Thioglycollate broth is a reducing medium. It contains sodium thioglycollate, which reacts with molecular oxygen keeping free oxygen levels low. The sodium thioglycollate in the broth creates a redox potential in the tube, with higher levels of oxygen at the top of the tube, and a complete absence of oxygen at the bottom of the tube. Fluid thioglycollate broth also typically contains a redox potential indicator such resazurin, which produces a pink color in an oxidized environment. As with the BHI media, organisms will only be able to grow where their oxygen requirements are met, and will localize to the area(s) of their oxygen requirements in the fluid thioglycollate broth