Nontypeable Haemophilus influenzas (NTHi) is a predominant etiologic agent of otitis media (OM) in chil- dren as well as a major cause of pneumonia and other respiratory tract diseases in adults. Although much has been learned about several outer membrane proteins and the lipooligosaccharide produced by this or- ganism, overall we have a poor understanding of the pathogenesis of Haemop/7/7i;s-induced infection. Type IV pili (Tfp) play a significant role in the pathogenesis of disease caused by numerous Gram-negative pathogens and some Tfp-based immunogens induce protective activity against these organisms. Although a cryptic gene cluster that might encode a Tfp in Haemophilus has long been recognized, expression of Tfp has never been demonstrated by this organism. In fact, in a review entitled DNA uptake during Bacterial Transformation, published in Nature Reviews Microbiology in March 2004, Chen and Dubnau state "Other competent organisms, such as H. influenzae or the Gram-positive bacteria B. subtilis and S. pneumoniae, require similar genes for DNA uptake, but do not possess filamentous structures that extend from the cell surface". Further, twitching motility, a non-flagellar-based Tfp-dependent motility has never been observed among the Pasteurellaceae. We recently demonstrated that NTHi produces Tfp under defined environmental conditions and further, that Tfp are responsible for twitching motility in NTHi. We hypothesize that Tfp will play an important role in pathogenesis. We have also determined that H. influenzae has a single copy of the pilA gene (encodes the major pilin subunit) and we have now sequenced this gene in 11 NTHi strains. Unlike the situation for N. gonorrhoeae, the derived amino acid sequence of these genes is highly conserved which is a considerable advantage for our vaccine development efforts that intend to target the H. influenzae Tfp. Experiments are thereby proposed herein to determine how the Tfp are assembled as well as how syn- thesis and assembly of these structures are regulated. Additionally, experiments are proposed design a type IV-pilin based immunogen that will induce protection against infection caused by the majority of nontypeable H. influenzae isolates. These experiments will lead to a better understanding of the pathogenesis of NTHi- induced disease as well as new approaches for the prevention of disease caused by NTHi. [unreadable] [unreadable]