Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Avian Pathogenic Escherichia coli is responsible for significant economic losses in the poultry industry by causing a range of systemic or localized diseases collectively termed colibacillosis. The virulence mechanisms of these strains that are pathogenic in poultry and possibly pathogenic in humans have not yet been fully elucidated. This work was developed to study if over-expressed genes in a microarray assay could be potentially involved in the pathogenicity of an Avian Pathogenic Escherichia coli strain isolated from a swollen head syndrome case. For this study, five over-expressed genes were selected for the construction of null mutants [flgE (flagellar hook), tyrR (transcriptional regulator), potF (putrescine transporter), yehD (putative adhesin) and bfr (bacterioferritin)]. The constructed mutants were evaluated for their capacity for the adhesion and invasion of in vitro cultured cells, their motility capacity, and their pathogenic potential in one-day-old chickens compared with the wild-type strain (WT). The Delta bfr strain showed a decreased adhesion capacity on avian fibroblasts compared with WT, in the presence and absence of alpha-D-mannopyranoside, and the Delta potF strain showed decreased adhesion only in the absence of alpha-D-mannopyranoside. The Delta tyrR mutant had a reduced ability to invade Hep-2 cells. No mutant showed changes in invading CEC-32 cells. The mutants Delta flgE and Delta tyrR showed a decreased ability to survive in HD-11 cells. The motility of the mutant strains Delta bfr, Delta yehD and Delta potF was increased, while the Delta tyrR mutant showed reduction, and the Delta flgE became non-motile. No mutant strain caused the same mortality of the WT in one-day-old chickens, showing attenuation to different degrees.45194105Fundacao de Amparo a Pesquisa do Estado de Sao Paulo [2010/50596-9]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)