PROJECT SUMMARY The human gastrointestinal (GI) tract harbors trillions of microbes that are collectively referred to as the microbiota. The resident microbiota play complex roles in human health. For example, the microbiota may effectively compete for nutrients against invading pathogens and thereby act as a barrier to limit or prevent infections. Conversely, certain members of the microbiota may enhance pathogen infection by producing metabolites that enhance growth and/or influence virulence gene expression. Because of the interplay between bacterial pathogens and the microbiota, manipulation of the microbiota may be an innovative means to treat infectious diseases. However, a more complete understanding of the specific roles of distinct members is required. To date, most studies have focused on microbes belonging to the domain Bacteria, which comprise the majority of microbes in the GI tract. Significantly, the microbiota is also comprised of microbes belonging to the domain Archaea. Of the Archaea, the methane-producing methanogens are the predominant members. Methanogens are present in the GI tract of nearly 100% of the world’s population and have been proposed to play an important role in human health and/or disease. Despite this fact, the functional contributions of methanogens to human health are virtually unexplored. Accordingly, Specific Aim 1 will assess how inhibiting methanogenic activity in vivo influences disease progression by an invading bacterial pathogen. Specific Aim 2 will investigate the impact of methanogens on enteric infection using a defined microbiota.