Thioglycollate broth (Fluid Thioglycollate Medium) is a medium designed to test the aerotolerance of bacteria. Along with nutrients to support bacterial growth, it contains sodium thioglycollate, thioglycollic acid, L-cystine, methylene blue, and 0.05% agar. The sodium thioglycollate, thioglycollic acid, and L-cystine reduce the oxygen to water. Methylene blue is an indicator that is colorless in an anaerobic environment and greenish-blue in the presence of oxygen. The agar helps retard oxygen diffusion and helps maintain the stratification of organisms growing in different layers of the broth.