the wild-type and complemented strains. B, C Colonization and survival of APEC strains in ducks during systemic infection. Seven-day-old ducks were infected intramuscularly (B) or intratracheally (C) with 108 CFU of each APEC strain. Graphs show bacterial titers recovered from the liver, spleen and blood at 24 h post-infection. The bacterial loads of the mutant strain were significantly lower in liver, spleen and blood compared with the wild-type and complemented strains. A nonparametric Mann–Whitney U Test was conducted to determine statistical significance (*P < 0.05).