| The goal of this work was to define the factors that inhibit growth of Bacteroides fragilis at oxygen levels > 0.05%. One important aspect of this growth arrest is the accumulation of endogenously-generated reactive oxygen species (ROS) like hydrogen peroxide (H2O2). We show here that H2O2 scavenging rates were reduced to 20% of the wild-type in a strain missing alkylhydroperoxide reductase ( ahpC), catalase, and thioredoxin-dependent peroxidase (tpx ). This strain was hypersensitive to room air and was found to generate ROS at a rate of ∼35nM/min/OD600=0.1 under these conditions. However, deletion of fumarate reductase gave rise to a strain that accumulated ∼19nM H2O2/min//OD600=0.1, indicating that this enzyme accounts for ∼47% of the ROS generated by aerated B. fragilis . Deleting frdC increased the aerotolerance of a Delta sod strain by ∼100-fold during 9 hours of exposure to room air.;Spontaneous mutants of B. fragilis capable of growth under 1% oxygen arise at a frequency of ∼10-6. These strains carried mutations in an orf designated oxe. Deletion of oxe established the O2-enabled phenotype, and this mutant could be re-sensitized to oxygen by providing a wild-type copy of oxe in trans. Microaerobic growth of the Delta oxe strain was characterized by several amino acid auxotrophies. Though Oxe is not a major source of ROS under room air, a Delta oxe strain was found to scavenge micromolar amounts of H2O 2 at a rate approximately triple that of the wild-type.;Fecal strains of B. fragilis did not grow under ∼1% oxygen. Ten clinical strains plated microaerobically with efficiencies greater than 10%, 14 plated with frequencies similar to the fecal strains, and 7 had an intermediate phenotype. The oxe locus was sequenced for 7 clinical strains that plated with high efficiencies microaerobically and four were found to encode a wild-type Oxe, while 3 were predicted to contain amino acid substitutions in Oxe.;Oxe may contribute to the utilization of amino sugars by B. fragilis , as a Deltaoxe strain bearing a mutant allele of nagB could not grow on N-acetylglucosamine or glucosamine. However, the role of Oxe in the anaerobic physiology of this organism is still under investigation. |
