CpxR contributes to the invasion of APEC to DF-1 cells. The ability of APEC strains to adhere to (A) and invade into (B) DF-1 cells was compared. The mutant strain ΔcpxR showed significantly higher DF-1 cells invasion than the wild-type and complemented strains. Results are shown as relative adhesion and invasion capacity compared with those of the wild-type strain. Error bars indicate standard deviations. Statistical significance analysis was performed by using one-way ANOVA (***P < 0.001).