* Methylene blue is a redox indicator: It changes color depending on its oxidation state. In its oxidized form, it's blue, and in its reduced form, it's colorless.
* Living cells have reducing power: Living cells contain enzymes that can reduce methylene blue, converting it to its colorless form. This is because they are actively metabolizing and have a high concentration of reducing agents like NADH and NADPH.
* Staining: When living cells are placed in a methylene blue solution, the dye enters the cells. However, the cells' reducing power quickly reduces the methylene blue, making it colorless within the cell. The dye remains blue only in areas where it's not being reduced, such as the cell wall or regions with lower metabolic activity.
Therefore, the blue color observed in living yeast cells indicates that the methylene blue has been reduced, and thus it's not a true stain but rather a visual indicator of the cells' metabolic activity.
Here's a simple explanation:
Imagine you have a blue paint that turns clear when it comes in contact with a specific type of substance. If you dip a sponge into the blue paint, it will turn blue because the sponge is absorbing the paint. However, if you dip the sponge in a solution containing that specific substance, the paint will turn clear within the sponge.
Similarly, methylene blue turns clear inside living yeast cells because they have a high concentration of substances that reduce the methylene blue. However, the cell wall and areas with lower metabolic activity will still retain the blue color, making the cells appear blue overall.
