Moraxella catarrhalis is the third most common cause of acute otitis media in children, the most common pediatric infection treated with antimicrobial agents. The outer membrane protein UspA1, an antigenically conserved high molecular weight adhesin, is a potential vaccine candidate. UspA1 was shown to be expressed by a majority of Moraxella isolates from children less than two years old. However, a minority of strains apparently lacked UspA1 expression or demostrated a downregulation of expression. Characterization of this subpopulation is important for the further evaluation of UspA1 as a candidate vaccine antigen. Furthermore, data obtained in our laboratory led to the hypothesis that UspA1 expression is regulated in concert with other outer membrane proteins of Moraxella. Therefore, the aims of our future research will be the investigation of regulation of Moraxellacatarrhalis outer membrane proteins. Subpopulations of Moraxella catarrhalis with reduced expression of outer membrane proteins will be genetically defined. In addition, surface expression of outer membrane proteins will be studied in vivo in the human nasopharynx and in vitro under various environmental conditions. A further aim of the studies will be the search for global regulatory mechanisms governing expression of multiple outer membrane proteins in Moraxella catarrhalis.