After in vitro infection of the human epithelial cell line A549, we found that the genes most frequently expressed among the gingivitis, moderate periodontitis, and chronic periodontitis strains were those related to adhesion (fimH, iha and papA), toxins (usp and hlyA), iron acquisition (feoB, fyuA, irp-2 and iroN), protectins (traT, KpsMT and ompT), and pathogenicity islands (malX). This evidence concerns the gene IREB2 and periodontitis.