Irritable bowel syndrome (IBS) is a disease that affects many people worldwide. Clinical trials using probiotic bacteria to treat IBS show promising outcomes, although the molecular mechanisms of probiotics and their effect on gut health is not completely understood. Multiple studies comparing different strains of probiotics show Lactobacillus plantarum and Bifidobacterium longum as the most effective strains in inhibiting the release of proinflammatory factors in the gut and promoting proliferation of healthy gut bacteria. The purpose of this study is to evaluate the protein profile of probiotic bacteria Lactobacillus plantarum and Bifidobacterium longum in both healthy gut and IBS gut conditions. To observe how Lactobacillus plantarum and Bifidobacterium longum respond to different simulated gut conditions, samples of the bacteria will be isolated from probiotic supplements. These samples will be grown in aliquots representing different simulated anaerobic gut conditions for 12-16 hours at 37°C. Lactobacillus plantarum and Bifidobacterium longum will be grown separately and grown combined within these conditions. To enrich secreted proteins, the supernatant from each aliquot sample will be clarified using centrifugation. Secreted proteins will be solubilized and denatured with sodium deoxycholate. The denatured proteins will be reduced with dithiothreitol and alkylated with iodoacetamide. The sample will be diluted using ammonium bicarbonate and digested by trypsin. Tryptic peptides will be analyzed using liquid chromatography and mass spectrometry. Tryptic peptides will be identified using the SEQUEST search engine with a reference database. I anticipate the probiotic strains will demonstrate differential proteomics when isolated and combined in the healthy gut and IBS gut conditions. The secreted protein analysis completed in this study will further the understanding of the probiotic molecular mechanism of promoting gut health.