We previously observed in mice that Bacteroides thetaiotaomicron (B. theta) significantly increased in abundance in the gut microbiome of mice when mice were fed egg and yeast dietary protein sources. We also observed that B. theta was expressing proteins previously connected to growth on mucin glycans when mice were fed an egg-white diet. To confirm that the bacterium were actually responding to the protein sources, we grew it in vitro using a defined medium, where the sole carbon source was dietary protein, mucin, or glucose. Our controls were glucose and mucin, and our experimental protein sources were soy protein, egg-white protein, and Torula yeast protein. We grew four B. theta cultures per carbon source statically at 37°C in a Coy anaerobic chamber (2.5 % H2 /10 % CO2 /88.5 % N2) in minimal medium (100 mM KH2PO4, 8.5 mM [NH2]4SO4, 15 mM NaCl, 5.8 μM vitamin K3, 1.44 μM FeSO4⋅7H2O, 1 mM MgCl2, 1.9 μM hematin, 0.2 mM L-histidine, 3.69 nM vitamin B12, 208 μM L-cysteine, and 7.2 μM CaCl2⋅2H2O) with one of the above mentioned nutrients added at 0.5% (wt/v) concentration. In order to aid the solubilization of the dietary proteins, we pre-prepared the proteins in 200 mM NaOH at 37°C for four days, the glucose control was also dissolved in 200 mM NaOH. After 8 hours, we enumerated CFUs to confirm growth, and we pelleted cells by centrifuging at 4,000 g for 10 minutes. We then removed the supernatant and froze the pellets at -80°C within 30 minutes.