H aemophilus influenzae is a Gram-negative pathogen colonizing the upper respiratory tract mucosa. H . influenzae is one of several human-restricted bacteria, which bind to carcinoembryonic antigen-related cell adhesion molecules ( CEACAMs) on the epithelium leading to bacterial uptake by the eukaryotic cells. Adhesion to CEACAMs is thought to be mediated by the H . influenzae outer membrane protein ( OMP) P5. However, CEACAMs still bound to H . influenzae lacking OMP P5 expression, and soluble CEACAM receptor ectodomains failed to bind to OMP P5, when heterologously expressed in E scherichia coli. Screening of a panel of H . influenzae OMP mutants revealed that lack of OMP P1 completely abrogated CEACAM binding and supressed CEACAM-mediated engulfment of H . influenzae by epithelial cells. Moreover, ectopic expression of OMP P1 in E . coli was sufficient to induce CEACAM binding and to promote attachment to and internalization into CEACAM-expressing cells. Interestingly, OMP P1 selectively recognizes human CEACAMs, but not homologs from other mammals and this binding preference is preserved upon expression in E . coli. Together, our data identify OMP P1 as the bona fide CEACAM-binding invasin of H . influenzae. This is the first report providing evidence for an involvement of the major OMP P1 of H . influenzae in pathogenesis. [ABSTRACT FROM AUTHOR]