A thioglycollate indicator (typically resazurin) is a redox dye used to visually map oxygen levels in bacterial culture tubes.
In its oxidized state, it turns pink when exposed to oxygen; in its reduced, oxygen-free state (typically at the bottom), it remains colorless.
Resazurin Indicator Characteristics
Pink Zone: Indicates the presence of oxygen (aerobic zone) at the top of the tube.
Colorless Zone: Indicates the absence of oxygen (anaerobic zone) deeper in the tube.
Reversibility: Resazurin is highly sensitive; if the broth is shaken, it re-oxidizes and the top portion will turn pink until the thioglycolic acid reduces the oxygen again