Fluid Thioglycollate Medium

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.

Oxygen is driven out of the broth by autoclaving, but as the broths sit at room temperature, oxygen begins to diffuse back into the tube. This is evidenced by the small layer of blue-green at the top of the broth. 

Obligate aerobes will only grow in this oxygen-rich top layer.

Obligate anaerobes will only grow in the lower areas of the tube. 

Microaerophiles will grow in a thin layer below the richly-oxygenated layer.

Facultative or aerotolerant anaerobes can grow throughout the medium but will primarily grow in the middle of the tube, between the oxygen-rich and oxygen-free zones.