Quantitative analysis of virulence gene transcription levels in APEC. qRT-PCR was performed to measure the transcription levels of ETT2 core genes and virulence genes associated with serum resistance in wild-type, mutant and complemented strains. Transcription levels were normalized to the housekeeping gene dnaE. The results are shown as relative expression ratios compared to expression in the wild-type strain. Two-way ANOVA was carried out to determine statistical significance (*P < 0.05; **P < 0.01; ***P < 0.001).