Nasal carriage of Staphylococcus aureus (SA) is a common factor that predisposes individuals to severe nosocomial infections, and acts as an important reservoir for harboring and spreading resistant strains. The disorder affects nearly a quarter of apparently healthy people and its molecular and cellular bases are unknown. Experiments from our pilot project revealed that SA nasal carriage may be due to impaired innate antimicrobial activity of nasal fluid. The protein expression levels of a major host defense polypeptide, lipocalin-1 (a scavenger of bacterial siderophores), was found to be reduced in human nasal fluid from donors whose nasal passageways were colonized by SA. In antibacterial studies, lipocalin-1 worked in concert with lysozyme to kill SA in vitro. Most importantly, lipocalin-1 could restore the intrinsic anti-SA activity of non-carrier nasal fluid selectively depleted of cationic polypeptides, and the restorative activity could be abolished by the addition of iron. In the aggregate, our findings clearly suggest an important correlation of lipocalin-1 deficiency with SA carriage. We hypothesize that 1) the expression of lipocalin-1 is dysregulated in the nasal mucosa of SA carriers, contributing to the progressive colonization of SA, 2) correcting the lipocalin-1 deficiency will reconstitute the antimicrobial activity of SA carrier nasal fluid against isolates of SA, and 3) SA augments the epithelial expression of lipocalin-1 and other host defense molecules, which contributes to preferential colonization of SA on carrier mucosa as compared with non- carrier mucosa. To test these hypotheses, we will: 1) Characterize the biological role of lipocalin-1 in SA nasal carriage, 2) Examine the influence of bacterial and host factors on the expression and anti-SA activity of lipocalin-1, and 3) Examine the contribution of human nasal epithelium to SA colonization. Our proposed studies represent a biologically relevant approach to identify and link causative factors of human airway disease (cationic polypeptide antimicrobials) with their effects (SA nasal carriage). Relevance to Public Health: SA carriage is of increasing clinical importance because hospital-acquired infections are commonly spread by people who carry antibiotic-resistant SA in their nostrils. Our studies will characterize factors responsible for SA carriage, and will continue to develop a very useful and natural model for studying the interactions of bacteria with a readily accessible mucosal surface in humans.