Bacteroides fragilis is an obligate anaerobe that resides in the gut. A particular isolate of B. fragilis, enterotoxigenic B. fragilis (ETBF), has been linked to colorectal cancer. The gut microbiome of colorectal cancer patients exhibited increased levels of ETBF and mice infected with ETBF developed colorectal tumors. While many antibiotics eliminate harmful bacteria, they also damage symbionts in the gut flora which allows for the rise of other pathogens. The objective of this project is to find ETBF-specific compounds that inhibit the growth of this microorganism and identify their targets and mode of action. After screening a small molecule library against B. fragilis, two compounds were identified to have sufficient selectivity and low toxicity to allow for in vivo experiments. Compound Bfr81 is highly specific for B. fragilis (MIC 8 ug/ml) and was inactive against all other species of the commensal panel tested, including Bifidobacterium longum, Staphylococcus aureus, Escherichia coli, and Clostridium difficile (MIC > 128 ug/ml). Compound Bfr44 displayed activity against Clostridia (MIC 4-16 ug/ml), but was able to discriminate between B. fragilis (MIC>128 ug/ml) and ETBF (MIC = 8 ug/ml), which are the same species though independent isolates. We will use Tn7 transposon mutagenesis to obtain mutants resistant to the tested compounds to identify the mechanism of action. This information can be used for the development of necessary drugs to prevent colon cancer, a major disease affecting 5% of the US population.