Uropathogenic E. coli strains are included in the recently defined pathotype ExPEC together with other E. coli isolates involved in various extraintestinal infections. These strains possess specific virulence factors such as adherence systems, toxins, iron uptake systems that distinguish them from the commensal strains. The expression of bacterial adherence structures requires a complex organization which involves a different number of proteins depending upon the assembly in fimbrial protrudings or afimbrial aggregates. However one could notice that their genetic determinants are usually similar. The toxins synthetized by ExPEC strains, mainly represented by hemolysin and cytotoxic necrotizing factor contribute to destruction of eukaryotic cells. Such siderophores as aerobactin confer the strains the advantage of capturing iron from the environment in which they must survive and replicate. Co-expression of virulence factors contributes to the host's defence system defeat and onset of infection. Quite often the codifying genes are localized in distinctive regions on the bacterial chromosome called pathogenicity-associated islands. The horizontal transfer of these DNA blocks among individuals belonging to the same species is one of the processes involved in the evolution of pathogenic strains.