PROJECT SUMMARY The National Action Plan for Combating Antibiotic-Resistant Bacteria identified critical research needs to confront the advancing threat of antimicrobial resistance. These include understanding the nature of microbial communities, determining the effects of antibiotics on these communities, and harnessing these communities to develop strategies for disease prevention. Staphylococcus aureus causes significant morbidity and mortality in healthcare and community settings. S. aureus carriage confers risk for endogenous infection. Previous studies investigated clinical and epidemiological factors associated with S. aureus colonization and infection, although the influence of the skin and nasal microbiota on S. aureus acquisition is unclear. We hypothesize that features of the commensal microbiota may confer susceptibility to, or provide protection against, S. aureus acquisition and development of subsequent infection. Further, in an effort to prevent S. aureus acquisition and infection, decolonization with topical antimicrobials is frequently employed in healthcare and community settings. However, broad-spectrum topical antimicrobial application may disrupt commensal microbiota, thereby promoting the acquisition or enrichment of multidrug-resistant and/or potentially pathogenic microorganisms. The impact of this potential adverse effect remains understudied. In this proposed project, we will analyze a vast and unmatched biorepository of >13,000 skin and nasal swab samples that were longitudinally collected over 24 months from a well-characterized cohort of 476 participants, including 99 pediatric patients with methicillin-resistant S. aureus skin infections and their household contacts, all of whom are at high risk for S. aureus acquisition and infection. The samples were collected during 12 months of longitudinal samplings (the natural history phase) followed by a randomized, 5-day decolonization intervention with topical antimicrobial application, and subsequent longitudinal sampling for 12 months (post-decolonization phase). We will perform metagenomic shotgun sequencing (enabling classification at the species and strain levels), to longitudinally characterize skin and nasal microbial communities in the context of S. aureus colonization and disease. We will compare microbial community features associated with susceptibility or resistance to S. aureus acquisition between household contacts with different phenotypic outcomes (e.g., colonization and infection), and integrate these data with detailed epidemiological data to predict individuals at risk for acquiring S. aureus colonization and/or developing symptomatic infection. We will quantify changes in microbial community structures following topical antimicrobial application, and correlate this disruption with subsequent acquisition and persistence of S. aureus and other pathogens. Finally, we will employ innovative targeted sequence capture to quantify genetic determinants of ant...