What does the appearance of a blue or pink color in a thioglycolate tube mean?

The appearance of a pink or blue color in a thioglycolate tube means that oxygen is present in that part of the tube. The different colors depend on which dye has been added. Pink color comes from resazurine, and blue is from methylene blue.

Thioglycolate broth with dyes is used to determine whether the microorganisms growing in the broth are aerobes or anaerobes. An organism that only grows in the pink band of the thioglycolate tube is a strict aerobe. Organisms that only grow outside of the pink band are strict anaerobes. Depending on where the microorganisms grow in the tube, it is also possible to determine if they are facultative aerobes, facultative anaerobes or aerotolerant microbes.