Hag directly mediates the adherence of Moraxella catarrhalis to human middle ear cells

Moraxella catarrhalis is a human pathogen that causes otitis media in young children and lung infections in patients with chronic obstructive pulmonary disease. In this study, the role of the surface protein Hag in the adherence of multiple M. catarrhalis strains was examined. The hag genes of four clinical isolates were disrupted with a spectinomycin resistance cassette, and the binding of isogenic mutants to primary cultures of human middle ear epithelial cells (HMEE), as well as A549 pneumocytes, was measured. These experiments revealed that the attachment of most mutants to both cell types was 10-fold less than that of their wild-type progenitors. To determine whether Hag directly mediates adherence to human cells, the hag genes from three M. catarrhalis isolates were cloned and expressed in a nonadherent Escherichia coli cloning strain. At least 17-fold more E. coli bacteria expressing Hag attached to HMEE cells than an adherence-negative control. Surprisingly, Hag expression did not increase the binding of recombinant E. coli to A549 monolayers. Our data demonstrate that the involvement of Hag in M. catarrhalis adherence to A549 and HMEE cells is conserved among isolates and that Hag directly mediates binding to HMEE cells.