| The opportunistic pathogen Bacteroides fragilis is a commensal organism of the human large intestine where it utilizes both dietary and host-derived polysaccharides as a source of carbon and energy. The studies described in this dissertation have focused on the characterization of a novel operon, designated o&barbelow;xygen-induced s&barbelow;tarch u&barbelow;tilization (osu), which encodes four genes, osuABCD whose translational products are involved in starch utilization. The first three genes of this operon, osuABC, are predicted to encode outer membrane/starch binding proteins, whereas the fourth gene, osuD has been shown to encode the primary alpha-amylase involved in starch hydrolysis in this organism. We have identified a gene, osuR, which encodes a transcriptional activator of this operon, responsive to maltose and higher glucosides. This obligate anaerobe has evolved a complex oxidative stress response (OSR) to allow it to combat the toxic effects of oxygen exposure outside of the anaerobic confines of the GI tract, where it is the most frequent isolate from anaerobic infections such as intra-abdominal abscesses. The results of this study demonstrated that expression of the osu operon was oxygen responsive, with expression of osuA induced approximately 20-fold upon exposure to air. Differential expression of osu results from oxygen stress or anaerobic starch utilization. The transcriptional activator OsuR, although involved in anaerobic regulation of this operon, did not play a role in activation during oxygen exposure. Consistent with this observation a shift in the transcriptional start site was observed during oxygen stress, compared to anaerobic growth on starch and this suggests the involvement of a different regulator under these conditions. Building on the hypothesis that expression of this operon was important for survival during oxidative stress, results from osu mutational studies demonstrated that expression of the osu operon was important for survival of B. fragilis during oxygen exposure. The mechanism of osu protection has not been fully determined but may involve providing a utilizable energy source to the cell during pathogenesis to maintain the OSR during oxygen exposure, thereby enhancing the pathogenic potential of B. fragilis. |
