Avian pathogenic Escherichia coli (APEC) infections are a very important problem in the poultry
industry. PhoP-PhoQ is a two-component system that regulates virulence genes in APEC. In this
study, we constructed strains that lacked the PhoP or PhoQ genes to assess regulation of APEC
pathogenicity by the PhoP-PhoQ two-component system. The PhoP mutant strain AE18, PhoQ mutant
strain AE19, and PhoP/PhoQ mutant strain AE20 were constructed by the Red homologous
recombination method. Swim plates were used to evaluate the motility of the APEC strains, viable
bacteria counting was used to assess adhesion and invasion of chick embryo fibroblasts, and
Real-Time PCR was used to measure mRNA expression of virulence genes. We first confirmed that
AE18, AE19, and AE20 were successfully constructed from the wild-type AE17 strain. AE18, AE19,
and AE20 showed significant decreases in motility of 70.97%, 83.87%, and 37.1%, respectively, in
comparison with AE17. Moreover, in comparison with AE17, AE18, AE19, and AE20 showed significant
decreases of 63.11%, 65.42%, and 30.26%, respectively, in CEF cell adhesion, and significant
decreases of 59.83%, 57.82%, and 37.90%, respectively, in CEF cell invasion. In comparison with
AE17, transcript levels of sodA, polA, and iss were significantly decreased in AE18, while transcript
levels of fimC and iss were significantly decreased in AE19. Our results demonstrate that deletion of
PhoP or PhoQ inhibits invasion and adhesion of APEC to CEF cells and significantly reduces APEC
virulence by regulating transcription of virulence genes.
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