Methylene blue is a staining reagent that is used to stain Gram-positive bacteria. It stains Gram’s stain pink and causes the bacteria to appear as blue in the Gram’s stain. Methylene blue has been used for decades in clinical microbiology laboratories and remains an important tool for the identification of Gram-positive bacteria. However, it has several limitations:
- Methylene blue is not absorbed by most types of bacteria, so it cannot be used to identify a bacterial species if another stain has been applied first, such as crystal violet or safranin O.
- Methylene blue stains some types of bacteria (e.g., enterococci) more intensely than others (e.g., Neisseria gonorrhea), so it may not be appropriate for all clinical strains of pathogenic microorganisms.
- Methylene blue stains both aerobic and anaerobic organisms equally well, which makes it difficult to differentiate between aerobic and anaerobic organisms on Gram’s stain slides unless special stains are used (e.g., methylene blue/pus diluent).