PROJECT SUMMARY Staphylococcus aureus is one of the most prevalent organisms responsible for nosocomial and community acquired infections in humans, and cases of community associated methicillin-resistant S. aureus (CA- MRSA) infection have continued to increase despite wide-spread preventative measures. j Another common resident of the vaginal tract in up to 30% of healthy women is Streptococcus agalactiae, also known as Group B streptococcus (GBS). During pregnancy GBS infection can be associated with bacteriuria, urinary tract infection, and upper genital tract infection, and following transmission to the newborn can cause serious neonatal infection. GBS surface adhesins that bind fibrinogen (Fg) are required for vaginal cell adherence and persistence in the vaginal tract in vivo, indicating that Fg is an important vaginal matrix component for bacterial colonization. Epidemiological studies suggest that there may be an association between MRSA and GBS during vaginal colonization, but these polymicrobial interactions have not been investigated. We hypothesize that to promote vaginal colonization, MRSA must express factors that promote interaction with Fg and vaginal epithelium, and cooperate with other common colonizers like GBS for niche establishment. These hypotheses will be addressed both in vitro and in vivo using a human vaginal epithelial cell line, a mouse model of MRSA vaginal colonization and novel bacterial mutants lacking regulatory factors and various surface adhesins involved in Fg interaction. The following specific aims will be addressed: AIM 1: Identify the molecular determinants that promote MRSA vaginal colonization; AIM 2: Assess MRSA interaction with fibrinogen as a determinant of vaginal colonization; AIM 3: Characterize polymicrobial interactions during vaginal adherence and colonization. The knowledge gained as a result of these proposed studies will provide fundamental and novel insights into mechanisms of MRSA vaginal colonization and polymicrobial interactions as well as inform treatment strategies to prevent MRSA vaginal carriage.